[amsat-bb] Re: Digital modes and antenna questions

[Bruce Robertson]

Quoting Stargate <stargatesg1 at verizon.net>:

> Hi,
> 	Can someone point me to a website that explains the current digital
> modes
> and ways to use them?
> Also decoding telemetry from the birds, is there a "one size fits all"
> software or does each satellite
> have specialized software for doing this?
> Can you use Sound cards, TNC, specialized modems?
> I keep seeing references to 9K6, 38k6, and the like and have no idea
> what
> they are.
> 
> 	Also, are TNC's really necessary anymore? I've been away from amateur
> radio
> for over 10 years
> and things sure have changed.
> 
> 	I'm still putting my satellite station together and am down to picking
> antenna's and feed line.
> I am trying to build the station up for the future HEO birds as well,
> so
> what kind of overall db
> receiving level should I be shooting for?
> 
> 	Has anyone used "Gulf Alpha" antenna's? The cross polarized antenna's
> look
> very good to me. It wouldn't
> be circular polarization but I would be able to switch between vertical
> or
> horizontal with a flip
> of the switch.
> 
> All help appreciated,
> TNX
> 73's
> RoD
> KD0XX

Rod:

Your questions are numerous enough that I'm of the impression you would 
enjoy general introductions. I've found that the books offered by Amsat, as
well as the ARRL's _Radio Amateur's Satellite Handbook_ are excellent
starting points.

The AMSAT webstore has its publications at
http://www.amsat-na.com/category.php?c=Publications
I own _Getting Started_ (you'll want the 2006 edition), and I suspect
you'll like the _Digital Satellite and Telemetry Guide_, too. Because AO-51
is a super test-bed for all this stuff, I'd recommend the book on it. If
you want the latest on what's happening, get the Proceedings of the 2006
symposium. And if you haven't become a member yet, please do.

While there are other more expert than I on the subjects addressed above,
I'll take a stab at answering them briefly. For digital modes, you can
demodulate with a computer soundcard and appropriate software very easily
at 1200 bps rate; the 9600 bps rate (short-form, 9k6) is more difficult to
do in software, partly because your average soundcard link box for, say
psk32, will likely use devices that limit the audio so that the signal
cannot be demodulated. Apparently nobody has yet written software that can
demodulate 38600 bps (or 38k6).  If you're using a TS-2000 as your rig,
there is more specific advice I'd be happy to give. So the need of a TNC is
determined by what birds you aim to communicate with, and how.

Modulation schemes, alas, vary in satellite work, but you can do quite a
bit with the FM modulation you might be familiar with from terrestrial
packet, which I believe is designated AFSK.

If you're aiming for antennas that will work on future HEO's, such as P3E,
I would look around the web for descriptions of systems that worked well on
AO-13 and AO-10. My impression from such research is that you really will
want fully circularly polarized antennas when that day comes, especially on
mode B downlink: it's 3 dB that would take a lot of boom length to buy
otherwise. Back in the day, it seems, people used 7 - 10 element circ.
polarized antennas. Of course, it depends on how far the bird is: I just
was playing around with the preliminary keps for P3E, and noticed that the
distance can be as far as almost 50,000 km away, but much fun can be had at
distances under 20,000 km. Plug that into the formulas in the books
described above, and you have the makings of a wonderful night of system
design!

You ask about a commercial antenna design. Might I recommend getting your
feet wet in satellite work first with very cheap home-made antennas, say
five elements on 145 MHz and eight on 435 MHz? I've had a blast with such
units on the LEOs. They require far less precise aiming (you can do away
with the elevation rotor), and are well-equipped for the job.  They'll let
you find the kinks in your system. You're also less likely to QRO the bird
with such antennas. If you want to follow this way of thinking, there are
many of us who can lead you to some dead-simple antenna designs that use
Home Depot parts.

Either way you go, I note that you don't mention preamps. They are crucial.
 My strong recommendation is that you buy a mast-mounted SSBUSA or ARR
brand preamp for each band that you plan to receive on. I think you'll have
more fun with one of these hooked up to a vertical, than with a 20 element
circ. polarized yagi that has no preamp. 

Finally, if you're thinking ahead to the launch of P3E, don't forget S-band. 

73, Bruce
VE9QRP