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Re: Features in FT-847 replacement (Ratio Tuning)


Thanks for the explanation. I had understood it to be
the way you described, but never actually thought
about it. 

My suggestion of ratio tuning, however, was
specifically for the FM sats rather than those with
linear transponders (where, as you pointed out, the
ratio tuning would not do any good).

If we do the math for Mode J sats, however, we can see
why ratio tuning would work:

1.) Doppler equation for stationary observer: F' =
Fo*[1/(1+(V/c))] (V relative to observer)

2.) Doppler Shift Magnitude (2m or 70cm) = F' - Fo =
Fo*[(1/(1+(V/c)) - 1]

3.) Shift Ratio (70cm to 2m) = (F' - Fo)[70cm]/(F' -

Since c and V are constant in (2) for 2m and 70cm at
any instant in time (or during the pass), the
bracketted terms cancel on top and bottom of (3),
which leaves:

Shift Ratio = Fo[70cm]/Fo[2m]

This shows that the shift ratio is only dependent on
the ratio of the uplink and downlink frequencies.

If Fo[70cm] = 435 MHz, and Fo[2m] = 145 MHz,  Shift
Ratio = 3

One could solve a ratio that best fit for the sat band
allocations, but 3 works alright since the max
deviation of the ratio at opposing sides of the
satellite up/downlink band allocations is less than
1.5%. If the shift were 10kHz for example, the
deviation would be 150 Hz, which is an acceptable
deviation for the forgiving FM voice transponders.

Sorry if I was not more specific the first time!


Matt Bennett

--- "John P. Toscano" <tosca005@tc.umn.edu> wrote:

> 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!
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are
> those of the author.
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Matt Bennett
UCISAT Communications Team Member
University of California, Irvine
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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