[amsat-bb] Re: Mode S advice

Greg D. ko6th_greg at hotmail.com
Sat Oct 4 16:50:42 PDT 2008

Hi Michael,

Circular polarization does wonders in reducing QSB due to the rotation of the satellite, but it's not a cure-all.  I run circular on both 1268 and 2401, and still experience some fairly deep fades from time to time.

My 2401 receive capability was created for AO-40, and involves a BBQ grill that I lined with metal window screening, and a 3 3/4 turn helix for the feed, with a Khune preamp mounted directly to it.  I use a Drake 2880 down converter modified to put the IF at 301 mHz, where my ICOM R-7000 all-mode receiver can pick it up.  I was able to work AO-40 at apogee with this configuration, thought it was a bit of a strain on the ears.  It's clearly overkill for AO-51, though I still don't hear needle-pinning signals.  Never have figured out why...

For 1268, I experimented with both a yagi and a helix.  The helix did seem to perform better, both in the QSB department as well as overall signal strength.  I have only used the antennas with satellites, as there is nothing terrestrial within line of sight on that band.  The only repeater that I might have been able to hit has been off the air for a few years.

Pictures at:  http://home.wavecable.com/~ko6th

Good luck,

Greg  KO6TH

> Date: Sat, 4 Oct 2008 16:08:27 -0700
> From: kd0ar at sbcglobal.net
> To: AMSAT-BB at 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
> 814-671-0666
> Chapter Chair, SBE-122
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