Maintenance on the NB Transponder!

  • It is always a tradeoff: we could have designed the transponder to work with a piece of wire only, but then all power would have to go into one single QSO. We had to define a minimum ground station for SSB and this was 60-80 cm (depending on location and weather). For CW, less is possible even today and for JT65/FT8 probably even less.

  • Hi Lucio,

    there must be another reason.. the transponder parameters on the satellite are unchanged during the last week..

    73s Peter

    Hi Lucio, sorry I was reading the wrong date (20-23 March), but my statement concerning "last week" still holds :-) As Achim pointed out, transponder gain was adjusted in several steps to find the optimum performance. More details about the analysis will be published soon on our website and future issue of AMSAT-DL Journal.. The WB is unchanged since we started transmitting the DATV beacon.

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

  • We had to define a minimum ground station for SSB and this was 60-80 cm

    Of course I agree with that - my remark was only to indicate that something has changed, not that it was for the worse.

    60cm dish is not so bad even for a portable station.

    (of course not portable like a handy, but easily transportable and setup to make qso from a rare location or for emergency communications)

  • Thanks all for clarifying. I expected such situation, just needed some confirmation.

    I remember the first day perfectly listening to SSB QSO with DRO LBN only. That was above my expectations.

    Yesterday I have received beacons and some SSB signal with LNB only, but signals were very weak.

    However this is still promising. I expect adding any small suitable antenna or reflector with 10-20 dB gain should be sufficient for nice portable or even mobile work.

    P. S. I have 1,2m dish, but it is not interesting. It is like working local repeater with roof mounted antenna ;)

  • SV1BDS George, it is not 'that simple' (because you/we? see the noise floor +5 dB 'in Greece'). Suppose -in your situation- the tpx gain will be decreased with another 5 dB, you will see (with your setup) 0 dB tpx noise.

    In other words, you can't see an increment due to the tpx.


    So the noise of the tpx will be the same as your system noise. However, these 'noises' are not correlated, but ... add up stochastically, meaning you've 3 dB more noise (system_noise + tpx_noise = 2 = +3 dB). That means that the detoriation of weak(er) signals is more than 3 dB (refer to the 'noise figure formula' of a receiving system).


    The tpx gain ('noise') is set up in such a manner that the given user requirements of the link budget (dish size, or better ... G/T, see ITU ES'hail-2 document) are met.


    (I commented a few times about this on this forum, but can't find it now .. ; -) Iirc the ground station requirements are around 75 - 80cm dish diameter in the 'average coverage area' (<-- my words) when the uplink power (using the same diam dish) is around 5 - 10 Watts for 20 dB C/N @ 2.5 kHz.


    So, if you use a 1.6m dish you receive these 5 - 10 W uplink with 26 dB C/N, and an increment of the tpx noise. As a consequence, with this 1.6m dish you can reduce your uplink power with 6 dB : -)


    Summarized: dish size around 75-80 cm diam, and 5 - 10W uplink power has to produce 20 dB C/N in 2.5 kHz BW 'averagely'.


    Having monitored the tpx since the beginning I think the reduction of 10 dB (in two steps, first 4 dB and then 6 dB) is a good choice and heeds the formulated requirements and ... improves dynamic range : -)

  • PA3FYM Hi Remco,

    With a 60 cm dish and a LSP04H LNB I receive the CW beacon with 34 dB SNR. With 3W CW and a 80 cm dish I am 4 dB under the CW beacon (30 dB SNR). When I reduce the output power by 10 dB I see 10 dB drop (20 dB SNR). Again if I drop my power another 10 dB I see another 10 dB rx drop (now 10 dB SNR). From these measurments I think that the real (maximum) dynamic range is now 34 dB.

  • SV1BDS I have no in depth knowlegde concerning the real parameters of the tpx (however, some people on this forum have ;; -) but from the ITU user requirements it is possible to do some calculations. If these calculations meet reality is difficult to say, Formally the link budget is part of the ITU/IARU satellite coordination procedure. However, my experience in a former life is that when these coordination parameters are violated/exceeded there is no strict enforcement regime 'because the bird is in the sky'.


    So, 34 dB might be plausible. What I know when the tpx sensitivity was maximal, there were more situations the AGC of the tpx was activated. Nowadays this happens rarely.