WEST SOMERSET AMATEUR RADIO CLUB

Last month’s meeting is the annual Surplus Equipment Sale. This was not well attended; however, due to the generosity of those who were there (both donors and bidders), well over _ 40 was raised for Club funds. Two items were destined to be sold on eBay as these had values way in excess of the contents of the pockets of those present.

the 2nd Tuesday of January - 8th.Gather at 7.00pm with Eating and Drinking to start around 7.30pm

G3OUC was much surprised at this failure of Araldite as a RF insulator. But subsequent checking with The Physical Laboratory Handbook (Pitmans, 1966) revealed that Araldite is unsuitable for use at high frequencies, having a dielectric loss factor of 270 (x1000) at 10MHz (compared with 0.3 for polystyrene and 0.2 for mica). The Hand-book also showed that other well-known substances and plastics are likewise unsatisfactory above 10MHz with PVC, cellulose and ebonite specifically noted. However, I seem to recall that a reader once pointed out that some PVC pipes are, in fact, much better RF insulators than others, and can be used at HF as large coil formers etc. Any plastic can be checked using the 'microwave oven test', in which a small piece is cooked briefly (remember always to place also a bowl of water in the oven). RF-resistant plastic will remain hard and cool. Poor RF performance will be indicated by the plastic heating and softening. RadComMay2001

CLUB CALL AFFILIATED

GXØOWX WEST SOMERSET AMATEUR RADIO CLUB R.S.G.B.

Last month’s meeting was the annual Construction Contest. the demonstration of which might make your hairs stand on end (or rather wilt!).

Next month’s meeting is the annual Surplus Equipment Sale. This should be a good session as Oli has kept back various donations from members. It is also rumoured that Ian Aldridge might arrive with more items from his garage where the contents increase miraculously by themselves.

1) Free of charge.

I have a Transformer, Large smoothing capacitor and heat sink from a Ten-Tec 20 Amp PSU - If anyone at the club is interested its FREE to pick up from Mac/G4OEC in Holford 01278 741527 or e-mail e.mcpheat@virgin.net

2) _ 15 Liner 2 2 metre S.S.B. rig. Contact Oli oli.g3nfy@virgin.net

South Sussex RAYNET group were present to help when a plane crashed at the Shoreham Airshow. Each year an emergency plan is rehearsed and refined, in the hope that it will never be needed. However on the weekend 15 and 16 of September that practice paid off. On the Saturday a Hurricane nose dived into the ground, killing its pilot instantly. The incident happened off-field, and no one else was involved.

GM3ZYE, and his son Richard were controlling the routine channel and looking out over the field at the time of the incident. Their shout of 'aircraft down' brought everyone in control to high alert. The emergency channel sprang into use. The pre-prepared messages and procedure for Incident Off Field was broadcast, and acknowledgments sought from all on that net. This resulted in all access and departure gates being closed, all senior safety and administrative personnel being briefed, the PA Caravan being informed, and members of the public kept aware of the situation. Shortly after the accident, Spitfires flew a 'man missing' flypast in tribute to the downed pilot.

HOW MANY FARADS?

It doesn't seem that long ago that a 100,000- microfarad capacitor was considered a huge value. The idea of a one Farad capacitor was practically unthinkable, but the 1mfd capacitor duly arrived.

Inevitably it was a low voltage device, the typical use being to iron-out fluctuations in power supply voltage and hold equipment up for a short while when power failed. Maxwell Technologies have now introduced a range of "ultracapacitors" with values up to staggering 3 kilofarads (3000 Farads, or 3 billion microfarads). The typically use for one of these capacitors would be to connect it across the output of a power supply, to deliver a pulse of energy that is not efficiently provided by a battery or power supply alone. Still low voltage units, they can only withstand 2.7 volts, but for higher voltage applications the company also provide modules containing several ultracapacitors in series.

It seems Toshiba might be at the gluepot, sticking a standard DVD back-to-back with an HD DVD so that standard content can be included for ordinary DVD players.

UNIVERSAL FLASH STORAGE. Digital camera and cameraphone users will be all too familiar with the variety of non- compatible memory chips and sticks there are. Although adapters are available to convert some types to others, the current situation is that there are numerous non- compatible memory types out there. It's easy to see how such a situation arose, with rival manufacturers each fighting to have their system adopted, but in most instances when rival formats slug it out one ends up being victorious. Although one or two types have fallen by the wayside this hasn't generally happened with flash memory devices, but hope is on the horizon because there is now a proposed specification for Universal Flash Storage (UFS). Currently it is being backed by Nokia, Samsung, Sony Ericsson, Micron Technology, Spansion, MTMicroelectronics and Texas Instruments. UFS devices are expected to require little power and operate at high speed. The drawback is that even the standard is not expected to be finalised until 2009, so UFS chips won't be appearing any time soon. I wonder if they will be compatible with present day equipment, via adapters.

PEE POWER. Japanese company Aqua Power Systems has released onto the market (in Japan, at least) a new generation of 'Non-Pollution Power' AA and AAA batteries that can be recharged with water (or bodily fluids!) via a pipette. The capacity of their AA cells is approximately 500mAh, making them broadly equivalen to zinc-carbon cells (alkaline cells can be up to 3000mAh). These so-called NoPoPo 'aqua batteries' can hold their charge for up to 10 years, but can only be re-charged 3-5 times before being disposed of. They could represent the beginning of a whole new generation of batteries that can be re-charged anywhere without a power source.

[The following is one of the best descriptions of how a V.S.W.R. indicator works - and I have 5 articles to choose from!]

DIRECTIONAL POWER METERS.

The HF directional power meter is based on a Bruene [1] type of directional coupler. The Bruene circuit uses a transformer to sample RF current on a transmission line and a voltage divider to sample the voltage across the line. Figure 1 shows a simplified schematic of a Bruene coupler. The primary of the current transformer (T) usually consists of a single wire through the centre of a ferrite or powdered iron toroid core. RF current in the secondary winding develops a voltage across R3. This voltage at point B is proportional to the current on the line. The voltage divider resistors Rl and R2 produce a voltage at point A which is proportional to the voltage across the line. When a transmission line is terminated by a resistance which is exactly equal to the line characteristic impedance, the SWR on the line is 1:1 and voltage on the line is exactly in phase with line current. The values of Rl and R2 are chosen so that for a given value of line impedance (usually 50Ω), the voltage at A is equal to and in phase with the voltage at point B. This means that there will be no voltage across AB. This circuit is essentially the same as in the Wheatstone bridge.

If the line is not properly terminated, reflected waves on the line will lead to out-of-phase voltage and current on the line which will unbalance the bridge and cause a voltage difference across AB. A sensitive detector at AB can be used to measure reflected power on the line. If the input and output connections are reversed, the bridge will indicate forward instead of reflected power. Swapping the input/output connections is a very inconvenient way of switching between forward and reflected power-it is much easier to reverse the phase of the current sample by using a double-pole switch. If the secondary winding of the current transformer is centre tapped, a single pole switch can be used instead. If the line is not properly terminated, reflected waves on the line will lead to out-of-phase voltage and current on the line which will unbalance the bridge and cause a voltage difference across AB.

The Space Pencil

Originally American astronauts, like the Soviets, wrote with pencils, according to NASA historians. Indeed, in 1965 NASA ordered 34 mechanical pencils from Tycam Engineering Manufacturing in Houston at $128.89 apiece: $4,382.50 in total. When these sums became public and caused an outcry, NASA scrambled to find a cheaper alternative. Pencils may not have been the best choice anyway. The tips could flake or break off, drifting in microgravity where they might harm an astronaut or equipment. And pencils are flammable—a characteristic NASA wanted to avoid in onboard objects after the Apollo 1 fire.

The Space-Age Ballpoint. Meanwhile the Fisher Pen Company had invested a reported $1 million (none of it from NASA) to create what is now commonly known as the space pen. The device, patented in 1965, could write upside down, in frigid or roasting conditions (down to -50 degrees Fahrenheit or up to 400 degrees F), and even underwater or submersed in other liquids. If too hot, though, the ink turned green instead of its normal blue.

Fisher offered the implement to NASA. Because of the earlier mechanical pencil fiasco, the agency hesitated. But after testing the tool—named the AG-7 "Anti-Gravity" Space Pen—the U.S. decided in 1967 to use it on future spaceflights. Fisher's pen makes up for a lack of gravity by storing ink in a cartridge pressurized with nitrogen at 35 pounds per square inch—more than twice as much force as sea-level atmospheric pressure on Earth. This pressure pushes the ink toward the tungsten carbide ball at the pen's tip. The ink, too, differs from that of other pens. It stays a gel-like solid until the movement of the ballpoint turns it into a fluid. The pressurized nitrogen also prevents air from mixing with the ink, so it cannot evaporate or oxidise. (Summarised from Scietific American Aug 2007)

. (I hope that some of you made the effort to contact them). GM3OXX, using just 1 watt, worked them on 5 of the bands that they were using. Great encouragement for QRP & Novices.

Also from the 3B7C expedition:-

They had the opportunity of directly comparing verticals with Yagis also mounted close to the ocean. Numerous comparisons carried out by 3B7C team members came to a number of conclusions with which everyone agreed. Firstly, in terms of both transmitted and received signal strengths, there was very little or nothing to choose between even a single vertical and a 3- or 4-element monoband Yagi in its preferred direction. On 30m, the single quarter-wave vertical often outperformed the full-size 2-elementYagi (which admittedly was mounted less than a half-wave above ground). Secondly, being omnidirectional, the verticals obviously picked up stations from all directions, whereas if the Yagi was beaming to Japan very few or no Europeans would call, and vice versa. The omnidirectional radiation pattern of the vertical could be seen as both an advantage and a disadvantage: it did allow 3B7C to be heard over a wider geographical area than was the case with the Yagi, but it also meant that the 3B7C operator had to contend with a higher level of QRM as stations called from all points of the compass! The background noise level was also noticeably higher with the vertical antennas. Yagis are therefore preferable to verticals in order to keep up the rate (number of QSOs per minute or hour) by focussing the pile-up in one particular area only and keeping band noise to a minimum.

[A couple of years ago RadComms interviewed the best British operators after a big Contest. Similar comments were received regarding the Vertical; that it was an very good 2nd aerial and sometimes out-performed the main station’s antenna]

CLUB CALL AFFILIATED

GXØOWX WEST SOMERSET AMATEUR RADIO CLUB R.S.G.B.

Last month’s meeting was a talk by young Steven on e-bay. It was a very well prepared presentation with plenty of questions and answers. It certainly removed the mystique of this form of Buying and Selling on Internet. It was a pity that not more members attended; they missed a most excellent evening.

Next month’s meeting is the annual Construction Contest. It is hoped that there will be a few more entries than usual! I have told that Oli is bringing along an item the demonstration of which might make your hairs stand on end (or rather wilt!).

At this meeting we will also discuss the venue for the Club’s annual nosh. It may well be the Valiant Soldier; if anyone who is unable to attend the next meeting has any comments on this choice would they please let Bob know.

For those who support our national society, the R.S.G.B., there is a change this month. (Not entirely due to the postal strike; the RadComms normally arrives in time for a quick scan for items of interest.But not this time) This time I have a couple of items from my archives:-

Zero-bias drain current,

gate cut-off voltage and

gate leakage current in junction f.e.ts can be measured with a high-impedance voltmeter (20,000 Ω/Vor better) using the circuits shown in figures (a), (b) and (c) respectively.

Using an external voltmeter, a simple tester can be built around three push-button switches as shown (right). With the source resistor value of lKΩ,. IDDS is read in mA on the voltage scales of the meter.

Two small 9-V batteries give sufficient voltage to test most f.e.ts. The f.e.t. is protected by the protection circuit (2N1711) which works on the loadline principle, and limits the power to the test circuit to about 200mW.

The circuit has an additional refinement:- By depressing the Vp button and the IDDS button simultaneously, the circuit shown in (d) is made. The meter reading now gives both drain current in mA and gate source voltage in volts, thus giving a third point on the IDVGS characteristic. The complete tester can be built in a box measuring 10 x 7 x 5cm.

Jostein Skjelstad,Alesund,Norway. (Published as a letter in Wireless World, Sept 1972)

Electronic engineers and designers have a special interest in the operating temperature of components/sub-circuits and even complete electronic and electrical apparatus. Stand-alone temperature meters as well as add-on units for use with DMMs (digital multimeters) are commercially available for this purpose. Most of these units offer a large temperature range of 200° C or more, and are fairly expensive. With occasional use in mind, however, the investment in a such a costly electronic thermometer is hard to recover. In most cases, a temperature range of -30° C to +120° C will be perfectly adequate. Moreover, that is easily achieved using common-or-garden components

MEASURING BRIDGE BASED ON P-N JUNCTION.

As you can easily see from the circuit diagram shown in Figure 1, the essential part of the T/V converter is a resistor bridge. Resistor R7 provides a constant current through the sensor, which is either formed by a transistor p-n junction or a silicon diode. An adjustable voltage divider, R1-P2-R2, is responsible for the constant reference voltage at junction B. The voltage between junctions A and B, also called the measuring gradient, is proportional to temperature change. Provided it is properly scaled to give a meaningful readout, the measured voltage is easily indicated by a high-impedance voltmeter. For this purpose, the circuit has a second preset potentiometer, PI, which is incorporated in the measuring bridge. The adjustment of PI will be discussed further on in this article. To prevent the sensor from heating up as a result of its internal current flow, the bridge section is operated at a low voltage and current level. Adjustable voltage regulator IC1, an LM317, provides a bridge supply voltage of only 3V. To ensure that this value remains stable, close-tolerance(1) resistors are used in positions R5 and R6. Because the current consumption is below 2.5 mA, and the battery is allowed to go as 'flat' as 5 V, the 9-V PP3 battery will last a number of years in this circuit.

A HOME-MADE SENSOR

The sensor may be a silicon diode (for example the ubiquitous 1N4148) or a silicon transistor. The ultimate part for this application is a miniature transistor type BC146 (from Philips or Temic/Telefunken) or the BC121, BC122 or BC123 from Siemens. Unfortunately, these devices are no longer manufactured, although with some luck they may still be found in the electronic surplus trade. The good news is that modern transistors like the BSX20 (having a metal case for good thermal contact), BC546B (plastic case) or the SMA-style BC848B (more difficult to use) may also be employed here. The thermal capacity of the transistor case determines the converter's reaction speed to temperature changes. A small metal case ensures fast measurements, while large plastic cases exhibit a sluggish response to temperature variations. Whichever you use, the sensor is connected to the converter by means of a sufficiently long, screened, cable. The wire connections to the emitter and base terminals of the transistor are properly isolated, while the collector remains open. The actual construction of the temperature probe depends on your application and requirements. For example, the transistor may be fitted in a pen holder whose tip is drilled to size, allowing the transistor body to be secured with two-component glue. A 3.5-mm jack plug may be fitted at the free wire end.

CONSTRUCTION AND CALIBRATION

All resistors should be metal film types which guarantee long-term stability of the circuit. Cermet multiturn presets are used for the same reason.

To calibrate the circuit you need a small amount of distilled water which is put in a freezer to make ice cubes. Put the ice cubes into a glass, and add water until they just start to float. Stir frequently, and allow the cubes to thaw about half-way. The temperature of the water/ice mixture is then very close to 0° C. Put the (electrically isolated) temperature sensor in the water/ice mixture. Set the DMM to the 200-mV range. Next, adjust P2 until the meter reads 00.0. If necessary, repeat this procedure several times, not forgetting to gently stir the water/ice mixture between measurements. Preset PI has virtually no effect on this calibration, and should be left at a mid-travel setting.

Next, heat the water to the boiling temperature, and immerse the probe again. Wait until the readout stable, then adjust PI for a meter reading of 100 (mV).

To enable the circuit to be accurately adjusted, the cermet presets have a relatively small range. If a particular range is not large enough, it is perfectly possible to change of one the fixed resistors in the bridge.

The boiling temperature of water depends on the relative air pressure. Fortunately, unless you are at the top of a really high mountain, or smack in the eye of a hurricane, the deviation caused by low or high air pressure is insignificant with respect to the normal measurement error of _ 1.5° C.

(Elektor Electronics 6/97 p 66-67)

CLUB CALL AFFILIATED

GXØOWX WEST SOMERSET AMATEUR RADIO CLUB R.S.G.B.

Last month’s meeting was a talk by Oli on the (lack of ) progress on his Slow Scan Television project. He had completely re-built the Power Supply. Two E.H.T. units had failed to work. (Much wasted time!). The 3rd attempt, with different circuit, was perfect (8Kv!). He also mentioned that he had spent the entire week (with the XYL away) on the unit but just failed to get a raster on the tube in time for the meeting. [Someone said "the XYL should go away more often"!]. He also described the picture sources that he hoped to show in a year’s time!

Next month’s meeting is a talk by young Steven on e-bay. (Oli says he is looking forward to this one).

I have the impression that not many of the Club members take RadComms which, for me, is helpful because that is my main source of copy. However, Kenn has kindly sent one item that lightens the tone of this edition.

The following are a few comments from the Enforcement Bureau of the F.C.C (USA equivalent of Ofcom). They have been written by a licenced amateur, K4ZDH.

To the contesters:

Be more courteous. You are responsible for the frequency you are operating on and realize that's true even when you operate split. All frequencies are shared. ( You do not have sole use ofany frequency)

To those who don't like contesters:

Lighten up! Contests are short lived.

Use the WARC bands.

Wash the car.

Cut the grass.

Leam from the contesters. Contesters are some of the best radio operators on planet Earth.

Ed. Whilst I am not a QRP operator, I would ask operators to deliberately look for QRP contacts; give them encouragement.

Unfortunately, the R.S.G.B. in their latest BandPlan has failed to give the recommended operating frequency for QRP operation.

To those who don't like QRPers:

Lighten up! Anyone can use a linear amplifier as a crutch.

To those of you who don't like Dpeditions:

Lighten up! If a group of people wants to spend a lot of money to go to a rock or sandbar in the ocean, live in a tent, and swat flies and scorpions for a week and talk over ham radio 24 hours a day, so what? Let 'em do it! DXpeditions, too, are short lived, and such operation must be important to someone. Scarborough Reef drew over 50,000 contacts, didn't it?

Liquid Tape is like paint-on self-amalgamating tape rubber compound. According to SOTABEAMS they have been using it for some months and have found it to be one of those products that really does what it promises on the tin. Liquid Tape penetrates the braid of coaxial cable connections and stops water ingress by capillary action. Full details are on the web at www.sotabeams.co.uk.

(Ed. Sounds good sticky stuff)

Even if no Club members ‘do’ much on 70MHz, they may other readers of this News who are not fully aware of just how many oither countries have access to this band:-

The Italian Ministry of Communications, in coordination with the Ministry of Defense, has authorised an amateur radio experimental campaign on the 70MHz band (4 meters) within the time period 11 July to 31 December. The technical parameters are 25W EIRF3 all modes, on the spot frequencies 70.1, 70.2 and 70.3MHz with 25kHz bandwidth. The operational area is the whole Italian territory except for a 30km strip from the Austrian, French and Swiss borders. Antenna types are omnidirectional and directional. The experimental campaign is open to all Italian radio amateurs and the results will be published when it has finished.

Sealed lead-acid batteries

Sealed Lead Acid Batteries (SLABs) or 'gel cells' are rather old fashioned in terms of battery technology and yet they remain a popular choice for portable radio operators. Here we explore why that is, together with some ideas for using them and getting the best results.

TYPES AND RATINGS.

Each type of SLAB has a stated capacity (eg 7Ampere hours or Ah) but beware: you can only achieve this capacity under specific conditions. The faster you discharge your battery, the less capacity it will have. A 7AH battery will give 0.35A for 20 hours but increase the current draw to 3.5A and you'll find it's flat in about an hour. There are two main problems with lead- acid batteries:

•Weight - they are heavy for the energy that they contain. A 7Ah battery weighs 2.65kg and has a specific energy of 32Wh/kg (20hr rate). This compares poorly with other battery technologies such as NiMH (>50Wh/kg) and lithium polymer (LiPO) (>100Wh/kg), thus you can pack the same energy into a much lighter battery by choosing a newer cell technology.

.• Discharge characteristics - lead acid batteries vary from about 12.8V for a fully charged cell down to around 10.2V for a discharged battery. LiPO and NiMH batteries have flatter discharge characteristics. So why do lettd-acid batteries remain popular?

•Availability-they are readily available and come at just the right voltage for most radio equipment (12V).

• Cost - they are modestly priced (_ 20 for a 7Ah lead-acid compared to _ 70 for a 5.4Ah 11V LiPO battery pack).

•Safety-sealed lead acid batteries are very safe when handled properly.

• Ease of use - this is perhaps the key factor and bears further examination.

EASY CHARGING.

Lead acid batteries can be float-charged from many of the standard "12V" PSUs used to power rigs. These PSUs are often set to around 13.6V which is just about perfect voltage for float- charging lead acid batteries. The only thing to note that the initial inrush current for a discharged 12V battery can be up to twice its capacity (14A for a 7Ah battery) for a few seconds so your PSU needs to be able to supply this current. 12V SLABs can also be charged directly from a cigarette lighter socket in a car. It is good practice to isolate the battery from its charger with a diode (a high current Schottky diode is a good choice as it has a forward voltage drop of only about 0.3V). I have never bothered with an isolating diode for float charging and have had no problems.

SLABs keep their charge quite well and will lose less than 20 even after 3 months at 20° C. With float charging they can be left connected to the charger more or less indefinitely but this will never quite fully charge the battery and will ultimately reduce its life somewhat due to sulphation of the battery plates. For a full (and faster) charge, a constant current charger with an open circuit voltage of about 14.5 volts is recommended. The charge current should not exceed a 1/4 of the battery capacity, eg 1.75A for a 7Ah battery. I normally charge my batteries at 800mA. With this charging method, you will need to disconnect the battery from the charger once the charge current drops to a few tens of milliamps. For rapid charging in my car I use a 12V switched-mode laptop power supply (readily available for about _ 20). I set this to 16v output and use a constant current source made with an LM317T a series diode and a few resistors. It's not pretty but it recharges my 4Ah battery quite well while I drive between activating different SOTA hills.

CONDITION MONITORING. It is useful to monitor your battery to see how much charge you have left. Steve Weber, KD1JV, shows a simple voltage monitor circuit on his website. CONNECTIONS. Sealed lead acid batteries often come with 4.7mm tabs for the connections. It is best to use proper connectors but if you solder a power lead on, make sure to heatsink the battery end of the tab otherwise you can melt the plastic surround. Insulate the connections well using heatshrink or several layers of tape - a short here may well start a fire. A fuse is an essential requirement given that the maximum discharge current for a 7Ah battery is around 50A! I use an inline fuse holder and tape a few spare fuses to the battery for use in emergencies. Some years ago I standardised my DC connectors to Anderson Power Poles and thus my SLABs have these connectors fitted. To stop the connections being accidentally pulled off, I always strain relieve the power cable using a cable tie (see Websearch).

LIFE. It is a good idea to write the date that you bought the battery on it using an indelible marker (cover the writing with transparent "Magic Tape" to protect it). This will help you to keep track of the life of your battery. One final tip is always to buy your batteries new, from a reputable supplier. Over the years I have bought several dud SLABs from less reputable suppliers. Ex-alarm batteries are seldom any good. I may have been unlucky but I always buy them new nowadays.

WEBSEARCH

Yuasa NP series datasheet

Radcom - Sept07-01 .notlong.com

SHETLAND ON 500kHZ. Looks like l00mW is enough to span the length of the British Isles!

Dx for the beginner or anyone else for that matter:-

One of the things for a newcomer to try is to listen at UK sunrise for North America on 80 meters. There will be lots of North American and Caribbean stations - and occasionally you'll hear some stations from South America, such as the Galapagos Islands at the same time. Listen for half an hour or so before sunrise and you'll hear the signals peak. Grey line propagation is really exciting and it's a great way to work some interesting DX on LF. If you've a suitable antenna, you can try the same thing on 160m - but the opening will be a lot shorter.

I came across this website whilst reading the October RadComms; looks interesting

www.porthcurno.org.uk

I should be grateful if readers would let Oli know at oli.g3nfy@virgin.net the locations of radio (and other technological) museums in the U.K. so that he can compile and publish these. (He went looking for one in the Isle of Wight and found that it had moved and then closed). For locals, there is the one at Tropicaria.

You know you are a Ham when........

The only time you get up at 6am is for a radio rally.

You know you can run all your home appliances on your equipment back-up battery.

Someone in a Music shop asks you what bands you like and you answer two metres and seventy centimetres.

In a conversation, you wait for the Roger tone before speaking.

A band opening is more important than a grand opening.

When you walk out of the house you not only feel your pockets for your keys and your wallet, but also your hip for your handheld.

Your child's schoolteacher calls you on the phone to ask why your child identified countries on the world map as JA, ZL, VE, G, UA and XE.

If you refer to your house as "Ohm, sweet Ohm".

You're talking on the phone and end every sentence with "(callsign) this is (callsign), over".

You realise that you've been studying code too long because you try and find out who did it in 'Inspector Morse' by listening to the background music.

You look at the Pennine Mountain Range and try to figure out how to get an antenna mounted on the highest peak.

You look at the Severn Bridge and develop a plan to get it to resonate on 160 metres

Thank you Kenn for forwarding that one; the bold items are those that your editor relates to!

I am wondering,is this Oli's car????

or do you know who is the owner?

CLUB CALL AFFILIATED

GXØOWX WEST SOMERSET AMATEUR RADIO CLUB R.S.G.B.

Last month’s meeting was the all important Rally at Allerford (Minehead). Thanks to all who helped. One of our Club Members came all the way down from Weston Super Mud (or somewhere up that way) not only on the Saturday to assist, but also on the Friday evening to help set up. Another came along on the morning and then disappeared!.

Progress was slow on Oli’s stand, taking some _ 20 (all for the Club). At Wimborne, most of our ‘stuff’ would go before the rally opened to the public; items being bought up by other traders. Oli did spend a little bit of money, some thermocouple (& electrostatic) meters, two heating mats and a powered jig saw.

I have one or two further comments from Oli.:-

He nearly sold 2 items at the rally. One was a monumental battery charger, some 80 Amps or so - there was insufficient room in the LandRover to transport it to the rally. He was very pleased that it did not go, it was required a week later! (More about this in another News). The other item was a mains motor driven 50Hz Variac. It was on the stand for _ 4 & still did not go. He later realised that this very low speed, reversible motor was absolutely ideal for driving the vacuum capacitor which will tune a mag loop aerial.

The Rally / Car Boot was great fun, and as our reporter says, we had some helpers the night before ,the night  was spent setting out the tables in the hall,

14 tables were booked inside. then it was down to sorting out signs for people to find us.

at about 19:00 on Saturday night, JAB Electronics  arrived to setup.

Simon and Bob then set off to put the signs up,and that was when the fun started,The day before Bob's car ran out of petrol,so some petrol was used from his garden tractor to refuel the car,but this is a warning,do not use second hand petrol.the fuel filter became blocked and the car then developed kangaro petrol.

Durring the night,our second trader arrived,he arrived at 01:30 in the morning,then at 06:00 am Bob was then out in the field,getting ready for everybody to arrive,

several traders / carbooters arrived and everybody had a great day,several of our local Hams came along,and I think everybody that came had a great time.

After all bills paid,we ended up with 40 pounds profit,that was double last year,but that was our first time,

anyway,put August 3rd 2008 in your diary,thats the date of the next one.

Next month’s meeting is a progress report (?) by Oli on Slow Scan Television. It will include a small sic demonstration of LDTV.

Have you ever had a device that worked on 24v and you needed it for a 12v supply?

Have you a mains transformer that required different secondary voltage?

It is not so much the number turns but the wire diameter that is the query. Oli has passed this information to me for the News. He apparently been hunting for it for years and finally found it. (Suprising what a clear-out can achieve!)

To halve the voltage:-

The device will take/supply twice the current.

The winding will have a quarter of the resistance.

It will require half the number of turns.

And here is the important bit:-

The wire will have twice the cross sectional area.

In practice, when you have the coil away from whatever iron core (if it is a mains transformer, probably E & I’s). Count the number of turns as you unwind it.

Now measure the wire diameter carefully. Various Club members will have a micrometer. Strictly, you should some allowance for the ‘enamel’ covering. There are various grades of enamelling and modern wires with self-fluxing coating tend to be thinner. However, for 10SWG I would suggest take off 6 thou; for 20SWG take off 3 thou and for 30SWG take off 2 thou. (i.e.Taking off an allowance for the two layers of enamel, one on each side of the wire!)

With the probable true copper diameter, look up the Cross Sectional Area of that gauge of wire. If you halving the voltage, you will need replacement wire of twice the Cross Sectional Area. Again, referring to wire tables, you can now determine the wire gauge needed.

As an example:-

Mains transformer which currently gives 24v and 12v is required:-

On stripping off the existing secondary winding, it was found to have 240 turns.

The wire diameter as measured was found to be 0.0146". Take off an allowance for the enamel giving 0.0144". From wire tables, the Sectional area is 0.000158 sq. ins. nearest SWG to this is 28.

Now refer to wire tables again. Double the sectional area is 0.000316 sq ins. This corresponds to a wire gauge of approx 25SWG.

A practical point:- The original coil will have been machine wound & it is unlikely that you will achieve such a neat job! i.e.your winding will take up a little more space so it is probably worth considering dropping a wire size. You will be putting 120 turns on for your new winding.

Bob & Oli (and several other members of the Taunton & West Somerset RAYNET) recently supported S.E. Dorset RAYNET with the Dorchester Ironman. This was a massive event with 1600 entrants and some 400 others who paid their non- returnable entrance money to be on standby. You may have seen the request in last month’s RadComms for radio operators to help. In the end, 65 operators assisted. To quote the S.E. RAYNET Controller’s (John Goodall GØSKR) comments "The proffesionalism with which everyone, regardless of age or length of service in RAYNET , undertook their duties during the Triathlon, was outstanding".. Amateurs came from Derbyshire - Leicestershire - London - Somerset - West Devon to name but a few attended. A slogan of RAYNET was Your Lifeline - Their Hobby. Still valid today; RAYNET provides professional standard communications without which many social events could not take place.

Do not think that that 70MHz is only available to us Brits (& Gibraltar) Great news that the Italians now have the band until 31 December. ±12.5kHz of 70.1, 70.2 and 70.3MHz, all modes and polarisation and 25W EIRP: Details are on the website (Web search). www.space.it/70mhz/

On the 12 June G4DEZ worked CT1HZE on SSB and FM and CT1FFU and on the 30th Bryn contacted OZ1DJJ, followed by the first ever G to HA contact using JT6m which took just 12 minutes with HA1YA and OY/G40DA (IP61). G4ZFJ worked OY1CT (IP62) on 22 June for a new country and grid and Colin reports that Nige Coleman, G7CNF (1081), worked VE9AA cross-band to 6m on 25 June.

At the Dorset Ironman RAYNET ‘do’, our newsletter editor was in conversation with another such ‘volunteer’ about items for publication. He said that such submissions were often late and was suprised to learn that I had to find everything! One of the problems is that I have no idea what you would like to see in the News (or to whether is even read!). My only source that is up to date is RadComms and that is a little unfortunate for those who support out National organisation and read the magazine themselves.

A Technical Topics item "Petrol-Electric Generator Lore" (June 2006, pp72-73) provided a number of suggestions on achieving reliable and safe running of petrol-electric generators. As these devices remain the prime source of power for field-days and emergency operations, it seems worth while, despite some repetition, quoting selectively from "More Power to You" by H Ward Silver, NOAX (QST, June 2007).

"All generators are not created equal. Along with the power rating, consider waveform quality and regulation. 'Contractor grade' generators for powering tools have poor regulation and distorted waveforms, particularly near full rating. Keying a radio can cause large voltage swings, risking damage to a power supply or improper operation. If you can, use a generator intended to power electronic equipment. Poor regulation can be helped by loading a 2kW generator with a pair of 100W light bulbs at all times. "Test generators well before field use unless you want to learn field generator repair! A generator with old fuel in the tank and carburettor is likely to run poorly, if at all. Generators should be stored with the fuel line and carburettor dry and a stabilising agent added to the stored petrol. Replace black and dirty oil. Inspect the air filter and silencer for clogging by dust or debris. Some sites may require a spark suppressor, so be sure your generator meets the [site] rules. "Monitor fuel consumption, devising a fuelling schedule so the lights don't go out unexpectedly. Your 'Generator Czar' should provide additional oil of the proper viscosity, fuel in safety containers away from the exhaust, a fuelling funnel, mopping rags, and a flashlight for night-time maintenance. Review the instructions for each generator, such as those found on the web at mayberrys.com/honda/generator/html/operation.htm.

"Take generator safety seriously. Never run a generator in an enclosed space - be sure there's plenty of ventilation. Keep flammable materials such as dry grass or cloth clear of the exhaust. Keep a fire extinguisher at each generator. Never fuel a running generator - insist that it be stopped first and try to have two-person crews do the job. If you operate near homes, consider the neighbours! If your generator is noisy, use plywood sheets to make a sound baffle. Try to direct or deflect noise up and away from people trying to sleep." NOAX also provides information on the AC wiring, battery operation and alternative power sources such as solar power, bicycle- and pedal-operated generation. Inter alia he suggests that "Of the different battery types, deep-cycle marine or RV batteries and gel-cells are the best choice for Field Days. ... Many radios do not operate properly at less than 12V. That means the batteries may need to be charged frequently or continuously, or you may have to use a more tolerant radio!" Personally, since 1944,1 have always viewed P-E generators with some misgivings. First, due to a stupid mistake, I managed, while at Nymegan with IS9 (M19) to write off completely my SCU9 Onan 150W P-E generator. Later, I was constantly frustrated by a repeatedly moist sparking plug in a replacement generator used close to the Rhine, often frantically pulling the starting rope repeatedly in an effort to meet transmission schedules. In one of my first post-war NFDs, the generator provided an output dropping to about 20HZ, burning out the receiver PSU transformer. It pays to follow the advice given above by NOAX and the additional hints given in the June, 2006 TT" Surprisingly, Onan 150W P-E generators were occasionally used, despite the noise of their two-stroke engines, by clandestine stations in Occupied Territory, including Norway. Dave Williams, G3CCO, has told me that he advised agents going into Malayan jungles always to dig pits for their generators in order to deaden the noise.

3B7C needs you or so a heading in RadComms reads.

3B7C is the call sign for (another) Dxpedition.

It is operating from 7 - 24th September on all bands; c.w and SSB

They aim to work every station that calls in. They recommend trying towards the end of the scheduled period, once the rush is over!. (Good opportunity for Bob; he has a good reputation for breaking log jambs)

Where is 3B7C? Ile de Sud, next to Coco Island at the southern end of the Brandon Group. None the wiser? Well, nor was I!! It is about 400 miles west of Panama, well out into the Pacific.

PROPAGATION TO 3B7C

The following predictions are based on a 100W station and simple antennas - if you have a beam and/or linear you will do better.

• 160, 80 and 40m. There will be a tough darkness path from about 1800 - 0200UTC.

•30m should open around 1600UTC, giving a fair path until the band starts to deteriorate after midnight.

•20m should offer good propagation from mid-afternoon, peaking at about 1800UTC and falling off later in the evening.

•17m is a good bet from late morn ing until reliability deteriorates in the evening.

• 15m is also worth checking throughout daylight hours, especially 1530- 1730UTC.

•12 and 10m are likely to be disappointing, although it is always worth looking, especially on 12m during daylight.

Most of the time, 3B7C will be working ‘split’; Tx’g on a different to Rx. However, the operator will announce this. Later on, after the rush, it is likely that Tx & Rx will be on the same frequencies.

See web site:- www.3b7c.com

Best of luck - try and work them on more than one band

CLUB CALL AFFILIATED

GXØOWX WEST SOMERSET AMATEUR RADIO CLUB R.S.G.B.

Last month’s meeting was poorly attended & was at Kenn’s QTH. 2 Club members arrived to carry out some ‘field day’ operation. Bob managed his usual trick of breaking into a pile-up whilst Oli had an excellent contact with the States. Ideas for a multi-band ‘field day’ type of aerial were discussed.

Next month’s meeting is the all important Rally at Allerford (Minehead). I trust you all have made arrangements with Bob concerning what help you can give.

This month’s News contains a complete Heat Sink design procedure. Heat sinks are expensive things to buy and the following uses flat sheet material. This follows an article on Lightning.

SUMMER FUN. For most people, summer is the prime time for portable operating. The thought of enjoying the early evening sun with a fine view and some interesting tropo or sporadic E DX on 2m is just about perfection. But this is a time of year with special hazards for the portable operator. Setting aside midges and wasps, lightning is perhaps the most prevalent risk. In Cheshire, for example, there is six times more likelihood of lightning occurring in the summer than in the winter [1]. My interest in lightning stems from nearly being struck twice in one year; an occurrence that forced a rethink on avoiding lightning. Lightning presents two distinct risks for portable radio operations:

• Personal risk due to direct or close strike

• Risk to radio equipment from the electromagnetic pulse caused by lightning

Between 30 and 60 people are struck by lightning in the UK each year. On average, three of these will die. The best way to avoid this risk is not to go portable when there is lightning in the area. In the UK, good lightning predictions are available from the Met Office [2] and real-time data is available at the Blitzortung website [3].

HOW CAN YOU TELL IF YOU ARE AT RISK? You can judge the distance of a storm from you by counting the time between the lightning flash and the sound of the thunder. If the gap is less than 30 seconds you should seek shelter. You should then remain there for 30 minutes after the last clap of thunder (sometimes called the 30/30 rule). If you are caught out, the Tornado and Storm Research Organisation [4] recommends the following:

Avoid wide, open spaces or exposed hilltops and don't shelter beneath tall or isolated trees. Seek shelter inside a large building or a motor vehicle. Check and take heed of weather forecasts of thunderstorms when planning a day walking in the hills, sailing and playing golf. If you are swimming, windsurfing or sailing, get to the shore as quickly as possible. Move away from wide, open beaches and seek shelter inside a large building or motor vehicle.

If caught out in the open during a thunderstorm, discontinue carrying umbrellas, fishing rods, golf clubs and other large metal objects. Keep away from metal objects such as motorcycles, golf carts, bicycles, wire fences and rails.

If your hair stands on end or nearby objects begin to buzz, move quickly away as lightning may be about to strike. These effects happen because the positive electrical charges forming at the ground are streaming upwards to try to make contact with the advancing downward negatively-charged 'leader'. Lightning does not always follow, as not all of the upward discharges make contact with the leader, but it is best to move away as a precaution. Seek shelter in a large building or motor vehicle. If caught out in the open with no shelter nearby, move to a place of lower elevation such as a hollow or dry ditch. Crouch down (to lower your height) with both feet close together. Do not place your feet wide apart or lie flat on the ground as this will increase the difference in voltage across your body, increasing the electrical charge you may receive from radial ground currents, if lightning strikes the ground nearby. Tuck your head in and place your hands on your knees. If inside a motor vehicle stay there during the thunderstorm. It will protect you as long as you do not touch the metal of the car body [or anything connected to it such as radio equipment!]. A lightning strike will normally be safely conducted over the metal bodywork of the vehicle before earthing to the ground over the wet tyres (that are sometimes damaged slightly). When indoors, keep away from windows, avoid touching metal pipes or radiators. If lightning strikes a television aerial, the cable may conduct the current into the building where it can jump to other wiring or metal piping circuits. Do not use a telephone except in an emergency. Finally, give first-aid (and contact paramedics promptly) to anyone struck by lightning to help them recover. You will not receive an electrical shock as they carry no electrical charge. Act promptly.

LIGHTNING DETECTOR. Clearly, "hill topping" represents a high risk even without an aerial system. With that in mind I decided to build a lightning detector to carry with me during the summer, on hilltop trips. The circuit I have used was developed by Charles Wenzel and full details are on the Web [5]. It functions well and is relatively immune to RF on the amateur bands - at least at low power levels. The circuit lends itself to more sophisticated applications. In my case I have interfaced it to a PIC to allow a more comprehensive user interface to be developed. There is no easy way of protecting portable radio equipment from a direct strike. Gas discharge tubes, the most common form of lightning protection for antennas, are unlikely to work well without a good ground system. For anyone wishing to investigate this lightning protection further, the ARRL has an excellent series of articles available at their website [6] although it relates primarily to fixed stations. As with the personal risk above, the best course of action is not to be out when storms are likely and at the first sign of thunder, to beat a hasty retreat.

WEBSEARCH

[ll www.metoffice.gov.uk/climate/uk/averages/lightning.html

[2] www.metoffice.gov.uk

[3] www.lightningradcom.notlong.com

[41www.torro.org.uk/TORRO/research/lightning.php

[5] www.techlib.com/electronics/lightning.html

[6] www.arrl.org/tis/info/lightning.htmI RadCom Aug 2007. P65

1 There are two approaches to the design of heat sinks. First, it may be desired to produce a heat sink which will be adequate for all normal applications, but it may not be imperative that the size is the smallest possible. This approach relieves the designer of considering a number of factors some of which may possibly be unknown. In the second instance, space may be at a premium, and it is desirable to use the smallest possible heat sink capable of dissipating the heat generated by the semiconductor device without exceeding the temperature rating set by the manufacturer. The heat sink should have a thermal resistance:

θ_r £ (T_case - T_a) ¸ P_tot ° C/watt (1)

where

θ_r = thermal resistance in ° C/watt

T_a = the maximum anticipated cooling air temperature in ° C

T_case = the rated case, stud or mounting-base temperature in ° C for the conditions under which the device is to be operated

P_tot = the power in watts dissipated by the device when used in the circuit. In the absolute maximum rating system all anticipated variations in device characteristics, circuit component values and supply voltages must be taken into account when assessing P_tot.

1.1 Heat Sink Design

1.1.1 Basic Design/or Natural Air Cooling

The curves in Figure 1 enable the size of the heat sink to be determined. If the smallest dimensions are not required, it will be safe to use curve U. If the smallest possible fin is required, the size can be reduced towards curve L but a heat run is essential to check the design.

The thermal resistance of a flat heat sink (fin) supported in free air depends upon its surface area, temperature, thickness, shape and orientation, the material of which it is constructed, the nature of its surface and the temperature of the cooling air. The above method applies to vertically mounted fins of ¹/₁₆ in. thick bright aluminium in which the ratio of the lengths of the major and minor axes do not exceed 2:1. The fin is assumed to have less than 100 sq. in. exposed to cooling air and its hottest point to be at a temperature at least 25° C above that of the surroundings. In the above method, heat dissipation due to radiation can be neglected. Thus, a safe design can be achieved suitable for many applications. However, when it is known that appreciable radiation can occur (e.g. with a single fin with matt black surfaces) and it is desirable to keep the size of the heat sink to a minimum, reference should be made to 1.1.3.

It is desirable to verify the design by means of a heat run, carried out under the most adverse operating conditions expected.

1.1.2 Basic Design for Forced Air Cooling

When cooling air is blown over the surface of a fin the area required for a given thermal resistance may be approximately determined from the equation

A = 1500 ¸ (θ_r^3/2 . V^3/4) ins²

where

A = the total surface area of heat sink exposed to cooling air

θ_r = thermal resistance as defined in equation above

V = the mean air velocity in ft/sec. Since the effectiveness of forced air cooling depends on the detailed arrangement used, a heat run is always necessary to verify the design.

1.1.3 Factors Modifying Heat Sink Designs

(a) Radiation

When a naturally cooled fin is free to radiate heat from its cooling surface, for example when only one fin is used and it is not near any other radiating surface, the heat radiated from it forms a considerable amount of the total heat transferred. If the surface is matt black, a reduction in area of about 30% may be made on the figure obtained from the curves.

(b) Fin Temperature

The temperature of the fin affects the heat dissipation by both radiation and convection. If it operates at a temperature substantially above that of its surroundings, the area may be reduced below the value derived in accordance with 1.1.1 (for example, by 30% if the differential is 100° C). On the other hand, if the differential is less than 20° C, heat transfer due to convection rapidly diminishes, requiring a larger surface area. In forced air cooling systems, where the air velocity is greater than 10 ft/sec, radiation effects can be ignored.

(c) Material and Thickness

If a large heat sink is constructed of thin material with a high thermal resistivity, its effeciency is low. In order that the effeciency of the heat sink shall not be seriously impaired, the material and thickness used should satisfy the expression

ρ/d < θ_r ° C/watt (3)

where

ρ = thermal resistivity (see Table I)

d = thickness

θ_r = thermal resistance as defined in equation (1)

If this expression is not satisfied, a larger surface area will be required than that determined from 1.1.1 and 1.1.2. On the other hand, there is little point in making

ρ/d < θ_r/2

since the heat sink efficiency is not substantially improved and metal is wasted.

Thermal resistivities likely to be encountered are shown in the table below:

The units used are ° C/watt for a cubic specimen having an edge of 1 in. or 1 cm long.

(d) Shape and Orientation

If a high efficiency fin is to be realized, it should be mounted vertically and should be roughly square or circular, with the semi-conductor device mounted in the centre. However, when a fin must be rectangular, its efficiency is impaired least if it is mounted vertically with its major axis in a horizontal plane, since maximum convection will then occur. If a fin is mounted in a horizontal plane, heat transfer from the lower face by convection will be reduced by a least 60% compared with vertical mounting, the upper face being unaffected. Consequently an increase in surface area of 20% or more may be necessary. When forced air cooling is used and the flow is parallel to the plane of the fin, shape and orientation are less important.

(e) Stacked Fins

When two or more fins are stacked side by side and naturally air cooled, the thermal resistance of each increases. In practice, if they are close together, a fin positioned between two others may dissipate virtually no heat by radiation. Convection will, however, be un-impaired if the distance between fins is larger than a certain critical value.

When designing stacked fins, the area of each fin should be taken directly from the curves in Fig. 1. The effect of radiation should be disregarded and the minimum fin spacing should be determined from curve P in Fig. 1.

Fins may be stacked side by side to great advantage when forced air cooling is used since the remarks above are not relevant.

1.2 Securing Semiconductor Devices to Heat Sinks

It is important that a good thermal contact is established between the base of the semiconductor device and the heat sink. This can usually be achieved by ensuring that the heat sink area is flat, smooth and burr-free, and by bolting the semiconductor device firmly to the heat sink. Overtightening of the fixing screws should be avoided, or the mounting-base of the device may be distorted reducing the effective contact area.

The use of a new soft lead washer of the same plan as the device mounting-base between the device and the heat sink is sometimes recommended. Such a washer may be about 0-005 in. thick (0-127 mm) and should never be used more than once, since on compression, it will work-harden and will not subsequently adopt the contours of the device and heat sink accurately enough.

Some manufacturers suggest that the device mounting-base be smeared with high thermal conductivity silicone grease before mounting, in order to minimize any air gaps due to surface irregularities. (Ed Definately advised)

Should it be necessary to provide electrical isolation between the device and its heat sink, a thin insulating spacer may be positioned in the same way as the lead washer mentioned above. Care should be taken to ensure that the spacer is not damaged during mounting, as a breakdown of insulation may damage the device or its associated components when the equipment is switched on. It may be desirable to check the insulation after assembly. The thermal resistance of such a spacer can be calculated from the equation

θ_s = (ρd_s) ¸ A_s

where

θ_s = thermal resistance of spacer

ρ = thermal resistivity of spacer (see Table 1)

d_s = thickness of spacer

A_s = surf ace area of one side of spacer

This thermal resistance will generally be a few tenths of a ° C/watt and should be subtracted from the value of θ_r used on the curves of Fig. 1.

(Ref The use of semiconductor devices 2nd edit. Nov -61 pubThe Electronic Valve & Semiconductor Manufactuers Association. -definately out of copyright!!)

CLUB CALL AFFILIATED

GXØOWX WEST SOMERSET AMATEUR RADIO CLUB R.S.G.B.

The old PMR (Standard) UK General Licence is no more as of the end of 2006. The new PBR UK General Licence has eliminated three of the old frequencies at u.h.f. They are 461.2875, 461.375 and 461.450. All three frequencies were very well used by a range of diverse organisations including shops, security guards, hospitals and some TV studio sets. The new UK General frequencies at u.h.f. are 449.3125, 449.400 and 449.475. These have been licensable since 2002 and have plenty of customers, but I can't see the users of the old u.h.f. channels rolling over that easily. If their kit is still working, then my money is on them carrying on with their old gear until either they are told to stop, or the radios break down!

A couple of submissions which are mostly concerned with restoring valve equipment. However, the comments also relate to restoring capacitors in general.

(From Geoff Darby, G7RTC, and, independently,Malcolm Perry, G8AKX:)

Firstly, service engineers of many years experience on valve equipment would not advocate changing components just because they might be faulty, or might give problems in the future, or just because they look a bit rough. When deciding to replace capacitors, it is essential to understand the function of the device within the circuit, as this will affect the type chosen for the replacement. Get this wrong, and you could seriously degrade a radio's performance or stability. Also, it is essential to dress the replacement component's leads exactly as the original manufacturer did as, again, this can be very important to performance and stability (Ed comment, see below). Secondly, there seems to be a problem with the maths regarding the replacement of high-voltage electrolytics. In his article, Steve, VK6VZ, states that he recently replaced a 16uF 500V capacitor, with two l0uF 450V capacitors in series. While this does produce a 'capacitor' with an overall voltage rating of 900V, it only yields a capacitance of 5uF - less than 1/3rd of the original value. In order to produce the original value, when series-connecting two capacitors to increase the voltage rating, each individual capacitor must have twice the capacitance of the original (or three times the capacitance, for three capacitors in series). So, to produce a capacitance of 16uF at 900Vwkg, Steve should have connected two 32uF 450Vwkg capacitors in series. Also, a voltage-sharing resistor of the appropriate power dissipation and voltage rating should be connected across each capacitor. (Ed:- The value of this resistor should be such that it passes a current of at least 10 x that of the maximum leakage current of either of the 2 capacitors. Both resistors (for 2 capacitors) should have the same value.

Replacing a rectifier valve with a semiconductor diode (or diodes) should be done only as a last resort. The output from a semiconductor diode appears immediately the voltage is applied and, because of the diode's lower forward voltage drop, the peak rectified voltage is almost certainly higher than that of the valve rectifier it replaces. The potential problems resulting from this are several; here is just one. All the valves in the set are cold, drawing no current at switch-on. This results in a power supply producing full output, because the supply's source impedance is not yet in play to reduce the HT voltage to its normal running level. It should also be remembered that some types of valve can suffer irreparable internal damageby having full HT applied before their heaters are at working temperature. Also, don't think of a Variac as being an ordinary transformer, as this may give you the impression that it provides isolation from the mains. It doesn't. Although it is a transformer, It is a special type called an autotransformer, which has only a single winding, the tap being a continuously movable carbon brush.(Ed:- Hence the output is not isolated from mains)

From Denzil Roden, G3KXF;Mike, G8DKW ('The Last Word',RadCom, May 2005), likes to live a little dangerously. For those of us who find enough excitement in life already, a Variac is extremely valuable for reforming electrolytics safely, and for other purposes. 'Reforming' is the important word. Electrolytics need a regular application of their operating voltage to maintain the chemically-formed dielectric film on a surface of one electrode. Absence of the voltage for long periods allows the dielectric to deteriorate. Sudden application of working voltage to a deformed capacitor can then cause excessive current, boiling of the contents and possible violent bursting of the case. I find it easiest, in the long run, to remove components and rejuvenate each one separately. Modern electrolytics have soft-burst cases, but beware older ones which can explode violently. To that end, you need a metal box in which to enclose and contain any possible eruptions. The other thing you need to know is the maximum permitted leakage current for an individual electrolytic. The following formulae are currently hard to find and come from an old RS publication Component Application Data, 6th edition, 1971.

Capacitors <100Vwkg: current (uA) = 0.08CV + l00uA.

Capacitors >100Vwkg: current (uA) = 0.05CV + 250uA.

C is in uF and V is the component's marked working voltage. Increase the voltage gradually, ensuring that the current neverexceeds the maximum. The measured current should then gradually diminish, whereupon repeat the exercise by increasing the voltage. It may take many hours, but eventually the current should stabilise at a safe value (*) at the specified working voltage. No matter how long the set has been unused, always try to use the original components.

Ed:- ‘Safe value’ The above leakage current values are for new components; probably at least 10x this would be acceptable. Also the data was for 1971 items; considerably less should be expected nowadays. However, they are usefull guidelines.

Ed- ‘Dressing’ wires to their original position can be very important. Years ago, this was particularly relevant with vibrator power supplies. Modern r.f. equipment can be similarly fusy; the advantage of p.c.b.’s

Oli says thanks for the return of his Lowe HF-125 general coverage receiver. It been on loan & he could not remember to whom!!

CLUB CALL AFFILIATED

GXØOWX WEST SOMERSET AMATEUR RADIO CLUB R.S.G.B.

A new version of 'Electronics, Mechanics, Maths and Computing V9.2' is available fc free download. This is the complete package without any install or time restrictions For more information log onto   http://www.eptsoft.com/StudentHobbyist/electrical.html

A special event is to be held on the August bank holiday called "Pumping Stations on the Air". You or your club, along with your local pumping station are invited to take part in the first International Pumping Stations on the Air weekend. This will take place on Sunday 26 and Monday 27 August from 10am to 5pm. The intention of the event is to set up amateur radio special event stations at as many of the pumping stations as possible throughout the whole world. The scope of your station is entirely up to you, as is the choice of pumping station. Burton upon Trent Amateur Radio Club will be at the Claymills Pumping Station on that weekend. For more information on how to register visit the Burton upon Trent ARC Website at www.burtonarc.co.uk/pump.htm.

THE QUIET SUN. A paragraph on the opening page of the Space Weather Newswebsite (Web search) on 14 April read: "BLANK SUN: The sunspot number is zero for the 11th day in a row. This marks the longest string of blank suns since 1996. 1996 was a year of deep solar minimum, featuring blank intervals as long as 37 days. 2007 is shaping up the same - a nadir of the solar cycle." This is borne out by the daily solar data report for the 30 days to 13 April which recorded 18 spotless days. The only period when there were some spots was 23 March through 3 April and then the maximum 10.7cm solar flux only reached 75. On zero days it was 68. Unsurprisingly only three new regions were recorded. The geomagnetic data recorded only five days when the mid-latitude A-index at Fredericksburg was 10 or more. The maximum value was 17 on 3 April so once again there were no reports of auroral activity in the British Isles. the mains cable round it.

LOW ENERGY LAMPS. (If you must, stock up NOW)

Proposals for energy-saving measures in Europe could include a ban on filament light bulbs by 2010, as has already happened in Australia. Forcing everyone to use low energy fluorescent bulbs is expected to benefit the environment by reducing carbon dioxide from electricity generation but Godfrey, G4GLM, raises the question of RFI from these lamps. He notes that it has previously been reported in EMC that some types of low energy light bulbs generate RFI.

Ed It is not only low energy lamps that can cause interference. Ordinary filament lamps can do so by means of Barhausen oscillations. Oli has experienced this once; has anyone else? Also audio oscillations can also be heard coming from a tungsten filament lamp. Because this is a physical movement of the filament of some amplitude, it can quickly lead to destruction. The method of supporting the wire has changed, presumably to prevent early fairlure by this mode

QRP ARCI ANNOUNCES CHANGE FROM 7040 TO 7030 KHZ.

On February 24, 2007, Dick Pascoe, GOBPS, and Ken Evans, W4DU, President and Vice-President of the American QRP ARCI announced: "After careful consideration based upon membership feedback, frequencies used by both RTTY and QRP operators, and recent changes and pending changes to band plans, it has been agreed by the Board of Directors of QRP-ARCI that with immediate effect the centre of activity for 40m QRP CW operation in North America shall be 7030 kHz. It should be remembered that operators will continue to use 7040 kHz as conditions and previously scheduled events dictate. However, the 'official' CW centre of activity for members and QRP ARCI operating events will now be 7030 kHz." This brings the NorthAmerican frequency of operation in linewith the rest of the world.

Last month, Richard Elford, GØXAY, left us with 2 documents. One was on the use of mobile radio whilst driving and the other is reproduced below. It has been OCR’d and I have not bothered to do the usual coorections to page layout.:-

The Big Bang

CLUB CALL AFFILIATED

GXØOWX WEST SOMERSET AMATEUR RADIO CLUB R.S.G.B.

If you are already licensed and do nothing, you WILL receive the new lifetime licence before your existing licence expires. Existing licensees who wish to receive the lifetime licence prior to its automatic issue can obtain it via Ofcom's online service (see below). ALL licensees will need to confirm their details at least once every five years, which can be done at no charge online, or by post I (which will be subject to a _ 20 administration charge). If you need to apply for a new I licence (e.g. you have recently passed an exam), you can apply for your amateur radio licence free online or by post (which again costs _ 20). The RSGB notifies Ofcom of all exam passes weekly so you can apply online shortly after the exam.

A very few licences will still need to be applied for by post, e.g. club licences, re-issue of lapsed or historical licences and overseas visitors applying for a reciprocal licence. These will incur the _ 20 administration charge. Postal submission is required so that Ofcom can verify documents.

For enquiries about amateur licences and Ofcom's online service please contact:

Ofcom Licensing Centre (OLC)

P.O. Box 56373, London SE1 9SZ

Iel: 020 7981 3131

Email: licensingcentre@ofcom.org.uk

Website: www.ofcom.org.uk/licensing

Application forms for amateur radio licences can be found at: http://tinyurl.com/2762up

It has now been clarified that the new Terms and Conditions only apply to those licensees in possession of their new licence. You must continue to operate under the terms of BR68 until you have the new licence in your hands. Ofcom have clarified that the new licence is not intended to permit emergency operation outside the amateur bands. RadCom apologises for any confusion that this article

Farnell Electronic Components, Canal Road, Leeds LS12 2TU Order line: 0870 1200 200Web site: http://www.famell.com/ukUse the professional Farnell catalogue, CD-ROM or web site. Credit/debit cards accepted on orders above _ 10+VAT. Delivery free. (RadCom Sep 2000)

Maplin Electronics, Freepost SMU 94, PO Box 777, Rayleigh, Essex SS6 8LU Order line: 01702 554000 Web site: http://www.maplin.co.uk The Maplin catalogue is available in major magazine shops (_ 3.99) and shows VAT-inclusive prices. There is also a CD-ROM. Credit/debit cards accepted. Small order charge: _ 3.95 inc VAT on orders up to _ 30 inc VAT; free above _ 30 incVAT. Maplin has many branches throughout the UK. Although none of the branches carries the full line of stock, you can order in advance for collection.(RadCom Sep 2000)

Combined Precision Components (CPC), Faraday Drive, Fulwood, Preston PR2 9PP Order line: 01772 654455 Web site: http://www.cpc.co.uk CPC sell a wide range of largely consumer items, and their particular strength is in spares for a wide range of household electronics. They deal mostly with the trade and their catalogue is not available to private individuals, but they will accept orders from 'non account holders' if you have somehow found out what you want! In future you will be able to use the catalogue on the CPC web site, but this was not ready at the time of writing. Credit/debit cards accepted. Delivery charge: _ 3.99+VAT.(RadCom Sep 2000)

MUSHROOM COMPONENTS Further to the item on page 7 of the July RadCom, Trade Orders Only', we have now received the following note from Paul Vaughan, G8EJQ, the Director of Mushroom Components: "Mushroom Components has a reputation as a global source of obsolete semiconductors. Whilst most of our business is done overseas, we are more than pleased to help our RSGB and amateur friends in the UK where possible as we have many parts in our warehouse ideally suited to the amateur and hobbyist. We would draw their attention to our general terms and conditions of sale on our website (www.mushroom.co.uk) and also to the special offers page which is updated regularly Readers will find an enquiry form on the website along with contact names and phone numbers. We may not be able to help you from our own stock, but we may be able to direct you to an associate who can. (Ed. Their web site is still active but there is a _ 25 min order charge and there is no e-mail address for contact- However, it is just possible that an enquiry, with an S.A.E.might ellicit a response).28, College St, Kempston, Beds, MK42 8LU. (RadCom Aug 2005)

Listening to Australia

According to Radio Australia, the best frequencies for their broadcasts in the United Kingdom and Europe are:-

9.475MHz 1100-1300 and 1430-1900hrs 9.500MHz 1900-2130.

For the keener listener, a full schedule is at www.abc.net.au/ra/pdf/frequency.pdf. These times/freqs should give you guidance as to when to try for Aussie.

Radio New Zealand International (RMZ1) also has a new broadcasting schedule to coincide with the introduction of its DRM service to the Pacific. In the a.m. mode, it broadcasts on:-

7.145MHz 1300-1650hrs, 6.095MHz 1650- 1850, 9.630MHz 1851-1950, 15.720MHz 1951-2235, 13.730MHz 223-0458, 9.615MHz 0459-0658, 7.145MHz 0659-1059 9.870MHz 1100-1259.

The thing to make clear first of all is that there's a difference between mains and r.f. earths. A mains earth is part of the safety system used with electricity supplies. An r.f. earth can be part of a radio station's antenna system. In all, there are three types of earth that can be found in the listening shack:- 1) The mains operator and equipment safety earth - this is essential. 2) A radio frequency earth - possible use with some h.f. antennas.3) Additional equipment safety earth - used for lightning protection.

The Mains Safety Earth - Essential This is part of the mains wiring system of every premises. It is there for safety when a fault occurs in a piece of electrical equipment that could render it dangerous to anyone making contact with it. This earth connection provides a low resistance route that the electricity will take instead of through a person touching the equipment. It should be taken as read, that the electricity supplier will provide a good safety earth and that all radio enthusiasts are responsible consumers and ensure that their equipment is properly wired and used in accordance with safety requirements.

Radio Frequency Earth This is about antenna efficiency. Some antennas need an earth, some don't. Balanced antennas, such as dipoles, don't need any additional connections as they are complete circuits. For v.h.f and above, antenna dimensions are sufficiently small for such balanced antennas to be constructed. At h.f., with some antennas, the earth forms part of the antenna. The classic being the end-fed long wire. It can be considered as half a dipole, with the earth providing the missing side. If you don't think the earth plays a part in such antennas, build a crystal set and listen to the difference in reception with and without an earth connection! For the connection to the earth to be effective, it needs to be close to the feed point of the antenna. If the distance between the connection to the antenna and the ground is a significant part of a wavelength long on the frequency of operation, it may not actually provide an r.f. link to ground. (Ed i.e. An upstairs shack). A good r.f. earth depends on the conductivity of the soil and how well the earth wiring connects to the soil. A metre long copper earth stake should be considered the minimum requirement. Water retaining soil conducts much better than fast draining varieties.

Additional Equipment Safety Earth Combined with a suitable static discharge device in an antenna feed line it provides an alternate route for static electricity to earth rather than through the radio equipment. Static can build up to quite high voltages and has the potential (pun intended) to damage electronic components, not to mention giving anyone touching charged items a little jolt. For the average hobby radio set up the static discharge device will consist of a spark gap. Any such earth should take the shortest route possible to ground, using substantial braid, bonded to one or more earth spikes. Whilst this may offer some protection against a nearby lightning strike, there are no guarantees that a domestic installation will survive a direct hit.

PME Protective Multiple Earthing is a safety system for mains electricity supplies that has been in use for the last 30 years or more. Where installed, all earth connections must be linked to the PME point by suitable cabling. If that applies to your premises, or you're not sure, seek professional advice before creating another earth system separate from the' mains earth.

(Ed What this article does not cover is ‘what to do to provide an r.f. earth for an upstairs radio room’. The answer is a counterpoise earth which is effectively the other half of the aerial, whether it be resonant or a random long wire.) [Even if already covered in earlier editions of this Newsletter, it will worth repeating]

Commercial loops such as the Racal Mini-Loop MLA IE, which we have measured, are tuned by a vacuum variable capacitor, and have no resistance loss associated with a sliding contact to the rotor of a tuning capacitor. DK5CZ has specifically commented on the fact that ordinary rotary-shaft capacitors need to be modified to realise a low loss connection to the rotor. His AMA loops (intended for the radio amateur) employ factory-constructed tuning capacitors. A disc is welded to the rotor, and a tension-spring wiper makes contact with this disc. It is very clear that the 'off-the-shelf broadcast-band tuning capacitor, combined with a fixed capacitor for the lower frequencies, used for the Boswell loop, is unsatisfactory to series-tune an electrically-small loop, since the other resistance parameters are so small. (RadCom Aug 2006)

[Ed. Ordinary variable capacitors do have this high loss associated with the wiper contact. However, assuming that you are using an old-style broadcast radio variable C* , then there may well be enough space to solder a short length of braid from the outer of some coax. onto the capac. shaft and the capac. frame. *this type of variable C assumed for the wider vane spacing required to avoid high voltage breakdown]

MOUNTING OPTIONS. The antenna can be mounted vertically or horizontally and gives very different radiation patterns in both configurations. When mounted vertically it gives a fan-shaped pattern in line with the loop. The deep nulls off the sides can be used to notch out local sources of interference and are very effective. You get a mixture of high and low angles of radiation - the latter may be useful for close-in contacts. If used vertically there is a lot to be said for using a rotator, although it isn't strictly necessary. When mounted horizontally the pattern is effectively omni-directional, although the minimum radiation angle will depend upon the height above ground - generally the higher the better.

Last month’s (July) meeting was a well attended Bar-B-Q at Kenn’s QTH. Cooked food was prepared by Bob and fine & delicious trifles:- Sue donated the large one, and Basil the smaller one. Carol and Mike gave the Salad stuff, and Bob donated all the rest.

The cost was £3 per person but many of those who were there gave generous donations as well, for which many thanks.

Next month’s is also at Kenn’s QTH. It is a fine opportunity for the whole membership of the Club (&as always, visitors are welcome) to have some operating experience and maybe Dx. Even though Oli’s rig has gone phut, he hopes that it will be back from Castle Electronics in time for this event. Oli will (or hopes to!) put up an inverted V fed by flat twin (balanced) and also a 132’ (top band) long wire with corresponding ribbon cable or similar counterpoise. No guarantee about the height of these Ae’s but the space available (courtesy Kenn) is BIG.

Mains is available; Oli will have some spare extension mains cables.

I think that I might have have indicated that the flavour of the year is Aerials/Antennae. Nothing much has surfaced as far as talks at the Club but I suppose that the Operating Session at Kenn’s is a good start. Bob has operated from the Community College so I suppose that was a start!

As opposed to Aerials, here is a bit about the other end the Earth or Ground. Every transmitting station needs a good R.F. ground (although this should less important when using a balanced aerial system). It is a definite no-no to use the mains earth; modern mains systems are just not suitable. Neither can you rely on water pipes providing a decent ground connexions; too many plastic pipes involved. (In a m,odern house you will see many safety earth connexions between just about any pipe, using stripped green/yellow wire). The mains earth should be used, as usual, for any of the mains sourced equipments such as power supply.

Here, we are referring to the R.F. earth. The main station r.f. earth should be to the Aerial Tuning Unit (A.T.U.). If you are using a set-up where no A.T.U. is required (i.e. V.H.F./U.H.F or H.F. with a resonant Ae) , then the R.F. connexion should go to the Transmitter.

The mains earth should not be used. So what do you use? Just sling a wire out of the window and attach to a ground rod? NO (This would do for a safety earth for isolated equipment but do not connect this to the mains earth). Not unless your window is very close to the ground and to the A.T.U. Even then the wire should be of some substantial size, preferably flat copper strip. (An alternative is to take a length of ½" coax, strip of the outer sheath, pull back the woven screen, flatten it and use this for the equipment to earth connexion).

The reason that only a short length of wire is suitable for an r.f. earth is that any length has an inductance. The greater the length, the greater this inductance and hence the higher the inpdeance offered to R.F. currents. The last thing that is required in a decent R..F. earth is anything that prevnts an uninhibited flow of R.F. to Ground.

Those people with any an upstairs shack or a place some distance away from true ground have a problem. (The lack of a decent R.F. earth to equipemnet can mean R.F. burns and R..F.feedback!). The answer is a counterpoise.

The following is from "Practical Wire Antennas", J.D.Heys G3BDQ, pub R.S.G.B. 1989

Using counterpoise wires

Simple end-fed wires have acquired a reputation for inducing RF feedback problems such as microphone 'howl' and RF in mains wiring etc, together with ATU matching difficulties. There will be some radiation from the wire where it enters the house but this may be largely overcome by using counterpoise wires a quarter-wavelength long. Such wires will provide a useful 'earthy' connection at the ATU which will be far superior to the usual earth-wire system.

An upstairs shack can be particularly bedevilled by earthing problems, for often the run of the earth wire, even when made with heavy-gauge wire or flat strip, has a considerable inductance and will be long in terms of wavelength on the higher-frequency bands. Excessive RF in the shack may give rise to 'hot' equipment, a nasty phenomenon where supposedly earthed metal cases can give the operator an unpleasant RF bum when touched during transmission. The various long wires used by the author over the years were 'tamed' completely when counterpoise wires were connected to the ATU. The arrangement shown in Fig 28(a) is adequate for most situations and it includes an inductance L (with tap points) and a variable capacitor C, either of which may be used in series with the antenna wire to remove unwanted reactance at the feedpoint end of the wire. The jumper (J) will be used on those bands where reactance does not present a problem. Reactance problems are usually revealed when it seems almost impossible to bring down the SWR between the ATU and the equipment to a sensible figure. It also may show as very 'sharp' tuning of the ATU. A 'sensible' SWR reading means one which is something between unity and 1.5:1.

The counterpoise wires are cut to a quarter-wavelength for each band, and are best made with PVC-covered multi-strand flexible wires. They may be hung down outside the shack window or instead arranged to run inside the house They can be put along skirting boards, up to the picture rail (a feature absent from so many modem houses) or under the carpets. Such counterpoise wires will have a considerable RF voltage at their ends when the band they are cut for is in use and, if the output power is in excess of 50W, it is suggested that their ends are bent over and taped.

Counterpoise wires cut to the formula length seem to be effective but they can be set up more accurately with a dip oscillator (DO). To do this, one end of the wire is connected to earth and at that end a half loop in the wire is loosely coupled to the DO coil. A receiver tuned to the wanted frequency, or better still a frequency counter coupled to the DO, will be more accurate than the calibration scale of the DO alone. If no dip oscillator is available three wires for each band could be used, two of these being cut either a few inches longer or shorter than the formula length.

Ribbon-cable counterpoises A more elegant way to fabricate counterpoises for several wavebands is to use a length of multi-conductor ribbon cable. This cable is obtainable in 10-way format (or even 20 or 30-way!). Such cable uses stranded 14 x 0.013mm tinned copper wires which are conveniently colour coded. The use of this ribbon as a four-band counterpoise is shown in Fig 28(b). A piece of ribbon is first cut to be a resonant quarter-wave on the lowest frequency band to be used, and it then has sections cut away to make quarter-wavelengths left behind for the remaining bands. Any spare wires can be used to 'broadband' the system by having additional wires that are a little longer or shorter than the calculated midband length.

It is best to splay out the end of each quarter-wave section for about 6in. (15cm) to minimise possible flashover problems at these points, especially when the transmitter power is high (more than 100W). A counterpoise system made in this manner can be easily hidden and 'lost' beneath carpets etc.

Counterpoises of this type do not contribute towards the radiation efficiency of the antenna, their purpose instead being to reduce or eliminate any RF feedback or matching problems. The 'earthy' ends of all the counterpoise wires must connect right to the earth connection on the ATU. They are very useful 'artificial earths', and the author cured an RF breakthrough problem with an AF filter used for CW reception by connecting the case of the filter to such a quarter-wavelength of wire. The problem only showed on one waveband so only one piece of wire was needed.

Here are the answers to last month’s quiz. Nobody bothered to respond, so no more quizzes!

1. (a) The depth of modulation is, by definition, the ratio of the peak value of the modulating signal to the peak value of the carrier.

2. (c) It is, of course, also this for the others.

3. (c) For any modulating signal which has a mean value of zero, the ratio (sideband power/carrier power) is half the ratio (mean-square modulation voltage/ mean-square carrier voltage). The square wave has the largest mean-square voltage.

4. (c) This follows from the formula given in ans. 3.

5. (d) For each sinusoidal component of the modulating signal the modulated-carrier spectrum contains a pair of side frequencies.

6. (a) This is true for any modulating signal which has zero mean value.

7. (d) Doubling the frequency of a square or sawtooth wave doubles the frequency separation of its sinusoidal components.

15. (b) Analysis shows that if the output is to follow the modulation while it is falling, the product of C and T /I R must be less than

Tm √ (1-m2 )

-----------------

2 x nm

where Tm is the period of the highest modulating frequency component. It is apparent from this that if envelope detection is to be used the depth of modulation can not be allowed to reach 100.