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 _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
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