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AO-40 tests



Howdy,

I did the final tests this morning on the L/s system in preparation for the 
expected transponder operation tomorrow. Copied 45/100 good blocks of 
telemetry when AO-40 was between MA 140 and 145. Range was 61,000 km from me 
here in Vermont. All failed CRC's were due to a momentary spin fade.

I'm quite pleased with the final Mode S receive system here. It's based on a 
Meyers dish, DB6NT preamp and Drake 2880. Signal strength was S4-5 with an S0 
noise floor. This translates to ~12 dB S/N on my IF.

My biggest concern with this system has been Mode S desense from my L band 
uplink. When I first brought the system online quite a few months back, when 
I was receiving _weak_ AO-40 telemetry, I'd see the signal be disrupted when 
I tested my L band. This was similar to the Mode J desense some of us are 
familiar with on the FO birds. It didn't really surprise me that much because 
of the proximity of the L/S systems.

When I was looking at designing the L band uplink, I used published figures 
of 10W into a 12T helix. My calculations showed 16dBic gain and 398W ERPc. I 
chose to use a Directive Systems 55 el loop yagi based on its gain of 21 dBi. 
I knew I'd theoretically loose 3dB going from horizontal to circular, so that 
cut its gain to 18 dBi. I setup and measured actual RF power at the feed 
point. I have a healthy 10W. The ERP is 631W. Theoretically 2 dB of headroom. 
What concerned me was this antenna was ~3.5 feet from the Dish. I wasn't sure 
of the impact of the second harmonic on the Drake front-end knowing that it 
was designed for ~2500 MHz. On my system I can see it affecting the Drake. As 
always, your mileage may vary.......

My solution was to add a DB6NT MKU232B preamp. This particular model has 
input and output filtering. I had it retuned for 2400. It comes stock tuned 
for terrestrial work at 2304. It spec'd at .57NF and 16.66 dB gain at 2400. 
This helped the Drake considerably. I do now find I need an additional 10 dB 
of padding in the shack in front of my 2M IF. I'm running a total of 20dB 
padding for S0-1.

I hope to work quite a few folks tomorrow morning. I plan to use L/s and 
occasional U/s tests as well! Look for me high in the transponder. I want 
avoid getting anywhere near the telemetry beacon!

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