Bob;
Carrying this thought a little farther leads me to wonder why the harness
of the DB 224 antenna made of VB83 and VB11 for example would not be a
problem also. Do you suppose it is the vapor block impregnation
that prevents microscopic movement?
Gran K6RIF
At 13:22 3/11/2005, you wrote:
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
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