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Notes on 2.4 GHz beacon



Dear Group,

The 400 bps GB (telemetry just like the V-Tx MB) has been set up on the S1 
transmitter (nominal freq. = 2400.2 MHz) from MA = 20 to 26. This is about 
2.77x6 = 16.6 minutes.  The MA chosen is a compromise of reasonable squint 
angle and visibility.  The V-Tx will remain on during this time, as 
well.  Keep a few things in mind.  First, the S1 antenna is highly 
directional.  At squint angles greater than 35-40 degs, I don't know how 
loud/weak the signal will be.  Also, at these relatively high offpointing 
angles the normally right-hand circular polarization, may be predominantly 
left-hand, which will also decrease the signal if you've got a right-handed 
antenna.  Finally, as has been mentioned, doppler at this frequency and 
this point in the orbit is going to be dramatic.  As a "guestimate", 20+ 
KHz down is not unreasonable if the satellite is moving rapidly away from 
you.  If you have doppler correction software, the current NORAD keps are 
very good and should get you close to the frequency, but scan around.

BTW, whenever a change is made in the transmitter or antenna status of 
AO-40, you may hear a change in the downlink signal (assuming the 
transmitter you're listening to hasn't been turned off!!).  This is due to 
two factors. First, the change in the loading on the 28v-buss caused by an 
additional transmitter coming on or off line causes a small momentary 
voltage change on this   buss and a momentary shift in the transmitter 
output frequency.  Second, if antenna switching is involved, transmitters 
are turned off for a fraction of a second for "cold" switching of the 
antenna relays.  Users of AO-13 will remember this phenomenon.


--
  ________________________________________________________________________
  Stacey E. Mills, W4SM    WWW:    http://www.cstone.net/~w4sm/ham1.html
    Charlottesville, VA     PGP key: http://www.cstone.net/~w4sm/key
  ________________________________________________________________________

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