James Miller G3RUH
Satellite working is one of the radio amateurs' most marginal methods of communication. The spacecraft performance is highly constrained by its power budget and antenna size, whilst most users are similarly bound by the amount of ground station hardware that can sensibly be mustered. The keyword is "marginal". Every ground station decibel squandered is a step nearer failure. Here are 10 easy ways to accelerate the process: 1. SQUINT. "Squint angle" is jargon mumbo-jumbo and can be ignored as such. WRONG! Squint, or Pointing Error is perhaps the single most important AO-10 user parameter. It is the angle that Oscar-10 sees your station from its antennas. A value of 30 degrees or more means you are well into its uplink and downlink sidelobes, a round trip reduction in performance of at least 10 db, plus added spin-fading. Squint is NOT minimised at apogee because the spacecraft's attitude is rarely parallel to the orbit major axis (Attitude Longitude ALON = 180). A rule of thumb is that squint is at its minimum around mean anomaly MA = ALON*512/360 - 128. The value of ALON is usually carried on the Beacon (q.v.). The proper calculation is done by all modern Tracking Software (q.v.). ELEVATION: When the satellites elevation is less than about 2 degrees above the horizon, reception of the downlink deteriorates rapidly. All you have to do here is increase your transmit power. This will now ensure you can hear yourself, and flatten the satellite simultaneously. 2. TRACKING Don't try to understand Oscar-10's orbital features. This way you will know nothing about its 19 day cycle, its daily motion across the sky, when the transponders come on and off, which parts of the world are in view and so on. With time, you can of course find these things out by trial and error. Then you could use the local orbital calendar, if there is one. You could build the AO-10 orbit model (O.N.44 Oct 1983 p.21 + updates) which in 30 seconds will tell you mean anomaly, geographic position, field of view and squint at a glance. If these old fashioned analogue methods are not to your taste, then there's a plethora of programs for calculators and home microcomputers. All you need to know is date/time, MA, range, elevation, azimuth and squint. If your program doesn't offer these minimum 6 parameters, find one that does. Additional numbers such sub-satellite latitude and longitude can be useful if you have a globe, and transponder Mode information saves time. Use your software as a tool; make it work for you. Plan ahead. Look back too. 3. LIFESTYLE By living in a city you can guarantee a much higher level of background noise, and so reduce the effectiveness of your receiving system. This noise is caused by industrial processes, switching apparatus, thousands of ignition systems, domestic appliances, thermostats, home computers and so on. In addition to this there can be many high level amateur signals in the rest of the 2 meter band busily cross-modulating each other in the front end of your receiver. The solution to this is not to increase your transmit power. Apart from moving out of town or giving up satellite operating, you can only really install better receiving antennas, a better front end, very high-Q cavity filters and turn off the shack computer. Also, make no attempt to adapt your habits to Oscar-10's. To maximise difficulties, try to operate only Saturday/Sunday from 10 a.m. to 11 p.m. Avoid using old-fashioned CW, even though it works fine on less than 50 watts e.i.r.p. most of the time. 4. MODE L BEACON Mode L is of course quite useless and can be safely ignored. FACT: the mode L beacon on 436.048 MHz offers you a spaceborne signal generator of accurately known power at a known distance. So it is ideal for testing any 70 cm antenna system. If your uplink antenna and feed is OK then the Mode L beacon should be a HUGE signal, virtually noiseless and quite free of spin modulation. If it is not, you've got serious problems. A rule of thumb; your 70 cm uplink antenna GAIN, including feeder losses is roughly the same as the 70 cm beacon signal-to-noise ratio in an SSB bandwidth. Assumed: Beacon power Pt = 10 watts, path 30,000 km, system noise temperature 1000 K, squint < 20 deg, elevation > 3 deg. 5. MODE B Beacon The 145.810 MHz beacon has no function other than to transmit a boring CW message, occasional RTTY and a gurgling called PSK (peculiar shift keying). FACT: the mode B beacon on 145.810 MHz offers you a spaceborne signal generator of accurately known power at a known distance. So it is ideal for testing any 2 meter antenna system. If your downlink system is OK then at squint angles (q.v.) less than 25 degrees, elevation above 2 degrees and range 40,000 km the Mode B beacon should be a splendid signal, virtually noiseless and quite free of spin modulation. If it is not, then you won't hear much of the transponder signals, and you MUST ascertain why. Checklist: Azimuth OK? Elevation OK and over 2 deg? Squint under 20 degrees? Spacecraft Hi-gain antenna on? (Beacon messages). If the three answers are all yes and the beacon sounds lousy, then your receive system is suspect. Is there a higher than usual external noise level? If not then it's the antenna/preamp/feeder/RX. 6. GASFETs A 2 meter GasFet pre-amplifier guarantees success. WRONG; a Gasfet pre-amp, correctly installed at the antenna end of the feeder will do wonders in overcoming feeder loss, and the generally high noise figures of "black boxes". IF the feeder loss (q.v.) is only a db or so then the pre-amp can be installed at the RX end. A GasFet pre-amp will also empty your pocket. The sky noise at 145 MHz is many hundreds of degrees K, so Gasfet noise figures of 0.5 db (35 K) are pointlessly low. A BF981 job giving 1 db (75 K) will perform just as well at a third of the cost. Spend the change on some new feeders. 7. FEEDERS Nothing to worry about here; that new stuff picked up cheap at a junk sale will do fine. And so it will - PROVIDED you check its loss first. If you find more than 1 db loss on the downlink cable and have no masthead pre-amp please refrain from using AO-10 until you've sorted it out. The effect of uplink feeder loss is ... well, can you really afford 3 db wastage here? To measure cable loss you do NOT look up the manufacturer's db/metre specification and multiply by the length. You do it by measuring the actual bit you intend to use, including connectors. One way is to attach a dummy load to the far end, send down power, and measure how much arrives at the load. This is fine provided you trust your power meter, the load is proven 50 ohms at the relevant frequency, and that moving the meter from one end to the other introduces no extraneous effects. Also, try reversing the power meter. If in doubt try another instrument. Bird Thruline owners can look smug. An alternative method is to open (or short) circuit the far end, squirt a few indicated watts into the near end and measure forward and reflected power. Their ratio is twice the cable attenuation - because the energy goes to the far end and back so is attenuated twice. Pitfalls; power meter may be innaccurate at high SWR; method doesn't indicate internal open or short circuits in the cable. 8. POLARISATION Don't trouble with circular polarisation. After all, linear polarisation works. Can you REALLY afford the 6 db round-trip performance loss? But don't assume that the penalty is automatically forgone by using "circular polarisation". It's easy to lose that 6 db again in phasing harnesses, due to mismatches, unproven components, poor workmanship etc., particularly at 435 MHz. 9. DOWNLINK ANTENNA. Your ordinary terrestrial antenna will be OK. WRONG. While your satellite system may be OK for terrestrial use, the converse is not necesarily true. The space communication antenna needs to be in tip-top condition (not five years old and falling to bits) with as much gain as you can afford, and preferably circularly polarised (q.v.). Azimuth rotation (and in general, elevation) rotation are essential. The antenna does not need to be as high as possible; it should be as low as is consistent with seeing over the horizon. This minimises local signal pickup. Antennas near ground level are also less likely to get destroyed by the next hurricane. Experience shows that the absolute minimum acceptable 2 meter RX antenna is an 8 XY yagi or equivalent. With this you should be able to hear the beacon (q.v.) well at optimum squint (q.v.). Ideally however you should try and have twice as much antenna as this, to allow a good margin in hand (10 XY, 2x17 ele etc). Do not stack two circularly polarised antennas closer than 0.8*SQR(IsotropicGain/pi) wavelengths otherwise you won't realise your 3 db gain; and use a decent combiner. 10. ALLIGATORS Of course the real reason you can't work Oscar-10 is called alligators, and we all know what they are. An alligator is "all mouth and no ears". Have you noticed how "mouth" and "no ears" is ambiguously specified. Some of the mouths do not even use Oscar-10; few have listened objectively; most simply can't hear. If you have explored all the criteria described above, you ARE probably enjoying our marvellous satellite. If you have failed on any of the above counts, then you've only yourself to blame. You're a Rotagilla.
Feedback on these pages to Webmaster. Feedback on the article should be sent to James Miller
Created: 1994 Nov 13 -- Last modified: 2005 Oct 29