Expected SN with a 80cm dish on NB transponder

  • Hi,


    I am completely new to QO-100 and I am starting for now with the receiving part.

    I am using a 80cm offset dish with an Octagon Single LNB Green, adalm-pluto & SDR Console.

    It's working fine but reception is quite weak compared to the Goonhilly Es'hail-2 Narrowband WebSDR.

    The best I can get adjusting my antena orientation and LNB skew is a around 30dB signal-to-noise ratio on the lower CW beacon.

    ¿Is this SN reasonable?, ¿what could I do to improve it?


    Thanks for your help.

    Nicolas EA4GQY

  • morning,


    The pluto is a little deaf!!!

    34dB with pluto

    41dB with my sdr play on a 1mtr dish and on the lower beacon

    Even my rtlsdr stick give bether results...or there is somthing wrong with my pluto!!


    greetings and stay safe and enjoy hamradio

  • Hello Nicholas,


    I believe that with a correctly pointed 80cm antenna and reasonable receive system you should be able to see the transponder noise floor and once you see that then the uplink S/N as "seen" by the transponder is largely the limiting factor. There is a thread here QSB on the beacon signal? where the measurement of the beacon S/N using SDR Console is discussed. Bandwidth settings have an impact on the figure but for his settings Peter DB2OS said 38dB S/N was usual for the lower beacon once you see the transponder noise floor. That's what I see with my 60cm with an unmodified Goobay GL202 LNB.



    73 Chris G4SDG

  • Hello Nicholas,


    I believe that with a correctly pointed 80cm antenna and reasonable receive system you should be able to see the transponder noise floor ...


    73 Chris G4SDG

    I also agree because the other measures are influenced by many factors. This is what I see with a 100 cm dish. They are more or less 6 dB.

    Antenna 100 cm unmodified Octagon OTSLO Twin LNB

  • My way to verify different equipments is to take a look at the ground noise

    I verify the ground noise of a unused part from the transponder with the ground noise 1 MHz beside the transponder.

    So I have no influence from stations and bandwith adjustments ...


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    On my 1,8m PF with OCAGON OTLSO Twin LNB (26 MHz GPSDO) i can see a transponder noise 9 dB over ground noise.


    What do you think about?

  • Hello all,


    here are some sample measurments from my rig:


    - Pluto with 30dB attenuation at the front end

    - also OCAGON OTLSO Twin LNB (26 MHz GPSDO) (same as DD4YR)

    - also 1.8m dish, however, not PF but offset.


    I get the following values:


    - lower CW beacon -73 dBm (this was adjusted manually by me, because it corresponds to S9 on a traditional S-meter)

    - transponder signal -108,7 dBm, so the beacon signal is 35,7 dB above the transponder noise

    - noise outside transponder -115,6 dBm, so transponder is 6,9 dB above the general noise leve.


    Simons SDR Console somehow calculates an SNR of the lower beacon to be 41,3 dB.


    DD4YR: Robert, good idea, but I was not able to measure the ground noise level, because I am glad that my 60kg 1.8m dish is pointed towards the satellite ;)


    EA4GQY: in my humble opinion 30 dB SNR is plausibel for a 80cm dish.



    Vy 73


    Holger 'Geri', DK8KW

  • Just as a reminder: two SNR readings from SDR console with different zoom settings (RX here is 88cm with Pluto, no GPSDO). Don't get me wrong: I love SDR-console but the SNR value is very misleading..


  • two SNR readings from SDR console with different zoom settings

    that is exactly the problem.

    It is difficult to assess signal strengths with different scalings, bandwidths and hardware, since each hardware configuration gives different results.

    I think this way we cannot compare our receiving equipment.

    We need an other solution ....

  • My way to verify different equipments is to take a look at the ground noise

    ...

    On my 1,8m PF with OCAGON OTLSO Twin LNB (26 MHz GPSDO) i can see a transponder noise 9 dB over ground noise.


    What do you think about?

    excellent, but let's say that for the NB transponder antennas greater than 100 cm are excessive. We say it for any new users who maybe worry, they think a big effort is required. For the NB transponder, a 40 cm antenna is sufficient for good activity. I repeat it only for new users.:)

  • excellent, but let's say that for the NB transponder antennas greater than 100 cm are excessive. We say it for any new users who maybe worry, they think a big effort is required. For the NB transponder, a 40 cm antenna is sufficient for good activity. I repeat it only for new users.:)

    Yes. If you can see the transponder noise floor there is little to be gained on receive with a larger antenna and even if you can't by as much as 10 dB you should still be able to copy easily signals at the same level as the beacon. That means a signal level about 19dB down on those reported by DD4YR above, i.e. a 20cm dish, should still be viable. I would not recommend that small but a re-purposed 45cm satellite TV dish should be sufficient.


    On transmit things are different. Smaller dishes need more power. To reach beacon level I need 200mW-300mW or so to my 2.4m dish. This would imply needing 10W to a 45cm dish, but that level of power is readily available, so it's not a problem.


    The reason I am using a larger dish if for digital television, which needs more power, but even then, 1.2m would be sufficient. The benefit of the larger dish here is the 6 dB reduction in power needed, around 5-10W instead of 20-40W. For DATV you need a very linear amplifier and that's more difficult, but not that difficult.


    In other words don't be put off by only having a small dish!


    Mike

  • For the NB transponder, a 40 cm antenna is sufficient for good activity. I repeat it only for new users.:)


    This is not what AMSAT recommends and how the Link budget for QO-100/Es'hail-2 was designed :!:


    A 60cm should be the minimum for receive...


    A 40cm dish might be sufficient for portable operation, but only if all components of your receive system are fully optimized and you are in the center/middle regions of the QO-100 footprint..


    Anyhow, we should always improve hearing performance and not become a crocodile... ;)

    Peter Gülzow | DB2OS | AMSAT-DL member since 1983 | JO42VG