[amsat-bb] Observations about AGC's on AO-40

["Howard Long" <howard@xxxxxxxxxxxxxx>]

Hello Folks

Maybe someone can help out here about some comments I have to make regarding
AGC's and AO-40.

Last night I had some QSO's and for the first time I managed an SSB QSO on
the L band uplink. Previously I'd only managed to hear my CW downlink with
an L band uplink, but certainly not enough for SSB. The reason for this is
simply down to the cable losses up to the antenna on the roof, so this time
I pointed a 20t helix directly out of the shack door on a tripod.

Here's some results. (Squint varied from 16 degrees  to 10 degrees during my
testing between MA 65 and MA 90 on orbit 404.)

Uplink  Power at    Gain   EIRPc   Downlink   AGC
Band    feedpoint   dBic     W     Strength   Supp
U          20        11     250       0dB     10dB
L1         30        13     600      -6dB      1dB

Of particular interest was that the strongest signals by quite a long way on
the downlink were uplinking on L band (except mine as you can see, but I
hadn't spent a great deal of time optimizing the pointing). Of two stations
I talked to on L band uplinks, one station was running 5W to a 2.4m dish
(est 22dBic gain, 800W EIRPc) and another 40W to four 18 ele linear yagis
(est. 19dBic, 3kW EIRPc). The AGC from the telemetry shows the L1 RX AGC not
flinching at all at under 1dB suppression, but the U RX AGC was at around
10dB during the entire test.

I don't believe LEILA is connected to the L1 uplink.

The U band Rx has a 15dBic gain antenna as does the L1 RX (or so I believe).

On the basis that the L band signals were so much stronger than the U band
signals, I wonder if (a) the AGC on L1 is working, (b) LEILA is perhaps
being a little too picky on U band, (c) the U AGC is being a little too over
zealous, (d) I'm not understanding something.

73 Howard G6LVB


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