At 3/11/2005 07:45 AM, you wrote:
>I would not use a mag mount for any thing.
>What would you gain by having a repeater
>using a mobile antenna that you could not
>do the same thing as just working simplex?
Actually, the best reason for not using a mag mount as a repeater antenna
is the feedline attached to it. I have yet to find a mag mount that uses
RG-223 or some other coax with silver-plated braid. Now if one were to
replace the feed with the proper coax, it might work.
This leads me to my recent experiences with copper or tin-plate-braided
coax in duplex service. I've serviced 2 repeaters in the past two
months. One did not have a desense problem at the time but had one of
those crossband diplexers with pigtail leads on the output of the (in-band)
duplexer. After reading all the recent postings on the subject of those
leads being a problem, I tested for desense by moving the pigtails around
while putting a weak signal into the system. Sure enough the scratchies
started right in. I removed the diplexer & replaced it with an identical
model that has no pigtails, then reran the test while moving all the other
cables (all RG-214). No desense! It's interesting to note that the "bad"
diplexer spent its entire life indoors, so it's not clear if any oxidation
is required for the coax to lose its linearity, or if sufficient oxidation
for failure occurs regardless of the environment.
The other repeater had a desense problem was was thought to be caused by
the rusty pole & tower that its GP9 antenna was mounted to. A thorough
shake & pounding of the pole revealed no effect on the desense (no static,
steady desense level). Checked the jumper between the hardline & GP9; no
problem there. Recalling my above experience with the crossband duplexer
pigtails, I headed back inside the building to look at the cabinet
cabling. Sure enough, there was about 8 ft. of RG-213 connecting the
duplexer to the hardline. Picked it up & the desense level went up by 20
dB. Replaced that with a shorter length of RG-214. Again, problem cured!
What I've learned from all this is that any transmission line that carries
transmit & receive signals simultaneously (duplex service) must use either
solid metal (Heliax) or silver-plated braid as the shielding material. The
interconnecting jumpers between the duplexer, RX & TX are far less
critical, since any low-level noise generated in the TX jumper will be
filtered out by the duplexer & there is no substantial TX signal (-40 dBm
or lower) in the RX jumper.
Bob NO6B
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/Repeater-Builder/
<*> To unsubscribe from this group, send an email to:
[EMAIL PROTECTED]
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
