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RE: Re: Question about TV on Ham satellite



Hello Grant

Thanks a lot for taking the time to explain that. Great to have a worked
example.

About those catches:

> 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.

I was thinking along the lines of 5W and 6dBic. A patch or short helix on
the side of the s/c facing the earth (assuming a gravity gradient boom
fitted or similar stabilising device).

> 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.

Again I was thinking along the lines of 6dBic on the face pointing toward
the earth.

> 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.

There's a restriction in the bandplan too on the uplink on 23cm of 10MHz.

> 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.

Yes. I noticed some interesting notes at
http://www.g1mfg.com/website/mods.html#Smeter which (if you scroll down a
bit) seem to suggest [at least for these devices] that you need a bit more
signal for a decent picture. eg:

-120dBm No discernable signal
-100dBm Noisy signal
 -80dBm P5

Do you think that these devices are just a little insensitive?

73 Howard G6LVB




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