Doppler Tuning on Linear Transponders
(For those Hams with older rigs).
To combat QRM from stations "walking" across the passband ...
Obviously, the best way to correct for Doppler tuning on a satellite is to use a tracking program in conjunction with a CAT ... (computer aided tuning) ... interface connected to your rig ... (see "THE ONE TRUE RULE" by Paul Williamson KB5MU, available on the AMSAT-NA website) ... "The one true rule" suggests keeping the frequency at the satellite CONSTANT by tuning "both" the transmitter and the receiver so that stations DON'T walk across the passband, causing QRM to the other operators. To implement this with any success one needs a fully automated doppler correction system.
... but I don't have a fully automated doppler correction system ...
Unfortunately, not all operators have access to a dedicated satellite rig or even a modern transceiver with a CAT interface.
However ... all is not lost ... there are still a couple of other options available to the resourceful operator. For those using the FT-290/FT-790 type of rig it's perfectly possible to achieve FULL DOPPLER TUNING by using the ITUNE program ... freely downloadable from the AMSAT-NA website.
This is an old DOS program that functions fine together with the tracking program INSTANTTRACK on IBM compatible PC's using operating systems from DOS 6.2 up to and including Windows XP.
Unfortunately, ITUNE can only automatically tune "either" a transmitter or a receiver ... (not both at the same time) ...BUT! ... by using two computers and two ITUNE interfaces it's possible to have FULL AUTO-DOPPLER TUNING when using older rigs. All you need are rigs with mic UP/DOWN pushbuttons, a computer, an ITUNE interface and the tracking program INSTANTTRACK.
The ITUNE interface is made in 10 minutes ... (15 if you have a teabreak).
Basically, the system works thus ... INSTANTRACK tracks the satellite while ITUNE detects the correct amount of Doppler and sends a pulse to the rigs microphone up/down buttons thereby tuning the rig automatically ensuring it's on the correct frequency at all times. YOU DON'T HAVE TO TOUCH THE TUNING KNOB THROUGHOUT THE PASS though you can "tweak" it if you want to ... ITUNE will always keep your frequencies correct whether the satellite is approaching you (faster Doppler) or receding away from you (slower Doppler).
The interface is very simple and connects between a PARALLEL port on your computer and your rig's microphone UP/DOWN buttons. A single ITUNE interface can be configured for either TRANSMIT or RECEIVE.
THE ITUNE INTERFACE.
Once you've built your interface, you edit the configuration file ...(read the documentation carefully) ... for the type of rig/system you're using and then operate it in conjunction with INSTANTTRACK ... available from most AMSAT affiliates for a small donation.
If you're using two computers and interfaces YOU NOW HAVE FULL AUTO-DOPPLER TUNING. If you only use one computer and one ITUNE interface you will have semi-auto Doppler tuning ... which is far better than no auto-Doppler tuning at all.
REMEMBER! ... due to the satellite's motion and the distances involved from the satellite to the ground-station, stations in different locations will experience different rates of Doppler.
For those who have no auto-Doppler tuning, the recommended Doppler tuning procedure is to tune the HIGHER of the frequencies used ... this alleviates SOME of the Doppler problem ... tuning your transmitter or receiver on each over will help ... though this is NOT a very good option and WILL cause QRM.