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[TowerTalk] Antenna Tuners

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Subject: [TowerTalk] Antenna Tuners
From: (Tony Brock-Fisher)
Date: Mon, 10 Mar 1997 09:13:12 -0500

1. If you use a tuner at the rig, it will present a perfect match to the
rig. This will allow the rig to deliver full power into the tuner. If there
is a mismatch between the feedline and the antenna, there will be reflected
power on the feedline between the antenna and the tuner. If the feedline
has losses, this reflected power will cause additional losses over a
perfectly matched or lossless system. If the feedline has low losses,
it is often acceptible to allow the mismatch between feedline and antenna.
For example, I have about 160 feet of 3/4 inch, 75 ohm feedline running
to a KLM KT-34XA, which provides a good 50 ohm load. Even though the feedline
is not matched to the antenna, the resulting losses are almost a DB lower
than if I used 50 ohm 9913 type feedline.

2. Putting the tuner at the antenna would result in lower losses in the
feedline if the feedline loss was an issue. The feedline would then see
a load equivalent to it's own characteristic impedance. So would the rig,
so you would need matching at the rig output as well. And a tuner at the
antenna is not practically adjustable, unless it's automatic and remotely
powered. Actually, most antennas include a tuner at the feedpoint; it's
called a matching network such as gamme, omega T-match, etc.

Bottom line: Use the lowest loss feedline you can get. Adjust the match
at the rig for convenience. In my case, I adjust the length of the feedline
to minimize SWR at the rig end (tuned feeder0).

-Tony, K1KP,

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