[amsat-bb] calling CQ on over the beacon

[Estes Wayne-W10191 <W10191@xxxxxxxxxxxx>]

Some people might call CQ on top the AO40 beacon because their receiver is tuned to some kind of harmonic/spurious signal instead of their actual downlink signal.  I had that problem when operating the Fuji satellites before I got computer-controlled Doppler tuning.  When tuning to find my downlink, I would "hear myself" on SEVERAL frequencies in the downlink passband.  I was never sure which signal was really coming from the satellite.  On the Fuji satellites, one can usually recognize a harmonic by absence of changing Doppler shift.  On AO40 it's not so easy to distinguish a harmonic from an actual downlink.

For me, the solution to this problem is computer-controlled Doppler tuning.  For the Fuji satellites, everything works perfectly once the correct value is set in the Doppler tuning setup file.  For AO40 it's a little more complicated because my downconverter L.O. frequency varies from day to day.

Before transmitting to AO40, I spend about 1 minute calibrating my Doppler tuning software to "today's" downconverter L.O. frequency.  To do that, I activate Doppler tuning and note how many kHz the beacon is away from the intial frequency when I activated Doppler tuning.  I apply that error to the downconverter offset frequency in the Doppler tuning setup file, re-start the Doppler tuning software, and confirm that the software tunes the radio to the beacon frequency.  Then I tune to an unused frequency and whistle in the microphone.  At this point, my downlink frequency is already so close to "correct" that I can hear my downlink signal.  I tweak the transmit frequency to make the return signal be the same pitch as my whistle.  The whole process takes about 1 minute, and it eliminates any frustration of tuning around to "find" my downlink.  Plus, it avoids QRM to other stations because my uplink signal is already VERY close to correct before I even transmit.

I've successfully done Doppler tuning using The Station Program, InstantTrack/InstantTune, SatPC32, and W6IHG Radio Tuner (for Nova).  All of these work with a simple serial cable between the computer and the radio.  No expensive interface is required.  Of the four, I found InstantTune and SatPC32 to be the easiest to use.  I like SatPC32 best because it has very nice frequency displays.  I didn't like Station and Radio Tuner because they forced me to tune across the transponder with on-screen tuning sliders.

Wayne Estes W9AE
Mundelein, IL, USA
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Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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