Re: [amsat-bb] Features in FT-847 replacement, wish list

["John P. Toscano" <tosca005@xxxxxxxxxx>]

Matt Bennett (KF6RTB) wrote:
> I agree that the linked TX/RX sat tuning is a cool feature, but I think it can be improved upon. The current linked tuning ratio on the 847 is 1 to 1 when the ratio of Doppler shift on 70cm compared to 2m is approx 3 to 1. Of course, you can use the sub tune knob to fine-tune the uplink, but its use could almost be eliminated if the VFOs tuned in the proper ratio. It seems to me that enabling the feature would improve the clarity of FM voice transmissions at lower elevations where the TX side is slightly over/under tuned. 

Uhhh...  I think that the "optimal" ratio actually *IS* 1:1, although at 
one time I did think differently, like you.

The uplink and downlink passbands have a linear relationship to one 
another.  *At the satellite*, moving 10 KHz up the uplink passband moves 
the signal 10 KHz down the downlink passband.

Yes, the Doppler effect causes the higher frequency to shift faster than 
the lower frequency (approximately 3:1 for a 2M vs 70cm pairing), *BUT* 
consider the following:

1) At AOS, the satellite is coming towards you at the maximum velocity 
of the pass.  Assuming an inverting passband VU transponder (e.g., FO-29 
or FO-20), your uplink signal on 2M will be shifted upwards by an amount 
equal to "X" KHz at the transponder's receiver.  This will cause the 
transponder's output to be sent on a frequency "X" KHz lower than 
"expected".  The satellite's 70cm signal will be Doppler shifted up by 
approximately "3X" KHz by the time it hits your receiver, so in essence, 
since it started out "X" KHz low and was shifted up "3X" KHz by Doppler, 
the net effect is that you hear your downlink about "2X" KHz higher than 
you would without Doppler.

2) At LOS, the satellite is moving away from you at the maximum velocity 
of the pass.  With the same assumptions as before, you transmit an 
uplink signal on 2M.  Your signal is shifted "Y" KHz down by Doppler. 
(Note that "Y" might be equal to "X" if the absolute value of the range 
rate was the same at both ends of the pass, but I don't know if that is 
necessarily true.)  Your downshifted signal is transponded "Y" KHz 
higher than "expected" for the no-Doppler situation.  The signal from 
the transponder to you on 70cm is Doppler shifted about "3Y" KHz lower. 
  Since it started out "Y" KHz high, the net effect is that it returns 
to you about "2Y" KHz low.

3) At the time of closest approach, the range rate is approximately 
zero, so "X" = "Y" = zero, there is no Doppler shift, and all the 
signals appear where you "expect" them to appear.

So what is the "optimal" tuning ratio between uplink and downlink? 
Well, during the first half of the pass, your downlink will be heard 
"higher" than expected, and during the second half of the pass, your 
downlink will be heard "lower" than expected, and at the midpoint, 
everything appears where expected.  Seems to me that the best ratio is 
still 1:1.  Furthermore, I think that if you do the math, you will find 
that wherever you are in the pass, once you have found your downlink 
from your uplink signal, if you tune 10 KHz up the passband, your 
downlink will move very close to 10 KHz down the passband, in spite of 
the Doppler effect.  Granted, 30 seconds or 1 minute later that won't 
necessarily be true, because the Doppler shift has changed, but the 
point of the linked tuning is that once you have the Doppler correction 
dialed in correctly, for that moment in time, as you tune around the 
passband, you will still be very close to synchronized in frequency.

I've used this property to good effect many times.  Tune to a quiet spot 
in the transponder passband, find my echo, then QUICKLY tune up or down 
the band to a busy spot, and if done quickly enough, your downlink will 
follow your uplink very nicely, and you can attempt to join the QSO in 
progress.

By the way, did you notice that the 3X difference in Doppler translated 
to approximately a 2X difference in "expected" vs. "actual" frequency? 
This is one of the reasons that an inverting transponder is used, it 
automagically reduces the impact of Doppler a bit!
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