[amsat-bb] FM signal on FO-29?

Zach Leffke zleffke at vt.edu
Sun Nov 8 22:13:27 UTC 2015

Hi Rick,
     I've been monitoring FO-29 on and off for the last few weeks as 
part of my AO-73 to FO-29 cross link experimentation.  I have 
consistently seen FM on FO-29.  In fact I played a video at the AMSAT 
symposium where I pointed it out in a spectral display (FFT and 
Waterfall) of the FO-29 downlink.  We can see the entire noise floor of 
FO-29's passband with the VTGS setup, and when the FM activity jumps up, 
you can see the entire passband of the transponder drop in power as the 
FM signal sucks up all the power of the transponder.

I have detected, but have yet to decode AO-73's tlm beacon via FO-29.  
Right now I'm operating on a weak crosslink signal theory as the reason 
for failed decodes.  I can hear FO-29 just fine (in fact VERY strong at 
VTGS), but AO-73 is barely making it into the uplink passband.  I've 
come up with a simulation tool thats a mixture of STK output data and 
python that tells me when chain events will occur (a chain being AO-73 
to FO29 to VTGS) and a whole bunch of other info like doppler offsets, 
crosslink path loss, etc.

In fact, this last Friday, around 1357 Eastern time, was one of the best 
opportunities to catch AO-73 via FO-29, in terms of minimum crosslink 
path loss, in a two week window.  At the beginning of the FO-29 pass 
there was LOTS of FM activity on FO-29.  I was cursing and pulling my 
hair out (to the dismay of the students who were watching) because I was 
worried that the FM activity would suck up resources on the transponder 
right when I'm hunting for a weak signal on the bird.  BUT... as the 
satellite continued through the pass (an ascending pass) the FM activity 
died down, which was good for my experiment because the AO73 chain event 
was about the last 300 seconds or so of the pass.  I still didn't decode 
(though I *think* I detected AO-73, I need to review the capture file 
more) because doppler shift had the AO-73 signal sweeping right through 
the center of FO-29's passband, where there was lots of normal SSB activity.

So yes, in the last couple weeks I've seen a LOT of FM activity on 
FO-29.  And based on my experience with Friday's pass, which was 
ascending, I'm leading towards the Central/South America QRM theory.  
Lots of strong FM activity as the pass started and the satellite was 
over the lower latitudes, but as FO-29 ascended over higher latitudes 
towards the north pole, the FM activity died down.

-Zach, KJ4QLP

On 11/08/2015 03:25 PM, Jim Jerzycke wrote:
> On 11/08/2015 07:03 PM, Rick Walter wrote:
>> I was just on the 18:44 Z pass of FO-29 and was certain I heard an FM
>> signal around 435.869. There was a loud squeal and it sounded like an 
>> alien
>> was talking! Should I get my hearing checked or did anyone else notice?
>> Thanks.
>> Rick - WB3CSY in FN10
> I've heard FM on FO-29 before, but it's been a while. I forget if it 
> was a Ham, or a Mexican/South American taxi or similar.
> 73, Jim  KQ6EA
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