Re: [amsat-bb] Re: Fixed-uplink tuning (was: Eagle Launch Campaign)

[Scott Townley <nx7u@xxxxxxx>]

Horse-hockey!
I have made and continue to make 100's of mobile--yes while driving--QSOs 
on RS-12/13, FO-20, and FO-29.
There are a couple of ways to minimize the Doppler pain:
1. Call CQ...then you only have to tune one radio (RX) to find replies.
2. To answer a stn, rig up something where you only have to use one hand to 
adjust your TX (I have a little switch on my keyer that sends a string of 
'dits').  Takes one (the same) hand to hit the switch, tune TX until I hear 
my downlink, then off with the switch and call.
Naturally some help from the other station is appreciated, and these ideas 
aren't specific to mobile stns: (1) stick to the "one true rule" so the 
other guy only has to adjust RX (and in only one direction as well), (2) 
when you've called CQ tune around a bit...like Paul mentions, everyone's 
Doppler isn't the same.


At 14:51 2004-07-31, Paul Williamson wrote:
>At 7:59 PM +0000 7/31/04, Mike Hooles wrote:
>>P.S. If you want to get those of us using IC706s etc when mobile, onto FO29
>>and successors, what about AMSAT nominating a specific single uplink
>>frequency for us to use. Then we can tune our receivers around the nominal
>>downlink  frequency accordingly and allow for the doppler shift ourselves.
>>I'm sure there is a 'spare' 20kHz that we could use for this. It's worth an
>>experiment anyway.,
>
>In this scenario, the various users' downlinks in a single QSO would be 
>scattered all over about a 6 kHz band as received at each station, because 
>Doppler is different at different locations. It would be awfully hard to 
>carry on a conversation with that kind of frequency split, especially mobile.
>
>Assume Mode J, 2m up and 70cm down, in low orbit (like FO-29). That works 
>out to about plus or minus 3 kHz of Doppler on the uplink, and plus or 
>minus 9 kHz on the downlink. From the point of view of a single ground 
>station in receive mode, every signal gets the same downlink Doppler shift 
>at any given moment, so the +/-9 kHz can be dialed out "easily" as it 
>varies smoothly through the pass at that groundstation. However, each 
>user's signal has a different uplink Doppler shift, anywhere from +3 kHz 
>for a station at AOS to -3 kHz for a station at LOS (of a high-ish 
>elevation pass). This frequency distribution at the satellite would be 
>repeated on the downlink, so the operator would have to tune madly around 
>a 6 kHz band on every "over".
>
>There'd be no way to know when all the listeners were finished tuning, so 
>each speaker would have to assume that the first few seconds of his 
>transmission were missed. That problem could be "solved" by starting each 
>transmission with a count to 10 for tuning purposes. That'd be painful but 
>perhaps tolerable, if it solved the problem. But it doesn't, because there 
>could easily be TWO operators transmitting on different ends of the 6 kHz 
>scatter, and each listener would be listening to only one. It would take 
>very careful operating practices to prevent this from devolving into chaos.
>
>The good news is that it wouldn't take 20 kHz out of the transponder, just 
>6 kHz. The bad news: it would still be nearly impossible to have a mobile 
>QSO without crashing your vehicle.
>
>73  -Paul
>kb5mu@amsat.org
>----
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Scott Townley NX7U
Gilbert, AZ  DM43di
http://members.cox.net/nx7u 
----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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