Digital Tests on NB Transponder

  • As I understand these are weak signal modes.

    For SSB about 1kW EIRP is estimated to be necessary.

    How much TX power is necessary to make a connection in the typical weak signal modes?

  • @db6rs: That will depend on the mode. People have already managed to get 100mW-200mW to work on CW, 500mW on SSB. Considering SSB requires ~6db more, that fits within the ball park.


    For a first *guestimate* take how much less S/N your chosen digital mode needs over CW, and calculate from that, that should give a good idea.


    However, you must take into account the antenna used. The 100-200mW CW station was using a 50x60m dish. the 500mW SSB station had a 1.2M dish


    Also, we don't know exactly what will happen when more folks start to use the transponder, figures may change if it gets very busy, It will be fun finding out...

  • For WSJT-X modes, it depends on the mode in use. Signals levels are normally referred to as signal to noise ratio in 2.5kHz bandwidth. JT65 (1m periods) works down to about -25dB, while QRA64 (1m periods) can go 2-3dB weaker. FT8 (15s periods) can go down to about -20dB. SSB becomes difficult to read at about 0dB, so with digital it would be possible to use very low TX ERP and still make QSOs.


    There are other digital modes that may also work well for satellite use. Some of the JT9 fast modes could be interesting, with periods of only a few seconds possible.


    Tests done so far here have been back to myself - one PC connected to uplink radio and another to the downlink radio. I set TX power to give me about -12dB for initial tests.



    73, Charlie

  • Hi Digi folks


    Following change of LO here to a clean one that drifts more, I am trying JT9F 10s periods and it is working very well. Drift is probably too much for FT8 as it is also problematic with JT65A. 10s periods are good too as it progresses QSOs faster!


    73


    Charlie

  • Hallo 17:28:15UTC GOOD RCV FT8 CQ DD4YR JN57 +5dB

    Jürgen, tnx for info!


    I made this afternoon a lot of tests:


    Finally it was necessary to stabilize the PLL of the up-converter with GPSO to read signals down to -15 dB stable ...


    (btw: 50mW in a 1,2m dish results 10 dB SNR FT8)


    73´s de Robert

  • Yes, agreed you need a very stable link to be able to work with FT8. My unlocked LO is marginally good enough for FT8 (and JT65A). I decoded you today a number of times with JT65A. My RX is GPS locked here.


    I found today that with my slightly drifty LO, I can work with JT9F fast with 5 and 10s periods. I received myself via the BATC Web SDR for these tests. It seems very stable. Also tests showed JT65C was better than JT65A.


    Also tested JT9E (fast), MSK144 (wideband), and ISCAT B. All seem fine with 5 sec periods.


    Given that in most cases S/N margins are good, we may not need to use modes with 60s periods, and use of these will be compromised if frequency stability is going to be problematic for many stations.


    I managed an easy QSO today with OZ1CTZ using JT9F. I also managed a QSO on FT8 but this was more difficult.


    Of course as technology improves, FT8 will still be interesting as you can pack so many users into a very small bandwidth.


    73


    Charlie

    g3wdg1@gmail.com

  • The drifty xtal oscillator chain has now been replaced by the original 10MHz locked synthesiser, with a redesigned loop filter. It now has a much cleaner spectrum.


    After switching back to the locked LO, I found that FT8 was then very reliable.


    73


    Charlie

  • Charlie, can you receive yourself? When, how deep can you receive in the noise?

    The best result during my tests was -15 dB. Nothing to see from the signal in the spectrum.


    73´s

    Robert

  • Hi Robert


    Yes - I have one PC connected to the TX and another connected to the RX and can decode myself with no problems. In the first round of tests (with the locked LO but poor signal quality) I got some FT8 decodes down to -20dB. I'll try some more tests tomorrow morning (I may have to find some more attenuators to put in the TX path :-). DT generally between 0.4 and 0.5s.


    I usually test with dial frequency mid band in the NB digital modes segment, QRM permitting (have not had any QRM yet).


    73


    Charlie

  • QRA64 Mode A

    found the problem on my TX side and solved it:

    The LF at the TX input was overdriven. I´m looking forward to a new attempt ...


    BTW: made yesterday after solving the TX problem perfect QSO in KG-STV with DL9DAC ... :)



    73´s de Robert

  • DD4YR


    I did some further tests here just now and was getting FT8 decodes reliably down to -20dB. This seems to be about the threshold for successful decoding here.


    This is about 10dB below what I can see on the BATC WebSDR waterfall. SIgnals decoding at -10dB are just visible.


    73


    Charlie

  • Hallo...


    i got an SWL report from ON7KGK on my QRP tests yesterday evening after my QSO with DK2RO... (see attached)


    MY TX:

    LIMESDR + GNURADIO GRAPH (by DG0OPK) + AUDIO via PULSEAUDIO LOOP from WSJTX 2.0.0 ...to GNURADIO


    Amplifier(?):

    MMIC DRIVER max.10dBm + DL2AM DRIVER 0.5W max.. LIMESDR TX POWER was set to 40dB (max is 60dB)...


    TX-ANTENNA: PF DISH 60cm , simple patch feed


    NO GPS LOCK !


    BTW: i decoded the same on my monitoring RX...