[amsat-bb] More thoughts on AO-51 USB/FM

[Emily Clarke <w0eec@xxxxxxxxx>]

It has been very interesting to operate AO-51 in USB/FM mode.  It 
seems to me the amount of power transmitted doesn't have very much 
effect on this mode at all.  I've been varying power from 2W to 6W 
and higher just to see the effect, and it doesn't matter much.

What seems to make all the difference in the world are four things:

1) Antenna polarity
2) Microphone Gain
3) Bandwidth
4) Doppler Tuning and Link Offset

I can't comment on polarity since my antennas are linear, however I 
have noticed more comments about polarity shifting than normal. I 
notice I get some fading on my uplink than normal so I attribute this 
to be due to this.  Perhaps people with switch-able circular polarity 
might wish to comment farther.

The next observation is microphone gain.  Even at 100% mike gain on 
the 910H, my signal is not nearly as strong as it would be at ~25% 
gain on AO-7/VO-52/FO-29.  Unlike the SSB birds, compression seems to 
have a little effect - some, but not very much.

I don't know the bandwidth of the SQRX but according to the specs for 
the Spacequest receiver it can be set to 4KHz, 15KHz or 150KHz.  If 
the SQRX is set for 15KHz to match the transmitted 15KHz wide FM 
signal, I wonder the effect setting it to 4KHz would be?  Would the 
analog output from the SQRX be stronger into the FM transmitter?

Since the notch filter and RIT on my rig have no effect on tuning in 
stations that are not in the center of the passband, I have been 
using my downlink set in FM-N mode (6KHz vs 15KHz) This has the 
effect of notching out stations that are way off frequency and it 
makes it easier to hear stations that are on frequency by filtering 
out QRM.  USB signals are typically 2.8KHz so concentrating on the 
6KHz in the middle of the passband rather than the entire passband 
seems to work very well.

On a very quiet pass at 1818 UTC I opened my receiver up to the full 
15KHz.  There were many more stations there, but they were either too 
high or too low in frequency to be heard. Some were drifting, which 
indicated to me they were not adjusting for doppler shift.  Others 
were staying where they were which indicated they were using doppler 
tuning but perhaps not compensating for any link offset.

I found that I needed a link offset of -1100Hz to be centered in the 
passband, though this dropped to -900Hz later in the pass.  It would 
be interesting to receive some opinions on this.  One thing that may 
give a clue was something I read about 60M operation. The NTIA 
indicates that you have to tune approximately -1.5Khz below the 
center of the 60M channel to keep your upper-sideband signal within 
the channel bandpass (2.8KHz).  It would be interesting to hear 
opinions about whether or not there is a similar situation going on here.

One last observation - it must be frustrating for stations who can 
hear the downlink signals of other stations clearly but can't hear 
their own uplink repeated.  Unfortunately it's also frustrating when 
you hear a station that is off-frequency above or below because you 
can't use RIT or adjust your downlink frequency to receive them as 
you would if you were on a SSB satellite.

73,

Emily

PS - Opinions are my own and not official AMSAT policy or position.



---------------------------------
W0EEC - CM87tm

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