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July 10th 03, 08:23 PM

(Jeff Y.) wrote in message ...
Hi all,

I've been trying to find some data on what happens to the feedpoint
impedence of an antenna when the thickness of the wire increases. I
know for a fact the bandwidth will increase somewhat but what does the
impedence do, say going from 14 gauge to 10 gauge copper wire? Does
it increase or decrease?

You can find books with graphs of dipole impedance versus length and
wire diameter that will give you a lot of appreciation for what
happens. You can, of course, use a program to generate such graphs
yourself, but it's already been done for you. I've found it helpful
to look at what happens to "short" antennas, as well as at resonances
at both odd and even multiples of a half-wave, and at points in
between the resonances. I've particularly liked the graphs (and
explanations) in the "Antennas" chapter of King, Mimno and Wing's
"Transmission Lines, Antennas and Waveguides" book. Since the graphs
are showing the value of a complex dependent variable versus two
independent real variables, it's nice that they are presented in
several different formats. It might be fun to put the data into a
modern interactive graphical presentation program to play with, but
the old 2-d plots can still tell you a lot.

The feedpoint resistance changes a lot for wire diameter changes with
a full-wave dipole, but as others have noted, not much at all for a
half-wave. You'll see similar things at other even multiples and odd
multiples of a half-wave. Of course, the same thing applies for
quarter, half, 3/4, full, etc. wave elements feed against a ground
plane.

Cheers,
Tom

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