|Subject:||Re: [TowerTalk] step away from stepIR|
|From:||"Rob Atkinson, K5UJ" <email@example.com>|
|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
On Mon, 11 Jun 2007 15:00:18 -0400, "Tom Rauch" <firstname.lastname@example.org> wrote:
It isn't how an antenna really works that matters. The only thing that really matters is how we feel about it.
------------ REPLY FOLLOWS ------------
Spot on, as they say.
SteppIR is becoming a religion, much like Collins.
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