Topband: [TowerTalk] RX 4 square
jim at audiosystemsgroup.com
Sat Sep 1 13:38:51 EDT 2007
On Sat, 01 Sep 2007 11:10:22 -0400, Roger D Johnson wrote:
>If this is
>true, then the phase shift through the antenna matching system,
>whether it be active or passive, would be irrelevant as long as
>all were stable and equal.
"Equal" is where the bodies are buried. To understand this,
remember that nulls in a pattern (that is, signal rejection) is
the result of the cancellation of signals that are equal and 180
degrees out of phase with each other. The DEPTH of that
cancellation is VERY sensitive to the PRECISION of that equality
AND the 180 degrees phase difference. If the amplitude is only
amplitude, there will be less rejection. If only the phase is
shifted, the nulls will move in azimuth.
It's also a stretch to assume that a receiver will terminate a
line in its characteristic impedance. Equipment designers are
sometimes careless about this. Try connecting an antenna analyzer
that measures R+jX to your receiver and see what you measure over
a wide range of frequencies. You may be surprised.
N6RK has outlined a much less expensive method of building an RX
array that does some pretty good engineering to avoid these bear
traps, and it does include good impedance matching. Mini-Circuits
(a product he recommends) is a very good company based in Brooklyn
that makes a wide range of both passive and active RF products.
I've used their 1x4 splitters for many years for wireless mic
Jim Brown K9YC
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