Re: [amsat-bb] Tuning uplink -> AO-27

["KF4FDJ (Mike in Ft. Myers, FL)" <seven77@xxxxxxxx>]

Fellow ops,

After receiving a private post from Steve Greene, KA1LM, a member of the
AO-27 integration team, I decided to try an experiment.

The basis for the experiment was to determine what effect tuning for
Doppler uplink while working AO-27 would have on the downlink signal.

I decided to try the experiment in my shack where I could monitor the
downlink more carefully than I could on a portable link.

I used a Yaesu FT-847 in full duplex "satellite" mode.  The downlink
signal can be centered in the receiver pass band, by keeping a careful
eye on the discriminator meter, and tuning to keep the signal centered.

My uplink antenna is a KLM 22C.  I believe it has 15 dBic gain.  I have
a little less than 1 dB loss in my switches and coax on 2 meters.  I
used The Station software, driving a Kansas City Tracker interface card,
which in turn drives the controller on a Yaesu 5400B rotator.

As I've intimated, the discriminator meter on the 847 makes tuning the
downlink easy.  I used The Station software to calculate the doppler
correction on the uplink, and tuned the sub-tune knob to control the
uplink frequency.

I am fairly confident the antenna was pointed within 3 degrees of the
bird during the entire pass.

I tried 3 different times to adjust uplink frequency within the pass
band of the satellite receiver.  Each time I tried 3 different power
levels.  For comparison sake, here are the statistics for the 3 power
settings:

1)	1 watt PEP	25 watts ERP
2)	4 watts PEP	100 watts ERP
3)	10 watts	251 watts ERP

All three times I tried the experiment, the doppler shift on the 2 meter
uplink was in excess of 3.1 Hz.  All three times, the results were
comparable.  Here is what I found:

At power setting 3, I could not discern any difference in the audio of
the downlink signal, nor could I see a change in the S meter reading as
I moved the VFO from 145.850 to 3.1 kHz off to compensate for Doppler. 
The signal did not vary on the S meter until I was close to 8 kHz away
from the calculated center of the pass band, including Doppler
correction.  This bird DOES have a wide pass band!

At power setting 2, the results were very nearly the same as for power
setting 3, except, while I could not discern a change in the audio
signal, a slight deflection of the S meter was apparent as I moved from
145.850 to the calculated center of the pass band, including Doppler
correction.  The signal dropped off noticeable when 7 kHz away from the
calculated center of the pass band, and in fact became noticeably
distorted.

The most dramatic results were obtained at power level 1.  Again, I
could not discern any change in the audio qualities of the downlink
signal, and this is probably a result of the AGC circuitry in the 847. 
S meter deflection was close to 1 S unit as I swished from 145.850 to
the calculated center of the pass band.  The same distortion and dip in
audio was noticeable as with power setting 2, somewhere near 7 kHz away
from the calculated center of the pass band.

So, with a properly pointed uplink antenna, if tuning for Doppler can
show a 3 dB (half power) increase in perceived downlink signal, there
should be an advantage to tuning for Doppler with a portable station. 
Off pointing of the antenna will decrease ERP, as will proximity to
metallic objects in the near field of the antenna.  

I will continue to tune for Doppler while operating portable.

Feel free to make your own extrapolations, I might be missing something,
but it seems that tuning for Doppler will give you an advantage on a low
ERP uplink, and striving to put your signal into the center of the
receiver pass band has merit.

Nothing will prevent your signal from being trampled by a more powerful
station, or ignored by other operators, but strive for every advantage
you can muster.

73, Mike in Fort Myers, FL

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