[amsat-bb] Re: RIT for FT-726R?

[John P. Toscano]

Scott Townley wrote:

> Now someone is likely to correct me on that last statement...but even 
> radios with tracking VFOs, track at a Hz-per-Hz rate, don't they?  My 
> IC-821 does...so if you manually tune, even with the VFOs in track, one 
> link is a little off because they need to track at 3:1 (at least for VU and 
> UV modes).

Yes, the VFO's track on a Hz-per-Hz basis, but this is not an error, it 
is by design. Remember, tracking of the VFO's has nothing to do with 
Doppler correction, where the amount of Doppler shift is proportional to 
frequency. The tracking of the VFO's has to do with positioning yourself 
within the passband of the uplink and downlink. On a linear transponder, 
once you are tuned in, moving 1 Hz on the uplink requires a 1 Hz move on 
the downlink (either in the same direction for non-inverting 
transponders, or in the opposite direction for inverting transponders) 
to stay tuned in. So if you hear your uplink coming down on the downlink 
in a quiet part of the passband, and tune the tracked VFO's to a new 
frequency where someone else is looking for a QSO, the downlink of your 
uplink should have followed you to the new frequency unless you took so 
long that the Doppler shift has changed significantly. But unless you 
tuned extremely slowly, you should be at least pretty close. Every so 
often, you have to adjust the frequency of the higher link without 
changing the frequency of the lower link to adjust for Doppler. On the 
FT-847, you either use the sub-tune knob to do that, or briefly un-lock 
tracking, re-tune, and re-lock tracking. And if the transponder is 
inverting and the ratio of frequencies is roughly 3:1 (mode VU or UV), 
the shifting Doppler is moving the two frequencies in opposite 
directions and so the net change in Doppler adjustment you need to make 
is closer to 2:1 than 3:1. Other modes have other ratios of changing 
Doppler, but at least for short periods of time when the net change in 
Doppler is small, a 1 Hz shift in tx causes a 1 Hz shift in rx, even 
mode V/S where the rx:tx ratio is over 16:1.

73 de WØJT