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
