Topband: Transformer Question

[K9AY]

Ford, Thomas

I quickly built and measured the situation you described.  The transfomer
is conventional (separate primary and secondary windings), but tightly
inter-wound for k~1, with the right ferrite core for the frequency range.
Measurements wre made at 2 MHz.

With this 4:1 Z ratio transformer, a 200-ohm carbon resistor transformed
to 45 +j6 ohms. With an inductor added to create a 200 +j250 ohm
load, the transformed impedance was 46 +j62 ohms.  Placing the right
capacitor in series with the low impedance winding could obtain a
45 +j0 impedance. I could not "zero out" the reactance using a
capacitor in series with the load, but it was certainly low enough to
be considered zero for practical purposes.

Conclusions? A good transformer simply changes the magnitude of
the vector representing the R +/-jX of the load without altering the
angle. This is what we want. There are many, many published RF
transformer designs that are OK at maybe 5 or 10 MHz but cannot do
this at 2 MHz.

I did not take the time to repeat the experiment with a transmission line
transformer.

Ford, with regard to your transmission line question -- The equations
are in Kraus, Jasik, Smith, Reference Data for Radio Engineers and
many other places.  I just use the Smith chart...

73, Gary
K9AY