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frequency jumps while listening to AO-40??



Hello, all!

I am writing with a question about something I observed listening to AO-40
today.  I was listening to the 2401 MHz downlink and periodically (perhaps
every few minutes) my received signals (whether my own uplink, another
station's downlink, or the MB) would shift erratically down in frequency by
about 300 Hz, and then return to the original frequency.  This happened very
irregularly, but persistently, until MA 220.  After the passband was turned
off, the beacon was of course all I could listen to, but whereas it had been
erratic as well, it was rock solid in frequency (with the expected Doppler
drift, of course).  There was no change in signal strength or in the
background noise level as might occur if there were a problem with the DC
supply voltage to the UEK-3000.  THe whole thing reminded me of how AO-10
has sounded when it gets 'flaky'.

I am trying to figure out if this represented a downlink problem here, or
something with AO-40.

I was not using any automatic doppler correction.

My receive setup is a BBQ dish with a UEK-3000 mounted right at the dish,
coming down to an ICOM IC-970.

I did the following to check it out.

1.	noted, as above, that it affected both transponder and beacon signals
from AO-40

2.	Checked my ICOM 970H receiver against a local 2 meter beacon for an
extended period.  NO frequency shifting observered.  Went back to the bird
using the same ICOM 970H as receiver IF.  AO-40 signals still jumping.
Thus, not the ICOM's fault.

3.	I used an alternate source of 12 volts and a bias tee to supply the
UEK-3000 to see if this made a difference; it didn't.

4.	Used my ICOM 970H 2401 MHZ module to transmit on my super looper while
receiving on the BBQ / UEK-3000/ICOM 970H combiination on 2401.  NO
frequency jumping or shifting observed.  This would indicate that it wasn't
the satellite receive chain here that was jumping.  By the time I did this
it was after MA 220, and so I couldn't confirm that the AO-40 downlink was
still jumping, as after this time the beacon was stable in frequency except
for doppler.

5.  I couldn't use the 2401 module in the IC970 and the super looper to
compare received frequency stability between this and the UEK-3000 setup
before MA 220 as the superlooper/2401 module were not sensitive to pick up
enough signal to do this until the bird was much closer.  (I had forgotten
how far superior the UEK-3000 and BBQ grill are to the 2 superlooper array
and the ICOM 2401 module in the 970H).  This would have been definitive, but
couldn't be done.

So I wonder if anyone else noticed this instability; I don't think I would
have noticed it if I was operating SSB, but with CW it was obvious.

There are two possibilities

1.	It was a real AO-40 phenomenon that for some reason stopped when MA 220
came around (power budget related??)

2.	I have an intermittent problem that just happened to stop when MA 220
came around.

Any thoughts / help would be appreciated.

73,

Roger Rehr
W3SZ   FN20ah
2 Merrymount Road
Reading, PA  19609-1718
http://www.qsl.net/w3sz

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