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

[Don Kirk]

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.

Rick N6RK said

> The way out of this dilemma is to use a smaller toroid core than the 1.14
inch diameter core you currently have.

But I saw the complex impedance phenomena even with smaller diameter cores
(using different grade of material), and here is my actual FT 50-43 data as
an example.

FT50-43 measured data as follows (windings on opposite side of the core
from each other) :
Pri = 5 T, Sec = 20 T, 1K load resistor
R= 42, X = 25, 1.8 Mhz, Capacitance = 5 pf
R= 51, X = 24, 3.5 Mhz
R= 57, X = 32, 7 Mhz

When I used two FT50-43 cores configured to make a binocular core, I
obtained the following results :
Pri = 3 T, Sec = 12 T, 1K load resistor
R= 38, X = 21, 1.8 Mhz
R= 50, X = 1, 3.5 Mhz
R= 52, X = 0, 7 Mhz

Rick also said :
> the small amount of leakage inductance you are seeing even with the large
core with separated windings is nothing to worry about.

I agree with this unless you are trying to phase RX antennas, and then the
complex impedance becomes a critical factor to deal with, and that's why I
feel it's important to understand (know) that this phenomena exists.

Rick also said :
> I would rather tolerate that and get the benefit of the reduced
capacitance from winding to winding.

Appears everyone agrees that a very low capacitance between windings
(between primary and secondary winding) is near (if not at the very the
top) of the list of desired properties of the RX antenna transformer.  This
is also supported by statements in the ON4UN Low-Band DXing book.

Carl (KM1H) goes to great lengths to reduce the capacitance between
windings on the binocular cores that he uses on his RX antennas per his
following statement : "Teflon tubes for each winding and that brought the C
way down by compressing each winding in the smallest tube that I could get
the wires thru and then forcing a toothpick thru to force the most
separation."

Thanks to Rick and Carl for the technical discussions we had.

73,
Don Kirk (wd8dsb)