James Miller G3RUH
Photo 1. A complete AO-13 S-band received system. Perfect beacon reception at all times is possible, and acceptable SSB even at 43,000 km range. CW is effortless. The pre-amplifier (NF = 0.6 dB, 43K) is coupled directly to the feed point and drives an SSB Electronic UEK-13-P3C converter (not visible). The antenna is shown mounted on a cheap camera tripod. Both are less than 50 cm long and the entire system goes easily into a small suitcase. Compare this with typical 145 MHz downlink equipment!
My goal was "the smallest possible antenna that will let me decode the PSK telemetry beacon error free at apogee with a low noise (0.6db) pre-amp".
Having measured the beacon SNR at apogee on a 60cm dish, I knew how much I could afford to throw away with a smaller antenna. Answer about 4.5 dB; so I determined that a 16 turn helix should be adequate. In fact, since the dish is about 3 dB overkill anyway, it seemed not unlikely that the little helix antenna would work on SSB, and certainly CW. And so it turned out to be.
Construction is exactly the same as per my dish article in Oscar News , except there are more turns, and of course it is right handed. They are supported every 5th turn (starting at turn 3/4) with three PTFE spacers screwed to the boom and drilled through for the helix. The matching strip spacing is slightly increased; 2 mm at the start, 8 mm at the end.
Photo 2. Close-up details of the 16 turn helix antenna for 2.4 GHz by G3RUH. Gain is 15.5 dBic, RHCP. The boom is 1/2 inch square aluminium stock, the helix is 1/8-inch copper wire (inner conductor of coax cable) and the spacers are PTFE (Teflon) drilled, tapped and screwed to the boom. The reflector is 5 x 5 inches and attaches by angle brackets to the boom. Matching to the 50-ohm N-connector is by a brass strip fixed to the first 1/4 turn; see text.
Testing a new antenna the first time is always exciting. Mode BS was on, and instantly there were the usual mode-B stations buzzing away. Next came mode-S exclusive, and the SSB was loud and clear! Noise-floor increase about 0.3 dB.
Finally the acid test, the beacon SNR. I went into the shack and checked the telemetry. Perfect, totally error free. The range was only 30,000 km, but a quick SNR check at the PSK decoder confirmed the margin would be just enough to cover apogee too.
Thus the gain is down about 1/3 or -4.8 dB, giving the helix a gain of say 20 - 4.8 = 15.2 dBic, roughly as expected.
In fact since the target for AO-13 mode-S reception is a system G/T of 0.45 K-1, and the figure of 0.3 K-1 achieved for the 16 turn helix isn't much under this, it's not surprising it performs acceptably on the transponder.
Its capture area is only 440 cm2, compared with a whopping 7 square metres for my KLM14-C 145 MHz system. That's 160:1 in size!
2. Miller J.R.; A 60 cm S-Band Dish Antenna, Oscar News No. 100, April 1993. Also: The Amsat Journal (USA) Vol 16 No. 2, March/April 1993. Also: Amsat-DL Journal, Jg. 20, No. 2, Jun/Aug 1993. CQ-DL 1993 Sept.
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Created: 1995 Jan 15 -- Last modified: 2005 Oct 31