[amsat-bb] Re: Ideal AMSAT FM Transponder

Robert Bruninga bruninga at usna.edu
Mon Oct 1 08:14:00 PDT 2012

> I presume the reason for UHF downlink rather than
> 145 MHz, with it's lower Doppler shift, is due to
> other mission constraints ?

Yes, avoid QRM to the 145.825 APRS transponder.

> BTW is the uplink in the amateur satellite segment
> of 10m (29.3-29.5) or down at the bottom of the band ?

Bottom segement.  Think of it as a UHF linked band monitor for existing
PSK31 operation on 10m usually between 28.120 and 28.123 MHz.  The entire
band is authorized by the ITU for satcom, though most band plans protect
downlinks above 29.3 MHz.

> This might also be a great project for balloon launches...
> to be able to test all the ground station setups.
> Any idea if these transponders will be made available
> to others after this project is completed ?

It will be up to Brno University and Mirek..

> It appears that the Small Wonder Labs' PSK device
> is no longer available, pending a replacement that's in the works.

Yes, we used a Brno module for the first flight back in 2006 on PCSAT2 but
then used a Small Wonder Labs PSK transceiver on the RAFT satellite in

The #1 hardest thing on making this system work, is getting the antenna
tuned and resonant.  This is a real challenge, because you cannot get
inside the 4" cubesat with any instrumentation to tune for resonance.
ANYTHING you connect to the spacecraft and antennas completely messes up
the affect of the counterpoise.

The best method we have found is to just build the transponder, and then
tweak a tuning element via a hole in the spacecraft until you can hear the
best signal from the transponder listening to normal signals on 10m.  Oh,
and having the whole set up FAR away from the ground and ANYTHING else...
It is a long and tedioius process.

We just did a preliminary EZNEC analysis, and the antenna impedance is
something like Z = 2 - J1100 using a 6 foot wire.


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