Re: [amsat-bb] S-Band Antenna Question

[Jonathan NAYLOR <Jonathan.Naylor@xxxxxxxxxx>]

Hello Adam

In terms of generating a RHCP signal, there is nothing to
choose between them. What really matters is the gain vs size
argument.

Assuming that you have a good downconverter with a noise figure
of around 0.6dB and adequate gain to overcome the noise figure
of the IF receiver, you should be looking at having an antenna
with a gain of at least 22dBic. This can be achieved with either
a dish of 60cms (2 feet) diameter or larger, or a very long helix,
or many helixes combined.

A ten turn helix provides a gain of approximately 13 dBic, while a
90cms dish (3 feet) will provide a gain of approximately 25dBic.

For simplicity I would go with the dish method, satellite dishes
are not expensive, and may even be free if you know of someone
throwing one away. Even brand new a satellite TV dish is not going
to be too expensive. The feed being either a patch or a small helix
depending on its f/D ratio. The amount of cunstruction work for a
dish feed is much lower than that for a long helix.

Those are my thoughts on the subject, of my short time on AO-40 I
have worked only one station using a helix and he was complaining
that his downlink wasn't good enough ! The rest used dishes from
60cms upwards.

Jonathan  HB9DRD / G4KLX
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