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TopBand: Transformers for pennants and flags

 To: TopBand: Transformers for pennants and flags pnesbit@melbpc.org.au (P&V Nesbit) Tue, 15 Sep 1998 09:06:29 +1000
 ```Recent discussions have highlighted the need for good transformer balance and low interwinding capacitance, for the matching transformers used for pennants and flags. Further examination shows that the problem is not trivial. The problem stems from common mode excitation of the antenna, in which it behaves like a bent monopole with an elevated feedpoint: * * * * * * * X * X = feedpoint * * * (bent monopole mode) * * * * * * * * -----------*----------- Ground K6SE reports the gain of the pennant and flag as about -36 and -30 dBi respectively, over average ground. My own modelling of these antennas confirms these figures and indicates a front-to-back ratio of 20 dB, meaning that the rear lobes are at -56 and -50 dBi respectively. To ensure that the front-to-back ratio is not degraded by more than 1 dB, it is necessary to keep the "bent monopole" gain at least 6 dB further down, i.e. at -61 and -56 dBi for the pennant and flag antennas respectively. Considering that the gain of a quarter wave monopole over similar earth is about +1 dBi, one begins to see the magnitude of the problem. In other words, to avoid filling in the rear null to any significant degree, it is necessary to attenuate the common-mode gain by some 50 to 60 dB! To determine the transformer capacitance needed to produce this attenuation, I modelled the pennant as a bent monopole, as shown above. I also modelled the flag on the assumption that it is supported by a single mast at the centre, with the feedline travelling horizontally back to the mast, and then vertically down to the ground. Over average ground, the common-mode impedance of the pennant was 0.76-j7507 ohms, and that of the flag was 0.68-j19175 ohms (calculations were done with NEC4WIN95, which uses the Mininec engine). Assuming a 50 ohm source, this produces mismatch losses of 43.5 and 51.7 dB respectively. Although the flag is within ballpark, the interwinding capacitance of the pennant transformer needs to contribute a further 17.5 dB loss (1). This is a simple potentiometer problem. To obtain a loss of 17.5 dB, Xc must be at least 7507*10^(17.5/20) = 56295 ohms. From this, we can calculate the maximum allowable interwinding capacitance as 1.54 pF at a frequency of 1.83 MHz. This is the effective capacitance, averaged over the whole winding. If each turn contributes equally to the capacitance, the maximum allowable total capacitance is twice the above or 3.08 pF. However practical windings are less than uniform, so the maximum allowable capacitance will be (say) 2 pF. This order of isolation is likely to be marginal or impossible to achieve with the fairly large balun core mentioned by W8JI, because of the close proximity of the windings. In my opinion it would be much better to use a toroid, with the primary and secondary well separated, and coupling through the ferrite only. I concur with W8JI on the absolute need to use the correct ferrite, and based on the curves published by Fair-Rite and Amidon, believe the best material to be #43 (ui = 850). This will produce a core Q of 4.25 and consequent loss of about 1 dB, which is reasonable for the application. I would avoid the use of a manganese-zinc ferrite like 73, as it appears far too lossy for this sort of transformer at 1.8 MHz. The fact that it works well for transmission line transformers up to 30 MHz (or beyond) is not really relevant, because the application here relies on magnetic coupling, whereas transmission line transformers do not. At this point, it must be emphasized that a transmission line transformer SHOULD NOT be used for this application, as their end to end isolation is much too low. They are really only suitable for low impedance circuits. My choice is an FT140-43 toroid with 8 turns on the secondary, and 34 or 35 turns on the primary (for 900 or 950 ohms respectively). Sorry to specify the larger and more expensive core, but one needs the higher AL value to get sufficient inductance (2). With this core, the shunt reactance will be +j689 ohms, leading to a nice conservative design. This larger core will also make it easier to keep the windings separated. This transformer should bring everyone's pennants and flags "to life", as reported by W7IUV. I would advise against stacking cores to increase the AL value, because the primary to secondary capacity increases in proportion to the number of cores. This happens because the wires run parallel for a longer distance. If a higher AL value is needed, choose a core with a larger outside diameter. Although this approach is more expensive, the interwinding capacity is not significantly increased. A final touch, although probably not necessary, would be to balance the low impedance winding by interposing a second transformer. The result would be to cancel the remaining capacity of the first transformer, as far as common mode coupling is concerned. Suggested details are: 10+10+10 turns trifilar, FT50-43 core, coax to one winding, the other two windings in series, and their centre tap grounded. Since this transformer steps the impedance up to 200 ohms, the main matching transformer will need twice the number of turns on the low impedance winding, i.e. 16 turns instead of 8. Losses should be low. Finally, with the antenna floating, it is imperative to provide a DC path for static discharge. I suggest grounding the opposite side of the antenna, at the centre. Don't be tempted to take the easy way and add a centre tap to the 900/950 ohm winding, because the first nearby strike will disaccomodate the ferrite and greatly increase its losses. 73, Peter VK3APN (1) The figures assume the bent monopole has the same directivity as a straight quarter wave monopole. It doesn't, but the difference is minor (only one or two dB). We are seeking ballpark figures here, not fractional dB precision. (2) AL = inductance of one turn in nH. This equals the mH per 1000 turns as quoted by Amidon. pnesbit@melbpc.org.au -- FAQ on WWW: http://www.contesting.com/topband.html Submissions: topband@contesting.com Administrative requests: topband-REQUEST@contesting.com Problems: owner-topband@contesting.com ```
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