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Re: Question about TV on Ham satellite



Howard

An FM ATV colour signal requires approximately 40dB more signal power than
an SSB signal for a good picture, mainly due to the increased bandwidth
(18MHz/2.4kHz = 7500 times the bandwidth, = 39dB).

Looking at the downlink first; if the satellite had an on-board ATV
transmitter generating 20W EIRP = +43dBm :-

Free space path loss at 2.4GHz over 700km = 92.5 + 20 log(f[GHz]) + 20
log(d[km])

= 92.5 + 7.6 + 57 = 157 dB.

Assuming the Rx is noiseless, it will need approx  -115dBm for a 20dB S/N in
an 18MHz bandwidth. But it won't be noiseless; assume 1dB NF, so Rx power
must be -114dBm.   Power at the antenna from the satellite will be +43 - 157
= -114dBm - an unbelievable co-incidence, but there you go.

Therefore, the 'dish' needs no gain at all - 0dBi, which even I could
achieve.

If we now assume that the satellite has a 2dB NF, then the power required at
the satellite Rx would be ~-112dBm.

Path loss at 1.3GHz is a mere 152dB, i.e. 5dB better.  Satellite Rx has 6dB
gain, so power at satellite antenna should be ~-106dBm.  Therefore EIRP for
ground Tx must be 46dBm, = 40W = 10W Tx into 6dBi gain antenna.

N.B. - This assumes that there are no atmospheric losses, and that 20dB is
sufficient S/N for a decent FM ATV picture.

So where's the catch?  Several :-

1) On the downlink, to generate 20W EIRP on the satellite would require an
antenna with 13dBi gain from a 1W transmitter.  This is highly impractical
for an LEO, and omnis would be preferred as then there would be no real
problems trying to point them to earth, and a directional antenna would have
a very small footprint.

So, the Rx antenna on earth would need to have approx 10-13dBi, which is
still not difficult.  But then you would have to track the satellite in real
time, and it starts to get a bit more complicated.

2)  For the same reason as above, omnis would be preferred for the
satellite's uplink antennas.  So you would need that much more EIRP on Tx.

3) Bandwidth - for FM ATV, you need about 16-18MHz for colour with sound;
there is very little difference between NTSC and PAL in this respect.
That's a lot of bandwidth for a single channel.  Digital techniques can
reduce the bandwidth, but then the whole thing gets a lot more complicated,
and you lose the benefit of a simple 'bent pipe' concept.

If you could accept a worse picture quality, eg. P3 (noise on picutre)
instead of P5 (perfect), then you would need about 6-8dB less signal power,
which would make the aerials that bit smaller and tracking easier.

Hope that helps; I think the sums are correct but I had to make some pretty
fundamental assumptions.

Grant

G8UBN@amsat.org
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