[amsat-bb] Need help understanding inverting transponder

Ronald G. Parsons w5rkn at w5rkn.com
Sat Jan 31 18:40:47 UTC 2015

The way MacDoppler and SatPC32 handles Doppler shift is by keeping the 
frequency a constant on both the uplink and downlink AT THE SATELLITE. This 
is commonly called Full Doppler Tuning.

As the satellite approaches you, the frequency you receive on the downlink 
is higher than the frequency at the satellite and decreases as the satellite 
passes the point of closest approach  (when the Doppler shift is zero) and 
continues to decrease as the satellite approaches LOS.

MacDoppler compensates for the Doppler shift in order to keep the frequency 
at the satellite a constant. This is advantageous as others can join the QSO 
and everyone will be at the same frequency at the satellite. It will appear 
to you that there is no Doppler shift -- the best method and easily 
accomplished by computer control.

If you tune your receiver up or down, MacDoppler will compensate by tuning 
your transmitter down or up by the same amount, and then compensate for 
Doppler shift as above.

Hope this helps.

Message: 5
Date: Tue, 27 Jan 2015 18:29:42 -0500
From: Jacob Tennant <jakewf8s at gmail.com>
To: AMSAT-BB <amsat-bb at amsat.org>
Subject: [amsat-bb] Need help understanding inverting transponder
<CAMM2CtW4axkair6JszYPaqXa7TEx=7V=wxF=3h8uVsHLo7iqMQ at mail.gmail.com>
Content-Type: text/plain; charset=UTF-8

I am very new to satellite operations and I am running MacDoppler on my
home Mac Mini computer and trying to get the modes setup as I am having a
little trouble with how / where the program should be tuning my Yaesu

As I think I understand it for a inverting transponder (FO-29 for example),
the downlink frequency range is 435.800 to 435.900 and that would be USB
and the frequency would move lower as the pass happens starting at 435.900
and going down to wards 435.800.

The uplink frequency range is 145.900 to 146.000 and that would be LSB and
the frequency would move higher as the pass happens starting at 145.900 and
going up to wards 146.000.

Is this correct?

Jake WF8S

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