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From: jhs%Mitre-Bedford@sri-unix.UUCP
Newsgroups: net.ham-radio
Subject: Re: antenna tuners for receivers.
Message-ID: <12425@sri-arpa.UUCP>
Date: Thu, 27-Sep-84 10:47:00 EDT
Article-I.D.: sri-arpa.12425
Posted: Thu Sep 27 10:47:00 1984
Date-Received: Sun, 30-Sep-84 04:43:57 EDT
Lines: 38

An interesting comment, that - it got me thinking!  I had always assumed you
cannot move the current distribution in an antenna by fiddling with a tuner
at the feed point.

For antennas terminating out there in an infinite impedance (the insulator at
the end), this is indeed true:  the current is always zero at the end and
always has its maximum a quarter wavelength in from the end.  No matter what
you may do with a tuner in your shack.  Or even a tuner up at the feed point!

However, you have educated me to the fact that if you bring the end of the
antenna back into the shack, as with a loop antenna, you can indeed hang
non-infinite impedances on its end, and can therefore change the standing wave
pattern.  Certainly you can short the end, getting a current maximum at the
shorted point.  You can also leave it open, getting a current zero there as
with the end fed antenna.  Presumably you can hang other (complex) impedances
there and put the current maximum anywhere you like.  Neat.

In particular, I agree that for a large loop (i.e. non-infinitesimal) this
should make it possible to place a current maximum near the highest point on
the loop.  By reciprocity arguments one can see that this should indeed make
the receiving loop perform better than it would otherwise.

However, I still claim that for the "good ole" receivers at least, and at the
lower HF frequencies, tuning the tuner will move the S-meter higher, but with
a decent AGC the audio won't sound any different.  I guess they don't build
AGC circuits like they used to.  (Gotta keep this controversy going somehow!)
Somewhere between 20 Meters or so and 2 Meters, the SNR limit moves from
external noise to front-end noise, and then I agree that the tuner is
important.

Of course it might be true that with the loop, changes in the directionality
of the loop due to current distribution changes of the type conceded above
might affect the signal to external noise ratio.

Thanks for educating me to a phenomenon I was unaware of!

						-73,
						John Sangster, W3IKG