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RE: S-band desense while UHF TX

This is my $0.01 worth.  I am running a Drake with simple mods and a DEM
13ULNA into my BBQ dish.  It is bolted about 2' away from my 435MHz antenna
(used to be a foot, but I move it) on my cross boom (non-conductive).  I
have the coax from the 435 antenna running right beside the coax from the
dish and the coax out of the Drake all the way to the shack.  With the 435
amplifier on and running around 100W I have no desensing problems at all.

Is it possible that the guys having desense problems may be due to a
radiating coax?  Did you guys check your SWR at the antenna and not the
shack?  Are your connectors to all devices all properly terminated and
TIGHT?  Is the coax new or old and possibly waterlogged?  I am just
blue-skying here and maybe spark a synapse in someone who knows more about
this than I (it wouldn't take much ;^) ).

Having said this, I am sure to have desense problems now....


> -----Original Message-----
> From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
> Behalf Of Greg D.
> Sent: Wednesday, July 25, 2001 4:15 PM
> To: amsat-bb@AMSAT.Org
> Subject:
> Hi folks,
> Last night was my first opportunity to test my updated antenna/feed
> in with the AO-40 Transponder.  MA 13, at 15 degrees elevation.
> With such a low MA, I was hoping for lots of strong signals, and
> an easy time getting into the bird myself.  The results were mixed.
> The beacon, as expected was very strong, although I didn't hear
> anything until the bird was up around 10 degrees elevation.  Trees
> and hills in the way, I expect.
> As others have discovered, I now have a mild interaction between
> the UHF uplink and the S-band downlink.  Transmitting on UHF gives
> a rise in the noise level on the downlink.  It's not real severe,
> but it was enough to mask my own SSB uplink, as that's never been
> very strong.  Since I didn't have this problem before, it must be
> due to moving the converter & preamp forward, from behind the dish
> to out in front.  The distance from the UHF antenna is smaller now,
> and towards the front of the antenna.  Also, the UHF coax passes
> right in front of the feed helix...  Probably not the best idea.  My
> plan is to first try routing the UHF feed around the back of the dish,
> then if that doesn't help, move the whole S-band contraption over to
> the 2 meter side of the arm.  If you want to see how it is now, look
> at http://www.jps.net/gregd/hamsite.htm
> But last night also brought some disappointing results in how the
> transponder sounded - very raspy.  My own CW - I can explain with
> the above.  But I also heard a few stations call CQ, a very few.
> One from JH1, and a couple from North America.  They didn't sound
> very clean.  There were also a few spots where there was some sort of
> digital/analog mixed wierdness that almost sounded like some of the
> digital stuff (non-ham) on HF, with some warbly tones and occasional
> CW mixed in.  Does anyone know what I was hearing?  Was there any
> other experimentation going on that might have affected the sound
> quality on the transponder, or is it just me?
> Since the bird was rising in the West on this pass, I am hoping that
> the lack of transponder traffic during the short time I was listening
> was due to there not being very many stations out over the ocean...
> Will try again in a couple of days.
> 73s,
> Greg  KO6TH
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