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Re: My new AO-40 2.4 GHz downlink isn't working



Eric,

It sounds like you're receiving should be up to the job.  I think you were a
victim of high squint angles.  Essentially, when the satellite comes through
the low part of the orbit (perigee) the antennas are pointing away from the
Earth.  At about the time you probably expected to start hearing the
satellite (1610 UTC?) the antennas on AO-40 were pointed 127 degrees away
from you.  As AO-40 made the swing back around to the West the squint was
still 90 degrees, so you were looking directly into the side of the
satellite's antenna.  The by the time the satellite went over your Western
horizon the squint angle was only down to about 40 degrees.  That makes it
almost impossible for stations to get signals into the satellite.  The
telemetry beacon is generally 10 db stronger than QSOs should be so you 'll
find it is not too hard to find when the conditions are right.

For frequencies, the only beacon that is used is the S2 Middle Beacon (MB).
The MB freq. is 2401.323 but doppler shift can be as much as +/- 50 kHz and
frequency conversion errors in the downconverter can add a bit more
uncertainty.  I'd suggest scanning as much as 75 kHz above or below the .323
value depending on whether the satellite is moving toward or away from you.

Saturday afternoon is looking pretty good with roughly a 4-5 hour period
from 1900-2353 UTC with decent squint angles for making contacts.

Good Luck,
Lee-KU4OS

----- Original Message -----
From: "Eric Rawson" <ericrawson@rawsonoptics.com>
To: <amsat-bb@AMSAT.Org>
Cc: <kn6kc@arrl.net>
Sent: Thursday, April 17, 2003 20:44 PM
Subject: [amsat-bb] My new AO-40 2.4 GHz downlink isn't working


> I turned it on this morning when AO-40 swung east over Central America (Az
~
> 170, El ~ 40; I am near San Francisco), and I couldn't hear any of the 6
> listed beacon frequencies.  I followed the predicted track as AO-40
reversed
> west and dropped below my western horizon several hours later.  But I DID
> hear two data signals (2401.266 MHz and 2400.892 MHz) that seemed
localized
> on that Az-EL bearing (the signal dropped near zero when I moved 40 deg
> off-direction). But I am disappointed not to hear any beacons at the
listed
> beacon frequencies.  Or any other SSB or CW signals in the down-link
> passbands.  Perhaps someone can suggest a fix, or explain what is
happening.
>
> Here is my setup. I have a BBQ antenna modified for circular operation, as
> follows. I covered the BBQ dish with aluminum screen and built a 5.25 turn
> helical feed (CCW twist moving away from the observer).  I added the flat
> shim-brass impedance transformer on the first 90 deg of helix, as people
> describe.  I have a DEM LNA mounted directly behind the feed-helix' small
> reflector plate, then 6 ft of good coax (TIMES LMR Ultraflex), and then
the
> DEM downconverter (to 144 MHz) is mounted about 2 ft behind the dish on a
> counter-balance boom. I run 12 VDC on twisted pair to the RCA-type power
> socket on the Downconverter, and I inject 12VDC into the 2.4 GHz coax just
> above the downconverter using a DEM BT. To carry the IF I use 15 ft of
> RG-58U to my receiver, an ICOM IC-706MkIIG.
>
> A circular-converted BBQ ought to be about 21 dBi, so if the beacons were
on
> I should have heard them if I was pointed in the right direction at the
> right time.  I searched around a bit, but still heard nothing.
>
> Any suggestions would be welcome.
>
> Eric, KN6KC
>
> ----
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