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AO-40 Update 2001-07-20

A couple of updates and comments on amsat-bb postings.

1. We are currently using Leila2, not Leila1 which was used previously.  It 
appears that the thresholds for Leila2 are slightly different than for 
Leila1, so some adjustments are needed to bring Leila2 to the proper 
threshold.  It does work, but the threshold currently appears to be a 
little too high.  That is, it is not triggering as often as it should.

2. The U and L-band receive antennas are right-hand circularly 
polarized.  This should be most obvious at very low squint angles.  As the 
squint increases, the difference between RHCP and LHCP will be less obvious 
and at some higher squints, LHCP may be slightly favored.  This was also 
observed on AO-13 and is as predicted in modeling of circularly polarized 
antennas at various offpointing angles.

3. AO-40 is not deaf :-).  In fact its receivers and antennas are 
incredibly sensitive.  It is, however, a limited resource.  It can only 
output so much power and if the passband contains numerous strong signals, 
the power available per signal will be reduced.  This may be reflected in 
the S2 Tx ALC setting.  More importantly, if multiple strong signals 
bombard a receiver, the receiver AGC will reduce its sensitivity by 16 dB 
or more.  This can very easily be seen in the P3T telemetry for the U/L1 Rx 
AGC's when the passband is on.  So if one strong station causes the U-band 
Rx to reset its AGC level, everyone's signal on the downlink will be 
reduced accordingly.  This causes everyone to raise their power, which 
causes the AGC to set even higher, and a vicious cycle ensues which the 
user can never win.  Hence the reason for Leila, which, as noted above, is 
not currently set optimally, but will be adjusted shortly.   ...so please 
bear with us, we are still in a "check out" mode.

4. There is no V->S2  passband currently in the schedule.  The V-Rx 
alternates with the U-Rx when the passband is not active.  This can be seen 
in the "N" schedule block.

5. RUDAK tests are currently scheduled for Orbit 339, which begins early on 
2001-7-26.  The passbands will be off for this orbit and the S2 beacon may 
be intermittently off as well.  Until then, the passbands will be active 
from MA = 10 to 100.

6. In the near future, we also will be testing the S1 transmitter passbands 
during times of optimal squint angle.  The higher gain of the S1 antenna 
should give better signals farther out in the orbit, as long as the squint 
is good.  This will probably be done as we move closer to 0/0.

Enjoy AO-40 this weekend!!!

  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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