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Re: AO-40 reception question




 Bill Howell wrote:

> When AO-40 was first turned on for amateur use, I was able to hear
> others' downlinks
> well enough for an SSB QSO (sigs were in the noise, but useable).
>
> Lately, it seems, (using the same equipment: Meyers dish, unmodified
> 3733, ft-847),
> all the downlinks seemed to be down deeper in the noise and not
> strong enough for a QSO.
----- some text snipped -----

I found that since AO-40' attitude has been moved to around 0/2 +- the parts
of the orbit before and after perigee ie. MA200 to MA50 have become very
dissatisfactory especially for SSB use. During this phase the main lobe of
the S2 Helix antenna is pointing away from earth too much for reasonably
good S/N signals. There are some squints where the sidelobes lift the
signals out of the noise somewhat, but generally one has to wait until after
the intermission at MA64 before S/N improves.

At what MA's have you made your observations?

Also when the sat is moving towards apogee stations on the eastern side of
the footprint are advantaged, from apogee towards perigee the western side
of the footprint has the edge. It is easy to envisage that with a polar view
of earth and the satellites orbit.

> I get S-5 background noise (away from the beacon freq), on the '847,
> and the beacon itself is at S-6.

To keep the noise at a minimum one should use an attenuator between
downconverter and receiver. Also some attenuation before the downconverter
may be useful if a preamp with high gain is used. Generally it is best to
only increase the RF signals enough so that the required MDS level is
achieved. Any more gain than that required is wasted and only increases the
undesired noise. It is better to have ample gain in the audio stages of your
receiver and only just enough RF gain to get the best S/N ratio.

Lets hope for the successful deployment of the momentum wheels, then we
should get the maximum availability of satellite use.

73! Helmut - VK4STR






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