[amsat-bb] Re: AO-73 QSOs on the beacon?

wouter weggelaar pa3weg at amsat.org
Sat Dec 7 04:06:24 PST 2013

Hi All,

>From a design perspective, the transponder would require about 30 to 40dB
more uplink power to produce a signal overlapping the telemetry. That
signal would then be a maximum of 9dB above telemetry, compared to the rest
of the transponder. Since the transponder is used, the power spreads
between users, further reducing the effect.

The telemetry signal and transponder are combined, with the telemetry
stepped down 9dB, and the filter rejection for the transponder at the
telemetry frequency is better than 30dB (40dB with some ripple, I measured
35dB rejection on the engineering model)

The AGC acts on the composite output, so if no-one is on the main passband,
the transponder will start running full gain (43dB extra). This will
possibly reduce the required uplink power to actually overlap the
telemetry. But this only happens on high power uplinks combined with no-one
running in the main passband.

As soon as someone triggers the AGC with a signal in the main passband, the
AGC will kill the overlap.
There also is FM-ing of the telemetry beacon, but this effect is small
(~100Hz), and the BPSK decoder can cope with it.

I think it is more likely that AO-7 was up, or some local intermod is
producing this effect, but this effect being on the satellite can not be
completely excluded. Thanks for sharing this information!

Wouter PA3WEG
FUNcube RF Board designer

On Sat, Dec 7, 2013 at 8:25 AM, Michael Fletcher <oh2aue at kolumbus.fi> wrote:

>     Subject: [amsat-bb] Re: AO-73 QSOs on the beacon?
>>    From: Michael Fletcher <oh2aue at xxxxxxxxxxx>
>>    Date: Fri, 6 Dec 2013 08:55:41 +0200 (EET)
>> AO-7, mode B, happens pretty often...
> As I noted earlier, the only few times I have heard traffic _on_
> the AO-73  beacon frequency is when AO-7 is above the horizon
> in the sun and in Mode B. VO-52 downlink traffic comes close too
> occasionally, due to Doppler and orbits, but not _onto_ the beacon per se.
> And my ancient FCD is running just fine, with a highly selective
> HB LNA and a very sharp multicavity notch for my local APRS
> repeater at 144.800 MHz.
> Michael OH2AUE
> --
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