[amsat-bb] AirSpy & LNA questions

[Hans BX2ABT]

Man, this sat stuff is difficult and I was getting a bit frustrated with 
lack of progress, to be honest. But just when I thought I didn't get 
anywhere, I went a big leap forward. But I'm not there yet, so I need 
your input.

My goal is to get some decent reception of the major telemetry sats, 
e.g. Funcube-1. I have two rigs: an ICOM IC820H and an AirSpy Mini SDR 
running on a Linux box with GQRX as front-end. I have a Chinese Arrow 
Alaska clone pointing 200 degrees with 20 degree elevation. The other 
antenna currently is a turnstile tuned for 137 MHz, but I have 2/70 DCAs 
as well (they had to come down this week because of a typhoon. Will be 
up again shortly).

The bare antennas do fine on terrestrial signals, but up till now very 
few sats were heard (mostly the Xiwang sats and Athenoxat-1) and signals 
were very very weak. I got myself some proto boards and PGA103+ MMICs 
and built two LNAs according to schematics found on the web. On the 
workbench they perform as they should and give some 15-25 dB gain 
depending on what frequency is used. I use an old AM/FM signal generator 
and an RTL-SDR as crude scope since I don't have a real one.

I mounted the LNAs as close as possible to the antennas and they are fed 
with a bias-T from Minikits.com.au. The first results were 
disappointing: I used the AirSpy and noticed a lot more signal but also 
an increased noise floor. With the AirSpy there is a lot of fiddling 
going on in the software and I managed to get some improvement by 
tweaking the LNA gain, Mix gain and IF gain. Still I couldn't hear any 
sat signals.

This afternoon I switched from the AirSpy to the IC820H and 
lo-and-behold very strong beacon signals from XW2A and F; the strongest 
I've ever heard. Even the AirSpy did detect a signal, although 
not-at-all as great as the IC820H. To get a cleaner signal from the 
IC820H I did have to engage the attenuator on the set, though. Using the 
IC820H I also heard a very weak beacon signal from CO-65, but the AirSpy 
failed there. Conclusion: the IC820H could detect signals that the 
AirSpy could not detect at all or detect them much better.

Pretty happy with this step forward, but not satisfied. Question is how 
to improve even more.

Q1: is the LNA desensing the AirSpy (and to a lesses extend, the IC820H) 
and if so, how to fix that? I read something about an attenuation 
network after the LNA to improve the front-end of the receiver. Might 
that help?

Q2: when not powered, MMICs seem to have a very high insertion loss. Is 
there a way to remotely bypass the LNA in order to compare reception 
with and without the LNA?

Q3: would filtering help? E.g. bandpass filters for 2m and 70cm. I do 
realize that this would mean another couple of dB of insertion loss, but 
maybe that disadvantage outweighs the advantages.

I would appreciate any input on the above and also would like to hear 
from others who use the AirSpy Mini for sat reception. I already did a 
lot of googling, but can't find any clear cut answers. Hope you all can 
help me go into the right direction. Cheers,

Hans
BX2ABT