Topband
[Top] [All Lists]

## TopBand: Transformers for pennants and flags

 To: TopBand: Transformers for pennants and flags w8ji.tom@MCIONE.com (w8ji.tom) Tue, 15 Sep 1998 10:03:14 -0400
 ```Hi Peter, What a well thought-out post with good analysis! I'm not quite ready to agree with the assumption common mode excitation directly relates to common mode impedance and feedline impedance, the issue is almost always more complex. Some factors make the antenna LESS sensitive to common mode problems than impedances alone indicate. One is the loop impedance of the Pennant, since it is not infinite some common mode current will harmlessly balance itself out. This is what happens in a small loop, where severe unbalance can be tolerated without ill effects IF the loop is isolated from earth. The larger the loop, the more important balance becomes. Let's just assume for now the common mode impedance requirements are complete. > 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! > 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. Here is the problem I see. We'll never achieve the required amount of common mode coupling capacitance with a single transformer. The ends of the cable and antenna will have more stray coupling capacitance than 2 pF, even a perfect transformer would not be good enough. Assuming the common mode isolation requirements are correct (I need to think about it a while, but my gut instincts tell me the requirement is much less stringent), the only real world system that would decouple the antenna and feedline properly would be a series of resonant common mode chokes along the feedline. In effect the solution would involve breaking up the feedline with resonant common mode chokes, and they would have to be high Q resonant chokes! A second problem is most people will install these antennas near other "stuff", and have coupling into other conductors. This antenna very well be site limited, and there may be MUCH better solutions. That's the problem with models, sources, antennas and even grounds are all perfect. We can design antennas that will be almost impossible to actually make work like the model predicts! > 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. That was a point I tried to make earlier. Most transformers are somewhere between the two types, and the mix selected really depends on the application. With transmission line transformers, the core material loss tangent isn't critical. All we want is a high impedance. With isolated windings, loss tangent becomes a factor. The more isolated the windings, the more critical loss tangent becomes. The core I suggested works very well for general applications, in particular Beverages and loops with better balance close to earth. With a "loop" like the Pennant balance issues become a real headache, especially if the antenna is elevated above ground with vertical feedline. The transformers I wound have a primary to secondary capacitance of about 18 pF on 160 meters. That is nothing when feeding a Beverage or "balanced shape" loop, but it is a problem with some "bigger" small antennas like these. > 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. IMO, based on 25 years of antenna experiments and RF design, they have very specific applications where they can should be used. Antennas are NOT one of them, unless it is matching a near-PERFECT unbalanced antenna (like a vertical with many many radials) to a coaxial line. For Beverages, elevated radial verticals, and almost anything else additional common mode isolation is advisable. They are not only suitable for low impedance systems, they are limited to certain applications where unbalance is near-perfect. > 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. I agree. But I think in this case the solution is to augment the matching transformer with a few common mode chokes along the feedline, so the feedline is "broken up" where it approaches the antenna. We may well be trying to "fix" and antenna that will never be predictable, because of its inherent sensitivity to common mode. These problems have always kept me away from loaded large ground-independent loops, that use the vertical wires as antennas. It's often better to use two small verticals or two small loops phased. 73 Tom -- FAQ on WWW: http://www.contesting.com/topband.html Submissions: topband@contesting.com Administrative requests: topband-REQUEST@contesting.com Problems: owner-topband@contesting.com ```
 Current Thread TopBand: Transformers for pennants and flags, P&V Nesbit TopBand: Transformers for pennants and flags, Earl W Cunningham TopBand: pennants and flags, Larry Molitor TopBand: Transformers for pennants and flags, Larry Molitor TopBand: Transformers for pennants and flags, P&V Nesbit TopBand: Transformers for pennants and flags, w8ji.tom <= Fwd: TopBand: Transformers for pennants and flags, W7lr@aol.com