Brian--

Your spice simulation looked familiar...VERY familiar. As it turns out,
I had plotted similar curves and happened to have used Microcap as well!
I was just not sure I understood the relationship between the transfer
function I saw and some kind of physical antenna.

Your comments make sense, though I do have one question: How did you
calculate the impedance of the antenna with the shape and dimensions
that you described?

73
Pete
AC7ZL


Brian wrote:

Pete, I was curious about the dummy antenna, too. Some time ago I
modeled it and checked the impedance looking back into the output leads
with a 50-ohm generator at the source. The results are he

http://users.tns.net/~bb/dummy.htm

Before I did the model I figured that the dummy antenna was trying to
simulate a certain length of wire on shortwave. But you can see that
the magnitude of the output impedance (upper plot) is around 400 ohms
throughout the shortwave part of the spectrum. Down in the broadcast
band the network becomes almost purely capacitive, which is what a
short wire will do.

In fact, at 500 and 1000 kHz (the two spots I checked), the dummy
impedance is almost exactly that of a wire that runs 15 feet vertically
and then 90 feet horizontally over perfectly conducting ground. I
imagine they had a 100-foot wire in mind, a figure I think I've seen
recommended for BC antenna length in the old days.

So I think the network is intended to be 400 ohms resistive at SW and
at BC behave as a 100-foot wire would. I align radios with the network
and then at installation I tweak the antenna capacitors, if they are
easily accessible, on the actual antenna.

Brian