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Amsat-UK's Oscar News, 1993 Oct No.103 p20-21
Entitled: "Small iS beSteSt"
Satellite Operator No. 33, June 1993. update ?
The Amsat Journal (USA) Vol 16 No.4, Jul/Aug 1993. Update Nov/Dec 93
Amsat-VK Newsletter No. 99, 1993 June, update No. 102, 1993 Oct

A 16 Turn S-Band Helix


James Miller G3RUH


This design represents the smallest practical antenna for Oscar-13 mode-S. Perfect beacon reception at all times is possible, and acceptable SSB even at 43,000 km range. CW is effortless. Tools required; ruler, hacksaw, file and hand-drill. Test equipment, voltmeter. Skill level, nil. Just do it!

 Testing the helix

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!

Own Goal

Readers will be familiar with my recent advocacy of a 60cm dish for Oscar-13 mode-S reception [1,2]. Since my dish is frequently down for demos, photos, talks etc, I decided to make another antenna for permanent installation, and particularly with the demise of mode-L, an interference free telemetry link for my command AO-13 role.

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.

To work

One Saturday afternoon, I got out my techie's armoury; ruler, hacksaw, file, scissors, hand drill etc and built a 16 turn S-band helix. I used no drawings. Made it up as I went along.

Construction is exactly the same as per my dish article in Oscar News [2], 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.

 S-band Helix Reflector

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.


I fixed a camera adaptor to the boom, clipped the antenna to my tripod and set up shop on the balcony. Coupled on the lo-noise pre-amp and converter, wired to an FT290R on the balcony and a FT736R in the shack, aimed vaguely in the right direction and waited for mode-S.

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.

Science fact

Next day I went out into the garden well clear of buildings, trees etc and measured the Sun noise power increase (see [2]). This was 1.16:1 (0.64 dB), equivalent to a G/T ratio of about 0.3 K-1, and compares with the G/T of 0.9 K-1 I obtain with the 60 cm dish.

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.


I had to wait a couple of months to test it for real at apogee range 43,000 km. This range of course also means no elevation, and that means some 2-3 dB of extra thermal noise from the ground, the worst case situation. No problem. The measured beacon SNR was exactly as predicted (Eb/No = 8), and most telemetry blocks decoded correctly. Next day the elevation was 16� and the SNR increased to about Eb/No = 12 with perfect decoding. Even at the horizon, SSB stations were easily copied, and the transponder noise floor was audible too.

Small is...

But goodness, this antenna is small. About the size of my forearm. Just 450 mm (18") long, 40 mm (1.6") diameter, and the reflector is 125x125 mm (5"x5"). Goes easily into a small suitcase. It is so small that, unlike 2 metre dishes and similar overkill, it goes unnoticed on the mast and has negligible windload.

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!


It's also cheap. The most expensive part is the N-type connector. The rest is basically scrap metal you can find anywhere. It could be mass produced for well under $50 sale price.

Bottom line

Given S-band's easily demonstrable performance advantages, do we really want a 145 MHz downlink on P3D? Not a lot of point, is there? Just makes life harder for newcomers.


1. Miller J.R.; Mode S - Tomorrow's Downlink?, Oscar News No. 97, October 1992. Also The Amsat Journal (USA), September/October 1992, and Amsat-DL Journal, December 1992.

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: 2023 Apr 13