Diamater of RX antenna

  • if you want to RX datv.... the bigger dish the better.

    the current transmission is using QPSK with 1/2 fec.

    only 1 dB c/n is needed to view it now. thats why many people can rx it.

    But if you use such a low spectrum density at 333Ks or worse 125Ks

    we are talking more about sstv then....instead of atv.

    So i go for quality.... and thus you need more db's at rx'ing point...

    So the bigger, the more transmissions you will be able to see....

    Even the higher quality ones.

    HD at 9mbits/s will not be possible... unless you had very very high power and a very very big dish!

  • I see some of the web sdrs are using 1,8 meter dish but I notices s/n was low so I became wary of if 0,8m size was enough for rx. However of course that has probably more to do with that the signal on the uplink has a low s/n. It is hard to say when we have no reference yet to what is normal on the downlink. I will do some tests with 0,8m dish.

  • Johannes

    i wil check it tomorrow evening, when i have time, on the SDR. Station is closed at the moment for today. I was using RTL SDR at first, but since i modified the octagon for IF 439MHz, i use FT847 for rx, whereas the noise level is S7-8 on the transceiver when using the small 30cm dish.I just switch on the ATT on the radio. One of these days i gonna try a variabel attenuator btween the biasT and the radio. With only the LNB(without dish) for rx, not yet the level, just by ear,hihi.

    73's Jerry

  • i have also tested only Scanner-RX and LNB in Germany ....

    ... it works :-)

    little Video from my test

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    vy 73 de Andy, DK5NAD

  • Lieber Thomas,

    ich würde mir über "driftenden" LNB erstmal nicht zu viele Sorgen machen, Du bist ja auch "Doppler-Shift" erfahrener OSCAR-User... Ich würde mir eher Sorgen um Deine 60cm Schüssel (für den Empfang) machen, die ist eigentlich unterhalb der Empfehlung für unsere Region.. da musst Du schon wirklich alles optimal ausrichten und auch einen performanten LNB haben... aber solange Du das Transponderrauschen noch siehst ist natürlich alles gut! Ein größerer Empfangs-Spiegel macht dann auch überhaupt keinen Sinn, da sich das Signal-to-Noise Verhältnis der Nutzersignale nicht verändert.. ;-)

    Dear Peter, DB2OS

    I know it's a bit out of topic regarding "Umbau Octagon", but...

    When you write that "as long as you see transpondernoise all is well" and "a bigger dish makes no difference to S/N ratio" this is not quite true :(

    I have seen this statement in other postings as well, but seeing the transponder noise floor, does not mean reception cannot be improved.

    In case of a 70 cm dish we see a typical transponder noise floor of around 3 dB - this is compared to the noise level outside the transponder bandwith. Actually what we see, is that the transponder noise - and the background noise within the transponder bandwith - are adding up and the sum of the two is 3 dB stronger than the background alone (or transponder alone if that could be measured).

    A bigger dish will receive more transponder noise, but the same amount of background noise (assuming identical LNB and antenna noise temperature). The background noise becomes less important and the S/N ratio increases. With a really big antenna, the transponder noise dominates over background noise and we can receive the satellite with up to 3 dB better S+N/N than in the "3 dB noise floor" example above.

    The noise coming from the transponder and from the background (LNB + galactic) are unrelated, so the noise power adds up in the transponder bandwith. This could also be shown on a spread sheet.

    A 3dB noise floor using a minimum size dish is in my opinion a good compromise between small setup and reduction of S+N/N ratio.

    73 Ole

  • Hi Ole,

    Many thanks for your comments..

    I indeed simplified it probably too much, but at the end we have the same conclusion and aside from physical correctness as a thumb rule it's still valid..

    Those who don't see the transponder noise either have a too small receiving antenna or bad adjustment. and they will have less receiving performance in general.

    The "3dB noise floor" compromise fits exactly to our pre-launch recommendation for the required dish size for receiving QO-100...

    For those who want to squeeze out the last of a dB can indeed have larger antennas, but at some stage it does not make sense any more..

    Indeed we all should try to optimize and aim for the better, in particular in ham radio.

    73s Peter

  • DB2OS

    Changed the title of the thread from “Diamater of RX antenna for Eshailsat?” to “Diamater of RX antenna”.