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Re: Doppler Tuning for AO40 with NOVA



Jim:

Yep, I have the same problem. But I don't necessarily
ascribe it to NOVA's calculations. There are so many
variables involved. Do we know that the beacon is exactly at
2401.323, and that the passband offsets are accurate? How
close to 144.000000 Mhz is your receive downconverter at
2400.000000 Mhz input? Is your downconverter stable? How
accurate is your radio?

So to solve these problems, I incorporated a tune-up feature
that works for both transmit and receive. First, I have an
Rx Sync button, and I "assume" the beacon is at 2401.323. I
manually tune the receiver for the beacon and hit the Sync
button. That locks in a receive downconverter adjustment
that is good for my setup. The next time the program is run,
the Rx Downconverter offset (somewhere +/- 2256.000000 Mhz)
has been set and the beacon should be right there. I say
"should" be -- it isn't always. Mine changes. I think the
downconverter (AIDC 2731AA) drifts with outside temperature.
Perhaps the satellite drifts as well. But in one click, that
takes care of the receive side.

On transmit, I have a Tx tune-up feature. Click a button and
the radio is sent below the beacon somewhere near the bottom
of the passband where there are few, if any QSOs. The
transmitter is placed in CW mode, and I key some dits and
tune the receiver until I hear myself. There's also a setup
option to tune not in CW, but in SSB mode, so I can
"whistle" myself on frequency. It usually starts out fairly
close, but I've observed it's not uncommon to be 3 Khz or
more off, and I can't explain exactly what is changing. Once
I've synchronized transmit to receive, I click the button
again. The transmit offset is stored, and the radio is
returned to the frequency I started at - usually a QSO up
above beacon.

The theory here is that too many people tune up on top of
other QSOs. Field day was wild. Dits and whistles on top of
almost every QSO. Carriers that swept the band. It was most
disturbing. But it's difficult to tune all the way down the
passband to where there are no QSOs, tune up, and then come
back to the conversation you wish to join. So I let the
software do that for me.

Once the offsets are recorded, I can tune to a QSO, hit the
Full Doppler Tuning button, and be locked into the
conversation (assuming they are using FDT). In FDT, the
software adjusts both the Tx and Rx as the doppler changes.
Or, if the person you wish to work is not using FDT, i.e. if
his receive is drifting, you can turn FDT off and and just
manually tune your receive to the QSO. As the receiver
changes, my software reads the frequency and adjusts the
transmitter accordingly.

Hope this helps.

73,
Art N3OY


Jim Cottle wrote:
> 
> Hi Art,
>   You mention you wrote a program for NOVA DDE and DOppler tuning. Did you
> find that the values calculated by nova were accurate? I have a problem. If
> I tune to the MB on S2 (2401.323) and, using the doppler correction enter in
> an accurate value for Local Oscillator, I don't end up predicting a proper
> uplink frequency for either U band uplink or L1 uplink. I suspect that I am
> not putting the proper frequency in for the matchin uplink passband for the
> MB, but I don't know.
>    Do you have to account for any uplink offset other than what is predicted
> by the program to make this work. In theory one should, with the frequencies
> locked at the satellite side, to accurately predict the uplink frequency. If
> it works, you should hear yourself by tuning to the predicted uplink. What
> am I missing?
> THanks in advance.
> Jim
> AC4EA
> ac4ea@arrl.net
> ac4ea@amsat.org
> jim_cott@earthlink.net
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