-----Original Message-----
>From: Jim Brown <jim@audiosystemsgroup.com>
>Sent: Jun 5, 2008 2:06 PM
>To: "towertalk@contesting.com" <towertalk@contesting.com>
>Cc: Bob C- W5BIG <BobC@ArraySolutions.com>
>Subject: Re: [TowerTalk] Analyzer and AIM comments to Jim Lux
>
>
>The AIM analyzer received a very positive review last year from Rudy Severns,
>N6LF. BUT -- Jim Lux's comments are entirely on target. EVERY measurement
>system
>has limitations, even the most expensive. Most antenna analyers make
>reflection-
>based measurements -- that is, they measure S11. The accuracy of such
>measurements can be quite good when the impedance of the unknown is within
>about
>a 3:1 (or 1:3) ratio with the characteristic impedance of the measurement
>system.
>Accuracy degrades rapidly (and wildly) for the reasons Jim Lux cited if the
>unknown is very small or very large. HP has published at least one
>applications
>note that says exactly this.
Heck, they've published multiple ap notes on it, and folks have written entire
books, and doctoral dissertations on it. It's a very big deal in the microwave
area where you are pushing the state of the art. For instance, last year, I
had to design and build a system that could generate pulses at 35GHz that were
phase stable to hundredths of a degree over a 1 millisecond time span. The most
challenging part was figuring out how to prove that it actually met the spec.
(and, to be honest, I'm not sure we were actually able to do that.. it's at the
limits of what's measurable from a laws of physics standpoint)
Jim, W6RMK
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