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AO-40 Test 05/08 (long)



I got into the shack at 1004 UTC MA 138 Tues morning. The night before, I had 
hooked 2 separate 2M IF rigs on the output of my S band downconverter. The 
idea was to copy/display telemetry while working the transponder. I 
specifically wanted to monitor the U & L1 receiver AGC levels. AO-40's range 
at this time was 60,000 km from me.

The first thing I noticed was how well I was copying the telemetry. Over the 
course of 2 hours I copied 94% of 600 blocks. The 6% failure was due to the 
fact I was tuning the rig by hand, and also talking with folks at the same 
time. I need a RAM upgrade to better multitask :-}

I used AO40rcv fed to P3T to monitor the AGC receiver levels. The idea being 
to use the minimum amount of power consistent with a readable downlink with 
minimal AGC impact. Shortly after I got on, I was joined by Bill, W0OQC. We 
proceeded to adjust uplink power levels up and down to see the effect on the 
AGC. With 20 watts into a KLM 40 CX Bill was a good 3 S units above my noise 
floor. This was quite good copy. FYI, Bill dropped to about 3 watts or so at 
one point and I could copy him!

Around 1019 we were joined by Dick, WD4FAB who also had good downlink 
quality. His signal also had minimal impact on the AGC level. Shortly after 
Dick joined us, I started seeing the AGC levels jump radically. Someone else 
was on the transponder. I could clearly hear Bill's signal dip on the 
downlink when the AGC peaked.

My next tests were on L band. My first L band test was at MA 151. Range 
59,800 km. With my 600 w erp linear signal, I could see no AGC effect. My 
downlink was barely an S unit. Squint at the time was ~20 degrees.

I went back to U band and proceeded to work W4AD, XE1/DG8NAO, N6ORS, N4QWF, 
ZL1AOX and LU4EBC. I tried L band once again at MA 165 Range 58,000 km. I 
still saw no AGC action.

Conclusions:

I filtered my telemetry and graphed AGC in P3T. It showed Bill, Dick and I 
averaged about 6dB. We probably could have dropped our power more, but I 
believe it would have been rough copy because of the squint on the S 
downlink. When Bill's signal was being effected, the graph showed some 16-18 
dB peaks with 10 dB averages. There was one big peak in the beginning of my 
test. I sent DITs for one complete telemetry block with my station at full 
power. AGC was 20.4!!!! This was at 60,000 km. I'll _never_ do that again!

The L band graph was a flatline. I never did move the AGC with my uplink. I 
was a little disappointed in this, but with the distance and squint I can 
understand it. Also, during my operating on Saturday, I found L didn't work 
well for me until about 55,000 km.

I want to do further tests on L band. I think my system will be more than 
adequate once AO-40's squint is reduced. I think we'll also see a stronger S 
downlink when S1 is turned on with reduced squint. S1 has 8 dB more gain than 
S2 (as I recall...)

UPDATE!

I worked AO-40 Wed morning 1045 UTC MA 218 Range 36,000 km with Jim, KK3K and 
Roger, W3SZ. I again tested L band and 600 w erp linear nudged the AGC 2 dB! 

Recommendations:
U band max uplink of 400 w erpc when AO-40 all the way out. This produced a 
solid 3-4 S units above the noise floor on my receive system. 600 w erp 
linear on L will begins to work at ~ 55,000 km yeilding an S or so of signal. 
600 watts linear (theoretically 300 w erpc) is too much once AO-40 begins to 
get closer than 36,000 km as AGC action apparent.

These numbers are bound to change as the squint reduces. I can't wait until 
AO-40 is stabilized and pointing right at us! I see great DXpedition 
potential for a small U/s system! When figuring your uplink budget, you must 
use ERP. Power output of your radio alone is meaningless, so brush up on your 
ERP calculations.

73,
Mike, N1JEZ
AMSAT #29649
Local Area Coordinator
"A closed mouth gathers no feet."
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