[amsat-bb] Mode S advice
kd0ar at sbcglobal.net
Sat Oct 4 16:08:27 PDT 2008
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.
Chief Engineer, Forever Broadcasting
New Castle PA
WKST WJST WWGY
Chapter Chair, SBE-122
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