[amsat-bb] Mode B doppler shift question

John Geiger af5cc2 at gmail.com
Tue Feb 14 03:39:33 UTC 2017

I wonder if someone can help me figure this out or think through this.  I
have Mode J doppler shift worked out-you start high in frequency, and the
frequency lowers as the satellite moves towards you, and then away from
you.  For example, if I transmit in the exact center of the FO29
passband-145.950-the received signal from FO29 starts out above the center
of the receive passband, around 435.859 or so at the beginning of the
pass.  When the satellite is overhead it is at the center of the
passband-435.850, and then moves lower in frequency as the satellite moves
away from me.  This is assuming I keep the uplink frequency at 145.950 the
entire time.

Is it my imagination or is the situation reversed on Mode B. It seems that
if I transmit on the center frequency of the uplink passband, the downlink
signal is below the center at the beginning of a pass, and then moves up
and ends up above the center of the downlink passband at the end of the
pass-assuming I keep the transmit frequency the same. Is that correct?  Why
would Mode B behave differently than Mode J?

You can see I still have some things to learn about the satellites.

73 John AF5CC

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