CLUB CALL AFFILIATED

GXØOWX R.S.G.B.

Last Month’s meeting was a not very spectacular activity. The noise levels at the College are rather high and both Bob & Oli were (trying) to use just 80m & 40m mobile whips. [Far too low a gain for the environment. Additionally, no real ground planes were available, there was feeder problem on one Ae and it was suspected that the counterpoise earth was non effective]. Much better results are achieved on the operating evenings when we are invited to Kenn’s QTH.

Next month’s meeting (1st Tuesday in April at the Minehead Community College, 1930hrs for 2000hrs start).

Several videos are available; one on how to make your own triode valve and others from the British Amateur Television Club.

BROADBAND BALUNS

These can be of one of two basic types: those which force the currents in the two halves of the antenna to be equal in amplitude but of opposite phase, and those which force the voltages to have this relationship. If the antenna is truly balanced, both achieve the same effect. One problem with wire antennas at the lower frequencies is that it is difficult, for many reasons, to achieve a fully balanced arrangement. The current balun ensures that in such cases, the currents in both conductors of the feeder are equal in amplitude.

Fig 4 shows two simple types of current-mode balun, both of which provide a 1:1 impedance ratio. These work on the principle of providing an impedance to restrict the flow of an out-of-balance current. In the case of Fig 4(a) this is achieved by coiling up the coax near to the feedpoint of the antenna. The coil has no effect on the normal signal flowing up the coax but looks like an inductance to any current trying to return via the outer. The same effect is achieved in Fig 4(b) by threading ferrite rings over the coax.

One difficulty with the arrangement shown in Fig 4(a) is that it can be difficult to make it work effectively over a wide frequency range - say 3.5 to 30MHz. Providing sufficient turns to cope with 3.5MHz is likely to result in the interwinding capacitance being too high for effective operation at 30MHz. The situation can be improved by winding the turns on a ferrite rod ring.

A current-mode balun can also be constructed by using a bifilar winding on a toroid or ferrite rod, as shown in Fig 5. It must be understood that these bifilar windings act like transmission lines, which can limit the performance of the arrangements in some circumstances and yield rather erratic results. Ruthroff [ 1 ] advocated the addition of a third winding to the simple bifilar winding of Fig 5(a) to yield the arrangement shown in Fig 6(a).Although this third winding overcomes some of the problems of the two winding arrangement, it has the effect of turning the balun into a voltage-mode device. Windings 1-3 and W3 act like an auto-trans former, so that the voltage at point A is half that of the input. The voltage at point B will also be half that of the input, but with a phase reversal. This arrangement tends to be regarded as the 'standard' for 1:1 impedance ratio baluns. The simplest form of voltage- mode balun, albeit with a 4:1 imped- ance step-up, uses the same basic arrangement as Fig 5(a), but with the windings connected in a differentway, as shown in Fig 6(b). Construction is as for the examples in Fig 5(b) and Fig 5(c). There are two, often conflicting, criteria associated with the design of voltage-mode baluns. Firstly, the inductive reactance of the windings should be high; and secondly, the leakage reactance should be low -compared with the load impedance in each case. The first usually determines the low frequency limit of operation whilst the second determines the high frequency limit. A current-mode balun can aiso be constructed by using a bifilar winding on a toroid or ferrite rod, as shown in Fig 5. It must be understood that these bifilar windings act like transmission lines, which can limit the performance of the arrangements in some circumstances and yield rather erratic results. Ruthroff[l] advocated the addition of a third winding to the simple bifilar winding of Fig 5(a) to yield the arrangement shown in Fig 6(a). Although this third winding overcomes some of the problems of the two winding arrangement, it has the effect of turning the balun into a voltage-mode device. Windings 1-3 and W3 act like an auto-transformer, so that the voltage at point A is half that of the input. The voltage at point B will also be half that of the input, but with a phase reversal. This arrangement tends to be regarded as the 'standard' for 1:1 impedance ratio baluns. The simplest form of voltage-mode balun, albeit with a 4:1 impedance step-up, uses the same basic arrangement as Fig 5(a), but with the windings connected in a different way, as shown in Fig 6(b). Construction is as for the examples in Fig 5(b) and Fig 5(c). There are two, often conflicting, criteria associated with the design of voltage-mode baluns. Firstly, the inductive reactance of the windings should be high; and secondly, the leakage reactance should be low compared with the load impedance in each case. The first usually determines the low frequency limit ofoperation whilst the second determines the high frequency limit.

CONCLUSIONS

This has been a fairly brief introduction into the subject and has (quite deliberately!) begged the question of which design is 'best'. The reason for this omission is that the use of baluns tends to result in compromises having to be made: what works well in one application may be a total failure in others. The sensible approach is to try a few different ideas and select the one which gives the best performance in your particular set-up. Fortunately, the components used are reasonably inexpensive and can easily be re-cycled for the different arrangements. For those wanting to have a go, [2 - 7] list articles and books containing more details of the different arrangements. You might be be musedby the fact that some authors will be enthusiastic about a particular arrangement whilst others regard it with horror. Take due note of any objections to the various designs, but do not let these put you off trying them.

REFERENCES

[1] C L Ruthroff, 'Some Broad-Band Transformers', Proc IRE, Vol 47, August 1959.

[2] lan White, G3SEK, 'Balance to Unbalance Transformers', Radio Communication, December 1989 (highly recommended reading).

[3] Radio Communications Handbook (RSGB).

[4] ARRL Handbook (ARRL).

[5] HF Antennas for All Locations (RSGB).

[6] Backyard Antennas (RSGB).

[7] Transmission Line Transformers (ARRL).

[8] Reflections Transmission Lines and Antennas (ARRL).

TOROIDAL CORES DEFENDED

The item Toroidal cores, baluns and ATUs' {TT, Feb 1992, p37) noted that the balun-type broadband impedance transformers with toroidal ferrite or powdered-iron cores of the type still found in manytransmatches (ATUs) are not suitable for use at high or reactive impedances: at high power cores are prone to saturate while at high impedance the RF voltage can cause arcs between the turns or between the windings and the core material. GW3DIX also noted the decision of the ARRL actively to discourage use of these components for such applications. This has resulted in several letters pointing out that it is wrong to condemn outright the use at relatively high powers of toroidal cores. Used correctly such components still have a useful role to play, several correspondents suggest. For example, Bob Pearson, G4FHU, writes: "While I agree with much of this item, it would be a pity if it were to cause unnecessary dismay among those who could nevertheless use ferrite or iron-dust cored baluns and transformers successfully. "It is often quite practicable to connect a balun on the transmitter side of an ATU, so that the balun drives a resistive impedance of a suitable magnitude. The ATU need not then be a balanced configuration even though it feeds a balanced line. But it does need to be fully insulated. For output powers up to about 100W PEP the ATU components can be physically small enough to fit into a plastic box of reasonable size, and shaft-insulation can consist of substantial control knobs with recessed and plugged grub-screw holes. "For example, a single variable capacitor and a tapped inductor can cope with a wide variety of matching requirements. The simplest arrangement uses terminals or wander plugs and sockets to permit a variety of configurations (see Fig 8). It is remarkable how small a suitable toroidal core can be, even for quite high power transfer, as long as the balun load impedance is correctly adjusted by the ATU before full power is applied. "The minimum core size can be estimated as follows: Most suitable ferrites have a saturation flux density of about 0.2 tesia to 0.5 tesia (2000 to 5000 gauss). Suppose we permit a peak flux density of no more than a tenth of the lower figure (ie 0.02 tesia or 200 gauss) and assume a winding of no less than about ten turns across a 50Ω load.

Results on this basis are shown in table 1. Commonly available toroids need be no longer than about 2-inch diameter to satisfy the highest figures shown. A 1-inch diameter core easily meets the 10OW PEP 3.5MHz requirement, though one could select a slightly larger core to minimise the number of turns of wire needed and to give a comfortable winding space".

TABLE 1: Minimum cross sectional area (mm²) for magnetic core with peak flux density of 0.02T (200 gauss) and 10-turn coil effectively in parallel with 50ohm resistive load. For higher flux or more turns, core area proportionately less. But required area increases as the square root of load resistance.

Power Lowest frequency of operation (MHz)

(WPEP) 1.8 3.5 7 14 28

1 4.42 2.27 1.14 0.57 0.28

2 6.25 3.22 1.61 0.80 0.40

5 9.89 5.08 2.54 1.27 0.64

10 13.98 7.19 3.60 1.80 0.90

20 19.77 10.17 5.08 2.54 1.27

50 31.26 16.08 8.04 4.02 2.01

100 44.21 22.74 11.37 5.68 2.84

500 98.86 50.84 25.42 12.71 6.36

1000 139.8 171.90 35.95 17.98 8.99