Topband: RX antenna transformer winding (pure resistance transformation issues)

[Tom W8JI]

>I had additional offline conversations with Rick (N6RK), and Carl (KM1H),
> and below is my official response to my original post about RX antenna
> transformer windings and complex impedance measured on the primary side of
> the transformer depending on how they are wound.

There are several things that cause the issue you noticed, and one of them 
is core magnetic path length and leakage. The ARRL in some publications, for 
example, teaches us toroids are "self-shielding". In fact, many toroidal 
cores have significant flux leakage. By squeezing or spreading turns to 
change mutual coupling between turns, a T157-2 core can have almost 2:1 
impedance shift! That tells us it has considerable flux leakage.

The primary issues, however, are the generally "impure" magnetic properties 
of very high permeability cores. If we tightly wound a single winding on the 
core and measured the impedance, we would find it is not a pure lossless 
reactance. These impure impedances modify the impedances seen through the 
transformer.

There is a great deal of stuff going on in a transformer.

Unless you have a system with pretty high common mode impedances, winding 
spacing means nothing. The primary reason I (and DXE) isolate windings with 
Teflon tubes is to reduce lighting damage, and to greatly reduce assembly 
damage. If the system has pretty high common mode impedances there might be 
some small advantage in pushing windings apart, but the primary-secondary 
capacitance is never going to be important in Beverages or other low or 
modest common mode impedance antennas. Balancing a small loop might be an 
issue.

I started using Teflon sleeves in windings because I ran out of small Teflon 
wire wrap wire I was using. Without Teflon on the wires, and with normal mag 
wire, the enamel was easily scratched. Not only that, lightning would punch 
through from enamel to core, or from wire to wire.

I'm glad the isolated primary transformers took over from the 
auto-transformers that were so common, but I think the need for low 
capacitance is being a bit overplayed in the vast majority of cases.

73 Tom