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Thread: RSSI Values compressed

  1. #11
    Passenger jackthom's Avatar
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    I tried taking the gain down to 10 initially and the range and number of messages per second both took a nosedive, as might be expected.

    Increasing the gain gradually up to max gave an improvement all the way as far as I could tell, the number of messages etc never seemed to go into reverse.

    I'm regularly seeing aircraft around the 150nm mark and the system is working reasonably well, it's just that the RSSI values rarely get better than -31dbFS. They certainly don't seem to be following anything close to an inverse square law in relation to aircraft distance, so I guess I'll try to filter and amplify until the 1090mHz signal levels are less dwarfed by the local cellphone transmissions around 960MHz.

  2. #12
    Captain abcd567's Avatar
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    960 Mhz is too close to 1090 Mhz, and there are chances that Flightaware filter may not fully clear it. If this happens, then you will need a Cavity filter or Ceramic Filter.

  3. #13
    Passenger jackthom's Avatar
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    I'm afraid that's right. If anything the FA filter caused the number of messages/sec to fall a little rather than improving things, presumably because of its ~2db insertion loss.

    I already ordered a RTL-SDR.com 1090 LNA/filter last weekend (coming to UK from China), so I'll give that a try when it finally arrives. They claim its triple filtering achieves around -50db at 960MHz.

    Hope that's not just sales talk.
    Last edited by jackthom; 2018-05-31 at 13:26.

  4. #14
    Passenger jackthom's Avatar
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    Just an update on the original problem which wasn't fixed by the RTL-SDR 1090 LNA/filter.

    I have set up an alternative Raspberry Pi 3B+ rig, this time using "dump1090-mutability_1.15~dev".
    Testing this, while using the same aerials as before, the RSSI values are now looking much more sensible.
    The previous odd compression of received signal strengths has gone and they now range between approx -1.5 and -33 dbFS.

    I have a cavity filter on order from Osmocom to try and sort out the old setup (too late to cancel it) although I'm no longer so sure its entirely down to high signal levels from nearby cellular masts.
    I guess in any case the cavity filter can't do any harm and it may well improve the whole system besides fixing this "problem", considering that no-one else seems to have have reported any similar issues with the previous version of dump1090-mutability.
    Last edited by jackthom; 2018-09-03 at 10:58.

  5. #15
    Purser
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    jackthom: Would like to hear how you go with the Osmocom cavity filter

    ylis

  6. #16
    Passenger jackthom's Avatar
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    Quote Originally Posted by ylis View Post
    jackthom: Would like to hear how you go with the Osmocom cavity filter
    Hi, it arrived yesterday and all I can say for definite is that it didn't cure the problem I had with RSSI readings which I've now put down to a Dump1090-mutability 1.14 software oddity.

    The filter hasn't had a noticeable effect on range or numbers of messages/second so I'm guessing it's actually doing it's job properly in a setup that probably doesn't need it.
    It seems the filters in the LNA's I've been using were good enough to counter any nearby strong signals at this site and the cavity filter is overkill.
    I would like to get together some broad scans of the spectrum around 1090MHz with and without it for comparison when I have the time.

    When I eventually get around to installing the LNA at the mast head, the cavity filter will be going in front of the amp just to make sure it's bombproof.

  7. #17
    Captain abcd567's Avatar
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    Quote Originally Posted by jackthom View Post
    Hi, it arrived yesterday and all I can say for definite is that it didn't cure the problem I had with RSSI readings which I've now put down to a Dump1090-mutability 1.14 software oddity.
    Have you tried dump1090-mutability ver 1.15~dev ?

    How to Install dump1090-mutability_1.15~dev on RPi
    .

  8. #18
    Passenger jackthom's Avatar
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    Quote Originally Posted by abcd567 View Post
    Have you tried dump1090-mutability ver 1.15~dev ?

    How to Install dump1090-mutability_1.15~dev on RPi
    .
    Yes thanks abcd567, wish I'd tried it earlier.

    I only began experimenting with 1.15~dev a few days ago and it did solve the problem plus it's possible it has also improved the number of messages/second.
    I'm now using it on my main receiver as well as a test rig.

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