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

[Don Kirk]

Digging back through the archives I found lots of discussion about the
"best" transformer to use with flags, pennants and beverages.  The
discussions included material type, and style (toroid vs. binocular), but I
found no mention about problems regarding the use of conventional toroid
cores in which a pure resistive load on the secondary appears to be a
complex impedance (resistance and inductance) on the primary side of the
transformer when having the primary turns on one side of the core and the
secondary turns on the opposite side of the core (which appears to be the
recommended method of winding if using toroid cores for this purpose).

I then noticed that the only recommended transformer configuration I
measured that would provide a pure resistance on the primary when having a
pure resistive load on the secondary was the transformer constructed with a
binocular core as recommended by W8JI and others, and creating a binocular
core using two side by side toroid cores also worked fine in my testing.

I then decided to try spreading my secondary winding (20 turns) on the
toroid core over approximately 330 degrees of the toroid core (FT 114-77),
and then overlay my primary winding (5 turns) on top of the secondary and
again spread it over the 330 degrees of the toroid core.  Doing this fixed
my transformation problem (now my primary was just resistive when the
secondary had a pure resistive load.


Here is some data to help explain the issue (the R and X values is the
impedance measured on the primary side of the transformer with the 1 K
resistance connected to the secondary of the transformer).

FT114-77, Pri = 5 T, Sec = 20 T, primary and secondary separated (opposite
sides of the core), 1K resistive load on secondary.
R = 57 ohms, X = 18 ohms, Freq = 1.8 Mhz
R = 58 ohms, X = 34 ohms, Freq = 3.5 Mhz

FT114-77, Pri = 5 T, Sec = 20 T, primary and secondary over lay (using 330
degrees of the core), 1K resistive load on secondary.
R = 56 ohms, X = 0 ohms, Freq = 1.8 Mhz, insertion loss = -0.54 dB
R = 53 ohms, X = 0 ohms, Freq = 3.5 Mhz
Note : measured capacitance primary to secondary = 5 pf, capacitance not
measured at RF

BN-73-202, Pri = 3 T, Sec = 12 T, 1K resistive load on secondary.
R = 59 ohms, X = 0 ohms, Freq = 1.8 Mhz, insertion loss = -0.30 dB
R = 57 ohms, X = 0 ohms, Freq = 3.5 Mhz
Note : measured capacitance primary to secondary = 9 pf, capacitance not
measured at RF

Has anyone else stumbled upon the above phenomena, and am curious why there
has been no mention of it before (or if there has been it's very hard to
find)?  I'm very happy with the performance of my binocular core
transformers, but I'm technically inquisitive.

73,
Don Kirk (wd8dsb)