[amsat-bb] AirSpy & LNA questions

[Hans BX2ABT]

Hello Ryan,

Thanks for your reply. The article was very interesting, but as stated: 
finding SAW filters suitable for other bands might prove difficult. Up 
till now I haven't found a suitable SAW filter for 2 meters. The helical 
filters are easier to get as Temell has a range especially suited for 
ham bands. They're a Taiwanese company, so this time I'm lucky and can 
order them locally.

As someone else pointed out: an FM band notch filter might also help, so 
that will get on my list as well.

73 de Hans

BX2ABT


On 07/13/2018 10:41 PM, Ryan Butler wrote:
> 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 
> <mailto: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 <http://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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