[AReU] AO40 downlink experiments

[w4tas]

I tried some different down converters and antennas tonight while 
the satellite was in a good position. It was almost due south and 
at an elevation of about 50 degrees. It was a very clear spot in the 
sky for me. 

The purpose was to determine the difference between an 18 inch 
offset fed dish with a W0LMD patch (with a 2 in. scalar ring) and 
an R. Myers BBQ grill. I found the BBQ grill to be better by about 
3 db. I didn't expect this to be the case but there it was clear as 
day. I want to try a 5 turn helix with the 18 in dish but didn't have 
one built up today. At last years AO40 antenna testing party at 
Davis park the 18 inch dish with a 6 turn helix appeared slightly 
better than the BBQ grill. The 18 in. dish still did well on SSB signals 
mainly because the satellite was about 42,000 km out and the 
squint was less than 15 degrees. The beacon was strong as 
was expected. With an attenuator in the line the beacon was 
S 5 with the BBQ grill and S 4 with the 18 in dish. It doesn't 
sound like much but it made a noticeable difference in the SSb 
signals. They were perfectly copyable with both antennas but 
closer to "armchair" with the BBQ grill. 

I also compared a Cal Amp converter with a newly acquired 
Transsystems 3731 unit. The 3731 had more gain but there 
was little difference in the signal to noise ratio between the 
two. The Transsystems converter was the lower noise unit
by a "smidgen". 

I also tried adding a Connifer 1033 preamplifier (thanks Jim!!!) 
in front of both converters. I found the preamplifier really helped the 
signal to noise ratio of either one but it was more noticeable on 
the Transsystem unit. The transsystem unit with the preamplifier 
and the R. Myers BBQ grill produced armchair SSB signals on 
a couple of QSO's. 

The Ku band LNB that came off of the 18 inch dish will also supply 
a low noise transistor that I will try in the Connifer preamplifier to 
see if it will improve the gain and noise figure. News on that 
project later. I will also compare the BBQ grill to the 18 inch dish 
with a 5 or 6 turn helix feed in the near future. 

That finalizes the downlink equipment that I will use at the 
BARS/TARC picnic. The uplink equipment will be an FT100 
with 20 watts output on 70 cm and an 18 element cross 
polarized Yagi. 

73

Tony