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