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Amsat-UK's Oscar News, 1992 Oct No. 97 p20-22
Amsat-DL Journal Dec 1992 Nr4. Jg.19
Amsat Journal (USA) Vol15 No. 5, 1992 Sep/Oct, p.14-15.
Amsat-VK Newsletter (VK), No. 90, September 1992
CQ-DL 9/93 1993 September, p.614-617.
73 Amateur Radio Today (USA), Issue #492, 2001 Nov, p.50-51

Mode S - Tomorrow's Downlink?


James Miller G3RUH

Government Health Warning; facts can seriously damage your health. This article contains facts.


I have never been given much to arguing matters of a speculative nature. If I don't know about something I keep quiet. If I do know I say what I know. I will tenaciously defend any proposition that is a demonstrable fact, that can be supported and extended by reasonable analysis. So......

A Modest Proposal

In this article I want to make a case for abandoning 145 MHz Phase III satellite downlinks. I will show why S-band (2400 MHz) is an infinitely better proposition. I will dispel the myths that S-band requires special knowledge, enormous expense, huge dishes and a "techie" mentality. These are totally untrue. S-band in practice is far simpler, cheaper and far more efficient than 145 MHz. I will not make my case with airy-fairy hypotheses. I will do it from practical experience. Show and tell.

Please don't read this article with a cynical smile and a "Oh here's just another wire-head evangelist on his soap box showing off". Read it and think about it. I have seen the future, and it works! Now it's your turn.

Brain ON

Time for a thought experiment. Consider this; Oscar-13 carries a mode-S transponder plus beacon. The transmitter outputs 1 watt to a 5 turn helix. That makes an e.i.r.p. of 10 watts. This is peanut power. Given that, what size antenna do you think is needed to receive the beacon comfortably? As comfortably as at 145 MHz. A 2 metre dish? 4 metres? Several 55 element loop yagis? No idea?

Skip the arithmetic until later. Will you believe that the answer is a puny little two-foot diameter dish! Yes, it surprised me too. So to prove it, I built a 60cm dish antenna from a spun aluminium lampshade I picked up when a local furniture store closed down. Cost me a couple of quid. OK, four quid; I bought two of them.

Sure enough, the Oscar-13 mode-S beacon was at least as strong as the 145 MHz beacon. And the transponder signals effortless copy, noisefloor included. (400 watts e.i.r.p. uplink saturates the AO-13 transponder). And will you also believe me when I tell you that this little dish still worked excellently indoors, through a closed window?

A Straight Choice

Now answer this question honestly. If you had to choose between your huge 145 MHz downlink antenna, and a tiny 60cm lightweight, compact, suitcase sized, no-tune, elegant dish that you could genuinely hitch to a balcony; which would you have? No contest, is there?

What Noise Annoys?

What's the reason for this surprising result? The answer is simple. Noise.

145 MHz is noisy. In some places it's so noisy as to be virtually unusable. I don't think there is much disagreement about this. Noise arises from almost everything electrical you can think of. Transport, appliances, weather, Sun, sky, QRM, splatter, tele-switches, computers; the list is endless. And it's getting worse every year. I haven't even included noise of the receiver as it's usually swamped by the forgoing.

A Measure of 145 MHz Noise

To quantify noise we use a measure called noise temperature. Higher is worse. It's the temperature of a 50 ohm resistor that, if connected to the radio instead of the antenna, would make the same racket. The noise temperature on 145 MHz is of the order of 1000K-1500K. In some conditions it may be less, but usually it's a lot more. Let's just use 1200K for now.

By the way, K means kelvins. Zero kelvin is total silence, water freezes at 273K, boils at 373K and 300K is a nice day.

The important thing to grasp is that there is nothing you can do about this noise. You cannot reduce it. Even with a hypothetical zero-noise pre-amp, you'll still have 1200K of noise.

2.4 GHz Noise is Rather Different

Now lets look at noise on 2400 MHz. Sky noise nil. Environmental noise nil. Cable losses nil (no cable). FT736R noise nil; there's a pre-amp/converter at the antenna. Antenna noise almost nil; just some pick-up of the warm Earth from the sidelobes, 20K maybe.

The only real source of noise is the 2400 MHz to 144 MHz down-converter. And this noise is totally under the control of the equipment designer.

A converter consists of a low noise pre-amplifier stage followed by a mixer. The mixer is driven by a 2256 MHz local oscillator, usually a 94 MHz crystal with x24 multiplier. Very simple. Very cheap.

The noise level of a typical low cost converter is about 100K. Let's assume this, although lower is achievable and add 20K for antenna sidelobes. That makes a total of 120K of noise for 2400 MHz. Guaranteed, everytime, everywhere, worst case.

The Big Payoff

So, where does that get us? We have established that 145 MHz noise = 1200K, and on 2.4 GHz it's 120K. That's a factor of 10x quieter at least.

It means that for a given spacecraft e.i.r.p., your ground station antenna on S-band can be 10 times smaller than for the 2 metre band.

Why? Because less noise means less signal needed. Received signal strength is directly proportional to the capture area of the antenna. Capture area is proportional to physical size.

That 10 times smaller means less mechanical engineering, less windage, less cost, less maintenance, less environmental impact, greater portability and so on and on.

Just as important, the noise level is controllable by the user (you), not by external influences beyond your control. Isn't that just what we want from a satellite communications system? I think so.

Exploding 2.4 GHz Myths

1. S-band is for "techies"

Complex, a mystery, deep stuff. Three days before writing this article I knew nothing about 2.4 GHz. I still know nothing. I don't need to know. And I don't know much about 145 MHz either. Do you? Do you honestly?

I made a small dish from materials I had to hand using no more than simple dish formulas available from the ARRL Handbook and elsewhere. I made a 2 1/4 turn helix feed from a bit of H100 solid coax inner, wound round an 40mm former (socket spanner from my car toolkit). I soldered it to the pip of a N-type panel connector, which was mounted on the 125x125mm reflector of 1.6mm aluminium sheet. I bought an SSB Electronic UEK-13 S-band converter from a regular advertiser, coupled it directly to the N-type, and fixed the whole assembly to a 1/2" square aluminium boom with two elastic bands! The boom ran through the hub of the "lampshade" dish, with some overhang for mounting purposes. The RG174 coax (3mm dia) and 12 volt feed ran neatly back inside the hollow boom.

"Scrap" it may be, but it looks professional enough to me. The most technical thing I used was a hand drill. Or was it a hack saw? Took 5 hours to build from a heap of bits to being on the pole ready to test with AO-13. And I was designing as I went. I did no electrical alignment of the helix feed. Like most, I have no test facilities for 2.4 GHz; probably never will have.

If this story labels me a "techie", heaven help blacksmiths!

2. S-band is Expensive
Successful 145 MHz downlinks require a large antenna and a pre-amplifier. If the system is used for transmitting, as is usually the case, it also needs low loss coax.

An S-band converter costs about the same as a good 2m pre-amp. The dish I described cost almost nothing. A commercial version (if it were available) would doubtless cost no more than a KLM14C. Coax can be RG58 or smaller.

So at commercial rates there is probably no difference. But build your own antenna which, as I have shown, is almost too easy, and S-band is a lot cheaper.

3. S-band requires a Huge Dish
As already explained, this is not the case. The size of a receive antenna is dictated by the size of signal you want to receive. This is dictated by what signal to noise ratio you want. This is constrained by the noise level. So, let's do an exercise for a very small lampshade dish and the "120K" converter mentioned. Fear not; what follows is arithmetic only! Calculators out now.

Calculator ON

Suppose the satellite transmits Pt watts. This spreads out in all directions over an imaginary sphere of radius R, typically 40,000 km for AO-13. The energy is intercepted on this huge sphere by a tiny antenna of effective diameter D, say 40cm for a typical 60cm dish. So the power it picks up, Pr is just the ratio of antenna area to the sphere area, that is:

Pr = Pt * (pi D2/4) / (4 pi R2)

If you plug in the numbers for AO-13 with Pt =10 watts, you get Pr = 6.3 x 10-17 watts.

Now let's calculate the noise power Pn received. This is easily found from the standard noise formula:

Pn = K T B


K = 1.38 x 10-23 w/Hz/K, Boltzmann's constant

T is the noise temperature in kelvins which for this article we took to be 120K on 2.4 GHz.

B is the receiver bandwidth; assume 2.7 kHz for a typical SSB receiver.

Plug in the three numbers, and the received noise power is
Pn = 4.5 x 10-18 watts.

So the signal to noise ratio, SNR = Pr/Pn. Dividing the signal and noise powers just calculated you get, for AO-13's beacon at 40,000 km on a 60cm dish in an SSB bandwidth SNR = 14 or 11.4 dB.

And believe me, it IS just that; I measured it. No tricks, no bull. Just simple arithmetic. So much for the "big dish" theory. A modest antenna is all that's required.

Outline transponder budget

You can extend these figures to a transponder downlink design by scaling.

Assume 40 users (100 kHz bandwidth), a 20 dB SNR (6 dB more) and you need 40 x 4 = 160 times as much satellite e.i.r.p. That makes 40 real watts to a 16 dB gain spacecraft antenna, say a 15 turn helix or 25 cm dish. Extremely practical. And remember, the ground station is still a 60 cm dish. Try a bit of number juggling to explore more options.

The ability to realise controlled 2.4 GHz gain on the satellite with physically small antennas, a task that's virtually impossible at 145 MHz, means we have the ability to design a system free from the constraints of the Ariane rocket third stage envelope. That's a priceless advantage.

More Bunkum

By the way I hope you noticed that this systems analysis failed to speak of "path loss", "noise figure" and "antenna gain"!

Nor should it have; these terms are just obscurations. There is no such thing as "path loss"; space is loss free. The term was invented by nomograph manufacturers as a convenient stepping stone on their gadgets.

"Noise figure" (in dB) is another meaningless number invented by pre-amp manufacturers to increase sales. Daft! If they used noise temperatures instead they would sell rather more, because those numbers are far more impressive. They are certainly more intuitive.

"Gain" is useful only when talking about transmitting - or if you are trying to sell antennas. For receivers, capture area is a direct measure and again, far more meaningful. For reference, G = 4 pi Ae/L2.

Disadvantages of S-band downlinks

So far as I have been able to discover, the only impediment to S-band signals is tree foliage. This seems to me to be a rather trivial drawback. I should like to hear if there is anything else technically negative. Perhaps some of the real techies (not the armchair ones) who have all the answers, but never tell, could come forward!

Disadvantages of 145 MHz downlinks

I don't want to labour the point! The honest truth is that AO-13's 145 MHz downlink does have serious shortcomings. Noise is high, and big antennas and highest quality receive technique are required. Many fail at this hurdle and give up. Others reach for their wallet. It is nothing like as easy as it was envisaged all those years ago. It doesn't have to be like this.

Phase IIID

We don't have to repeat the mistake with P3D. Cranking up the 145 MHz downlink power is no solution. It's a short term delusory crutch that will simply hobble us for another decade.

Isn't it time we took a careful, unbiased look at the options? And surely, before jumping to contusions, budding analysts should at least try AO-13 mode-S?

I promise you will be astounded at its simplicity and at its performance. You don't have to habitually use it for ever after. Indeed its experimental hard limiting design won't support hoards of users. Just try it, so help me! The equipment can be put by for the future, or lent to others. It won't be wasted. Are you willing to stand up and be counted?


(Composed 1992 Sep 09)

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Page created: 1995 Jan 15 -- Last modified: 2005 Oct 30