[amsat-bb] AirSpy & LNA questions

[Ryan Butler]

Yes you are on the right track, check out the following two links (similar
designs) but also provides a description of why bandpass filtering is a
must with an SDR.

https://www.rtl-sdr.com/designing-low-noise-uhf-front-end-sharp-filtering-sdr/
https://sivantoledotech.wordpress.com/2015/01/27/a-selective-and-robust-uhf-front-end/

Ryan, NF0T


On Fri, Jul 13, 2018 at 9:30 AM Hans BX2ABT <hans.bx2abt at msa.hinet.net>
wrote:

> 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
>
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