One of the local repeater operators used an antenna at the top of a 100 ft
tower that got bent over during last winter storms. He put up a temporary
antenna at the tower base and is experiencing some really bad desense with the
low antenna.
He is using a GE Mastr II base station repeater and had reasonable operation
with little desense on the antenna 100 ft above the equipment. The antenna
only 15 ft or so above the equipment now and has the bad desense problem. It
would appear that the antenna is flooding the equipment with more RF than the
shielding can handle.
BTW, take a look at some of the previous posts on modifying a DB-224 by adding
a 2 inch extension to each end of each dipole to bring it down into the ham
band. The SWR does not go completely to 1:1, but does hit a minimum in the
middle of the 2 meter ham band. No change to the harness was required to move
the antenna frequency.
73 - Jim W5ZIT
--- On Tue, 9/1/09, tahrens301 <tahr...@swtexas.net> wrote:
From: tahrens301 <tahr...@swtexas.net>
Subject: [Repeater-Builder] Antenna SWR = Desense?
To: Repeater-Builder@yahoogroups.com
Date: Tuesday, September 1, 2009, 2:03 PM
Hi folks,
Just a bit of an update... got the 6 cavity Telewave
duplexer tweaked up - looks like it pretty much hit
the specs in the data sheet.
With a dummy load at the 'antenna' port, I used an
"iso-tee" to inject a signal at both the receiver
input, and between the antenna port & the dummy
load. With a weak signal, both places showed me that
there was no desense. Very weak signal would hold in
the repeater.
However, putting the system on the antenna (a 150-160 mhz
DB-224 100' horizontally & 10' vertically separated)
through a metal building fed with 7/8 heliax, there
seems to be no end to the desense!
The wattmeter shows 30 watts forward & 3 watts reflected
at the antenna port, if my math serves, it's less than 2:1.
Can the less than 1:1 match be the culprit?
Thanks,
Tim W5FN
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