[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

calling CQ on top of the AO40 beacon

Some people might call CQ on top the AO40 beacon because their receiver is tuned to a 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 transponder frequency 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 AO40 frequency and whistle in the microphone.  At this point, my computer-controlled uplink 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 in the audio passband of an "empty ch!
nel" before I even transmit.

I've successfully used Doppler tuning with 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
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org