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Re: Question Regarding Doppler Correction (LONG)

Paul Yeager (W4SKI) wrote:
> I'll admit it up front, I'm a newbie!

We've all been there at some point in our ham "careers"!  Welcome!

> The problem may be in fact due to problems with my spreadsheet, but
> I also want to make sure that by basic understanding of doppler
> correction is correct.  If I am correcting "up" on RX (tuning
> higher than the actual transmit frequency of AO-40), then I need
> to correct "down" on TX (tune lower than the actual A0-40 RX freq)
> on TX, right?

The best way to remember it is to understand it.  For those of you
that already know all of the following, I apologize for the bandwidth.
But I feel that there may be more folks than just Paul who can be
helped by the somewhat long-winded explanation that follows.

As the satellite is approaching you, the signal it transmits is
Doppler shifted UPWARDS in frequency, so you tune your receiver
HIGHER to hear it.

As you transmit to the satellite, you are approaching it, and
the signal you transmit is heard by the satellite at a HIGHER
frequency, so indeed you would need to tune your transmitter
to a LOWER frequency to stay in the same place in the passband.

After the satellite passes through maximum elevation and starts
to move away from your location, the opposite situations are
present.  The signal you hear from the satellite is shifted
down in frequency (so you tune lower), and the satellite hears
your transmitted signal lower (so you would tune higher on
your transmitter to stay in the same spot in the passband).

One of the advangates of an inverting transponder is that it
tends to partially cancel the Doppler instead of magnifying it.
Let's say, for the sake of argument, you tune your uplink and
downlink to the exact center of the passband.  Obviously, this
is a VERY BAD IDEA on Oscar 40, since that's where the Middle
Beacon is located, but think instead of FO-20 or FO-29 where
the middle of the passband is a good place to start.  And in
any case, this discussion is just to help with understanding
the principles involved.  The theory applies anywhere in the
passband.  Starting in the middle simply makes it easier to
talk about it.

During satellite approach, your transmitted frequency is heard
higher than you are sending.  So the satellite hears your
signal ABOVE the middle of the receive passband, let's say 10 KHz.
That means that it will re-transmit your signal BELOW the middle
of the passband, in this example, 10 KHz below the middle of
the downlink passband.  So the signal coming out of the satellite
starts out low, and is Doppler shifted upwards in frequency on
its way to you.

Because the magnitude of the Doppler shift is proportional to
not only the relative speed of motion between space station and
earth station, but also directly proportional to the frequency
of the radio signals, the two Dopplers can't cancel exactly.  In
the case of the FO-20 & FO-29 birds (Mode J or, to use the modern
terminology, mode V/U), your 145 MHz uplink will have about 1/3
as much Doppler shift as the 435 MHz downlink.  So, in my example,
if your 145 MHz signal was Doppler shifted up 10 KHz, the inverting
transponder would move the downlink to 10 KHz below the middle of
the passband, and that signal would be Doppler shifted up 30 KHz,
with a net effect that the uplink appears 20 KHz higher than you
might have expected without any Doppler effect.  If the transponder
had not been inverting, a signal 10 KHz above the middle of the
uplink passband would come out 10 KHz above the middle of the
downlink passband, then get shifted 30 KHz further up by the
Doppler shift on the way to you, and would end up 40 KHz "high"
instead of 20 KHz high.  So the inverting transponder appears to
demonstrate only half as much Doppler as a non-inverting one in
this case.

In the case of a mode U/S satellite like Oscar 40, if the bird was
approaching you at a rate that created 10 KHz of uplink Doppler,
the corresponding downlink Doppler would be approximately 55 KHz
(2400/435 is approximately 5.5).  With an inverting transponder,
the net shift would be 55-10=45 KHz.  With a non-inverting
transponder, the net shift would have been 55+10=65.  So in this
case, we only reduced the effective shift by about 1/3 instead of
1/2, but that's still a good thing.

In any case, this also points out that when building a general
purpose spreadsheet for calculating Doppler shift, you need to
know if the transponder is inverting or not, and take that shift
into account also.

I hope that helps to clarify it in your mind.
73 de KB0ZEV (John Toscano)
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