John, Very good explanation and after that I played with it a bit more last night and got it working almost fine as you can attest when we had a QSO on VO-52. So I stand corrected, and will from now on use full doppler computer control whenever possible :).
73 Mike K5TRI On Jul 24, 2011, at 9:00 PM, John Papay wrote: > The purpose of full doppler correction is to keep your signal in > the exact same spot on the receiver in the satellite. It has a finite > passband and if you don't correct your uplink, you move within that finite > passband. Theoretically a single linear transponder can support many > conversations but it will not support the maximum unless everyone stays > put at the satellite receiver. Those who don't doppler correct slide > within the > passband and run into those who are staying put. Of course if you adjust > your uplink so you stay put at the satellite, you have to adjust your > receiver to hear yourself because of the doppler shift in the other direction. > Both uplink and downlink require doppler correction in this case. > > If you don't have a rig that can be computer controlled or if you are > operating portable and can't readily incorporate a laptop into your > setup for whatever reason, then by all means manually adjust the highest > frequency since that is the one that changes three times faster than the > lower frequency in a v/u satellite. You simply do the best you can. > > However, if you have a home rig that can be computer controlled, then by all > means use a computer to adjust your doppler. Stay in one spot in the > satellite's passband. You can make faster contacts since you only have > to pick up the mic to talk or use the key to send cw. If you are working > dx and the window is short, being on frequency and staying put will help > put that distant grid in the log. > > The program does all the work but you must be sure that your keps > are current AND your computer clock is accurate to the second. On a > high pass when the satellite is overhead, the doppler changes very fast > and even a few seconds of clock inaccuracy will have you not hearing > yourself in a normal voice. At lower elevations the doppler is not as > severe so a few seconds won't make much difference. > > Use the HO-68 beacon to test your doppler correction. Set your radio for > CW with a 500 Hz or better bandwidth. Pick a high pass where the bird will > get up over 45 degrees. Tune the beacon in at your AOS and then don't touch > the dial. If the CW note stays constant over the entire pass, your system > is operating perfectly. Make sure your program settings are such that there > are frequent frequency updates. I have mine set at 1 second no matter what. > Some older radios will not accept frequency changes while you are > transmitting. In > that case, make sure you only key down for short periods of time so the > radio frequency can be updated. > > Of course you must adjust your uplink calibration on SatPC32 so that you > hear yourself on the downlink. Hearing yourself in a normal voice will > be an indication that you have your calibration set properly. > > Most of the birds stay on frequency, but I've noticed that AO-7 Mode B will > be off by 1700Hz when it changes from Mode A to Mode B. Normally we would > not observe this phenomenon but since lately we have had this mode change > while > over North America we are able to experience it. Within a few minutes, the > satellite is down to a 1200 Hz offset. I'm not sure what happens on the next > pass because it is out of my footprint. The next morning, however, the bird > is back to my normal setting. This means the frequency drifts about 1700 Hz > back down to it's steady state value. The other birds may have some slight > drifting, probably due to spacecraft temperature but it is minimal, less than > a few hundred Hz. So when you are trying to make a contact on the first > AO-7 pass in Mode B, be prepared to readjust your uplink calibration so you > can > hear yourself. It may be as much as 1700Hz at the start. > > In my opinion, everyone that can control their radio for doppler should do so. > Doppler correction gives us the maximum usage out of our linear transponders. > Why go through the agony of fiddling with your radio when your computer can > make your life so much easier. You will never have to apologize for not being > on frequency. If I wasn't able to have doppler control on my radio, I > wouldn't > spend much time on the linear birds, unless of course there was a rare grid > to be worked. <grin> > > 73, > John K8YSE > > _______________________________________________ > Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. > Not an AMSAT-NA member? Join now to support the amateur satellite program! > Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
