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
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Topband Reflector
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