On 9/23/97 11:04 AM, rohre at rohre@arlut.utexas.edu wrote:
>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.
Not quite. In my book, a transmatch produces a "matched" or resonant
condition on the antenna "system" -- which includes the transmission
line. The resulting conjugate match is just as resonant as any other type
of antenna system.
True, there are so-called "standing waves" on the transmission line. But
the presence of these waves has next to nothing to do with the resonance
of the antenna system. I'll give you an example -- a "resonant" dipole in
free space has a feedpoint impedance of around 70 ohms. Hook 50 ohm coax
to it and you have a mismatch that results in about 1.5:1 SWR.
Standing waves are only a problem in lossy transmission lines -- such as
coax. Open wire has so little loss that the standing waves can be safely
ignored.
If you had a lossless 50 ohm coax, you could hook it to that 70 ohm free
space dipole and safely ignore the mismatch. Of course, with lossly coax,
that 1.5:1 SWR is probably much lower (feedline losses apply equally to
the reflected energy) at the transmitter -- probably around 1.2-1.4:1.
Low enough that you might ignore it anyway.
>I hope this helps divide the concepts into their proper areas. Antennas
>resonate depending upon their physical structure and dimensions.
No, antenna "systems" resonate depending on their ELECTRICAL
characteristics. A shortened dipole is no less resonant than a full-size
dipole, yet its dimensions are different.
There's nothing magical about certain antenna dimensions or "resonance."
The overriding issue is concern for energy LOSS. Once you understand the
nature of energy loss in an antenna system, it is a simpler matter to
reduce such losses.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
Quote: "Not in a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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