The most gain, best F/B and best SWR do not all occur with the same element
settings on a yagi antenna. There is a compromise setting and you can set
it for whatever is most important to you - usually gain. If you have a
reasonably good feedline there is very little loss with even a 2:1 SWR.
However, SWR is easier to measure than F/B or gain and I guess having no
reflected power makes some people think that their antenna is working great.
Subject: Re: [TowerTalk] step away from stepIR
From: "Rob Atkinson, K5UJ" <firstname.lastname@example.org>
Date: Tue, 12 Jun 2007 14:42:41 +0000
Well, I must say, I have had some questions about the steppIR fluid motion
antennas since I came back from Dayton.
It seems that one of the overriding draws to the Fluid Motion brand antennas
is that you can adjust the element lengths so that on any frequency above
the low limit, you can achieve zero reflected power. There are two things
about this that bother me, but I do not know if they matter or not since I
have never owned one of these antennas.
It's safe to assume that the feedline in a typical installation is at least
60 or 70 feet long, probably longer in most cases. This is usually a
significant length in terms of wavelength above 30 m. I also assume that
reflected power, or vswr, is being measured in the shack. One thing that
bothers me about this is that this doesn't guarantee that the Z at the
feedpoint is actually 50 j0. It's probably close enough on the lower bands
but on 10 or 12 meters, where the feed is likely to be mulitple lambda in
length, I wonder if using the antenna as a tuner gets the user into
something like the "my feedline tunes my antenna!" type of situation i.e.
the meter in the shack may show true unity along the line, or maybe you just
happen to have adjusted the antenna so that the length of feedline you are
using and your frequency result in a nice looking match at the shack but the
feedpoint, if you could measure there, would tell a different story?
The other question i have is, how do you know the element length that gives
you a match to 50 ohm feedline, is the antenna dimension that gives you
optimum gain and F/B? Is the element length for 50 ohm Z always the length
for best performance?
I essentially question giving no reflected power the highest priority which
seems to be what happens with these antennas. Or maybe most of the time you
get close enough to having your cake and eating it too so that my concerns
are minor issues?
rob / k5uj
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