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[TenTec] "Tuning" antennas; baluns

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Subject: [TenTec] "Tuning" antennas; baluns
From: rohre@arlut.utexas.edu (rohre)
Date: 23 Sep 1997 10:04:47 -0500
Hello all,
Well first off I refer the list members to the books that together discuss
this issue much better than we can.  But especially to Walt Maxwell's book, or
the Antenna texts like John Kraus's "Antennas".
And as to Baluns, Jerry Sevick points out that baluns are very efficient up in
the 90% range or better if properly designed, thus if one had a problem with
an Amidon one based on Jerry's designs, I think it was misapplied with the
wrong core for the powers involved or the frequency range to be covered. 
Weatherizing is also a big factor in most antenna failures and their
associated hardware.

Now, to try to summarize a complex subject:

The concept of resonance, and tuning to resonance for an ANTENNA, must involve
changing its Physical Dimensions.  Once you have cut it to some size, it will
have certain resonant properties.  No amount of "tuning" of L and C circuits
at the shack end of the transmission line will alter the resonant dimension of
the antenna up in the air.  Tuning is defined as affecting resonance; and the
antenna resonance depends on its dimensions in terms of wavelengths or
fractions thereof. 

The only thing you are "tuning" in the Antenna Matching device is actually
"adjusting"  the values of the L and C of those tuned circuits.  However, you
are MATCHING the transmission line to the rig, but you have not matched the
transmission line of the open wire type to the antenna, for you still will
observe standing waves upon the transmission line.  If it were matched to the
antenna, it would be a "flat" line with SWR of 1:1.  But, balanced open wire
line, and windowed line are low loss transmission lines, and they can operate
with considerable standing wave ratio, while still presenting lower losses
than coax under similar conditions.  If you weather proof the lines with wax,
or other water repellant, they should be more stable in wet weather.

In fact, I hope all will agree or will measure,  that even with an antenna
matching device, (which for years was mistakenly called a "tuner"), you still
have standing waves on the transmission line, and the matching device is just
there to TRANSFORM a complex impedance at the shack end of the line to a value
that more closely matches your transmitter.

I hope this helps divide the concepts into their proper areas.  Antennas
resonate depending upon their physical structure and dimensions.

73, Stuart K5KVH

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