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FT-847 AO-40 and Doppler Uplink Correction?



Hasan wrote:

Listening for the beacon is easy..just subtract doppler from 2401.332 and
bingo...there it is. My question seems a bit more complicated....what
adjustment do I apply from a non-doppler corrected uplink freq so that I
come out on the "predicted" 2401 downlink freq?

Wayne replies:

With an inverting transponder, the NET Doppler shift is proportional to the
DIFFERENCE between the RX and TX frequency.  A general rule of thumb would
be that the NET Doppler shift for mode U/S is 80% of the "beacon Doppler".
And the NET Doppler shift for mode L/S is 50% of the "beacon Doppler".  It
doesn't matter how you INITIALLY adjust for Doppler (RX or TX) unless you
are trying to get on a precise frequency as seen by the satellite
transponder.

Obviously, Doppler correction during a QSO can be problematic if you are
talking to a station that uses a different uplink band.  Stations can also
experience different Doppler shifts simply because of their widely spaced
locations  (i.e. the satellite could be moving toward one station while
moving away from another station).  The ideal solution to this problem is
for all stations to use computerized "full Doppler tuning" which keeps your
frequency constant as seen by the transponder.

Once AO40 returns to 0/0 attitude, we will have many hours of transponder
operation near apogee with minimal Doppler shift.  So if Doppler is really
bugging you now, you can just struggle with it now until we have transponder
operation available near apogee.

Hasan wrote:

The SUB tune function of the 847 is made for VERY slow freq adjustments...I
don't want to have to crank the knob for five minutes trying to get close.

Wayne replies:

The FT-847 Sub Tune control isn't too bad if you press the FAST key first.
Then press the FAST key afterwards to return to the "normal" tuning rate for
the main tuning knob.  In my opinion this is a software bug in the FT-847.
I can't think of any possible reason why the default Sub Tuning rate should
be so excruciatingly slow.

Wayne Estes W9AE
Mundelein, IL, USA

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