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Re: Mode S advice



Michael,
I would recommend a small 18" Dish with a patch or 3 turn helix feed.
Helixes at 2.4 are somewhat tricky and long helixes are effected by the
plastic used to hold the turns. Self supporting helixes are difficult to
make beyond 3-4 turns. Feed should be opposite rotation however I recall
AO-51 is linear polarization on 2.4 GHz.

Art

-----Original Message-----
From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On
Behalf Of Michael Heim
Sent: Saturday, October 04, 2008 4:08 PM
To: AMSAT-BB@amsat.org
Subject: [amsat-bb] Mode S advice

Last month I was experimenting with mode S receive antennas, and I think I
posted some questions on here as to what you guys who are on mode S use.
Although I have operated microwave, I have never done so via the satellites.


I have 2 options and both appear to work about equally.  one involves a
linear yagi, the other a helix, RHCP.  The last time the bird was in mode S
analog, it used mode L for the uplink.  I noticed, while listening in using
either antenna, a noticeable amount of deep fades on the bird.  I'm not sure
if it was caused by reflections nearby, or uplink fades on 1269 mhz.  Both
of these antennas were mounted on a camera tripod about 4 feet off the
ground, and there was some metal within about 25 feet or so, which is
unavoidable.  The antennas will be placed about 15 feet above the ground,
and the metal in the vicinity will be below the antenna.

So, your advice... should I go with a 16 dbi linear yagi, commercially made
for the MMTS band, or the RHCP homebrew 12 turn helix?  I am having trouble
getting much longer than 12 turns, winding the helix evenly.  Is there
really an advantage in using either?  I guess I'm looking for whatever will
do the best as far as fading goes.  I am impressed, as the signals were
quite strong while using only 2 foot long antennas.

By the way, I am building a 1269 TX converter from scratch.  I happened upon
a brick oscillator that I recrystalled for the proper LO for a 144 IF.  The
mode S IF is 123 mhz, so there should be no IF leakage problems.

 Michael Heim
Chief Engineer, Forever Broadcasting
New Castle PA
WKST  WJST  WWGY
814-671-0666
Chapter Chair, SBE-122
ARS KD0AR
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