POTY and return loss plot

  • I repeated the measurements with a short 20cm pigtail from the patch to the miniVNA Tiny and according recalibration. This is what i get now:


    This looks much more consistent with what I would have expected. Thanks for the hint DL9SEC . Those larger loops are not observed anymore (although they were to be expected as I've learned here) and the measurements are less noisy. I simply wasn't aware of the impact of the cable length. I put markers 1 and 2 where I would suspect the double dips to be located and marker three around 1.4GHz.


    G0MJW Indeed, the real test is what counts. Unfortunately my PA is not ready yet, so I can happily induldge in more or less meaningful measurements.


    To me everything looks okay-ish so far and I think I can leave my patch as it is and start doing some careful tests on the air as soon as my PA is finished.


    Just for my own understanding: taking the simplified two antenna model into account and assuming that in my measurement above marker 1 corresponds to the inductive and marker 2 to the capacitive antenna: their difference in phase, should be 90° in the optimal case. Is this correct?


    Thanks to everyone so far already

  • G0MJW we do have a link budget with is in very good agreement with the observed performance. But as you feared, this has too much NDA'ed information in it and for the sake of staying clean, I will not publish any numbers myself (sorry for being paranoid).

    However, if anybody else is generating a spreadsheet which can be verified by the QO100 community, there is nothing we can do about...

    OK - understood. Not everything in the spreadsheet will be under NDA though. I see the noise floor at ~10dB. Assuming antenna gain of 45 dB and a noise temperature of 400k I have a G/T of 19 dB. PSK Beacon SNR is 31.5 dB. This is according to SDR Console, I don't know if it is right but as long as it is consistent it does not matter.


    To equal that SNR in the same bandwidth with a similar signal, I need to transmit about 200mW.


    This is where errors start to creep in as I find if I TX at this level I am a lot weaker than some other stations who are apparently sending at the same level as the beacons.


    My dish gain on 2.4 GHz is (assuming Willi's simulation is correct) is 32.5 dB so an EIRP of 25.5 dBW is what I need to reach the beacon level. It should be mostly the same for everyone else.


    Now we could say at this point job done. To have a sensitivity such that the transponder noise is equal to the antenna noise one needs a dish with 7 dB less gain (not exactly, close enough though) other things being equal. -7 dB equates to 1/5th of the area, about 1.1m. The equal noise is an arbitrary target by the way but a fair one.


    There are some improvements to the ground station that could be made, I assumed 50% efficiency and a noise figure of 3 dB and about 120k dish temperature. Realistically, this could be improved to 200k with a lower noise figure and better side-lobes with 70% dish efficiency. That's probably the limit without going to extremes. That would allow another 3 dB and a 75cm dish.


    Part 2 after lunch

  • To get a specific SNR with known EIRP tells us what the satellite G/T is. Unfortunately I do not know what the SR bandwidth is in SDR Console but luckily I did some spectrum analyser measurements recently.



    The beauty of this is I can go back and do analysis on the data. This tells me the Resolution bandwith and other settings and allows me to measure stuff - the result of which is the PSK beacon is 18 dB above the transponder noise floor in 3 kHz. The beacon is 400 baud BPSK and from observation the majority of it's power lies within 1-1.5 kHz so we may have a correction to make.


    However, looking at the data the analyser was set to peak so the noise is peak detected too, and video averaged so it's level is somewhat uncertain. What I do know is it is at least 18 dB, probably 22 dB, possibly more. Too much uncertainty. Send contrary arguments to my server at /dev/null.


    If it were 22 dB, plugging this into a link budget gives a G/T of something like -6dB or -5.5 dB. That's not so unreasonable as a global satellite beam is about 22 dBi. However the errors in guessing the beacon EIRP based on my 200mW to equal it and not being quite sure where the noise floor is are too much.


    Time to try a measurement I think -

  • To get a specific SNR with known EIRP tells us what the satellite G/T is. Unfortunately I do not know what the SR bandwidth is in SDR Console but luckily I did some spectrum analyser measurements recently.


    The beauty of this is I can go back and do analysis on the data.

    Hi Mike & all,

    Maybe we can ask Simon G4ELI, how does the software work in this regard.
    On the latest release 3.0.13

    https://www.sdr-radio.com/Software/%F0%9F%92%BEDownloads

    he added a few niceties, notably an analog S-Meter, wich he describes here:
    https://www.sdr-radio.com/Console/SMeter

    73 de CT2IRJ

    Salomão Fresco

    Callsign: CT2IRJ

    GRID Locator: IM59re -- CQ Zone 14 / ITU Zone 37

    QTH: Fazendas de Almeirim, Portugal

  • Quick test with G4JNTs Signal-Noise monitoring package - 1W CW TX, SNR 58 dB Hz which suggests an uplink G/T of -11 dB/K. That is much worse than I expected so either my dish is not good or it's not very sensitive. I do recall much better results earlier on before the gain was reduced, but I don't think it could be that bad so perhaps a combination of both. It would be nice to have some other measurement datapoints.