QO-100 setup for transceive operation

  • An example: My QO-100 setup

    Main goal was to achieve a transceive operation with a 70 cm TX/RX, my already existing FT857D. I wanted to avoid tuning in procedures, and the digital mode equipment setup with my FT857D should be usable for FT8 and other modes too.

    The SG Lab. transverter is stabilized with a GPSDO at 10 MHz and the modified LNB is fed with a 2nd GPSDO at 25.7884615384615 MHz to get a transceive setup: -> 10,489.550 MHz - (25.7884615384615 x 390) MHz = 432.050 MHz

    TX: 432.050 - 432.300 MHz to 2,400.050 - 2,400.300 MHz

    RX: 10,489.550 - 10,489.800 to 432.050 - 432.300 MHz

    The modified Megasat Diavolo Twin LNB sits behind the 55 cm offset dish in a waterproven box. I cut the waveguide and mounted a short semi-rigid cable with a SMA plug, to feed the RF input directly via a suitable 1 pF microwave SMD capacitor. There are in total 40 cm of 3.6 mm semi-rigid cable and 60 cm of 10 mm Hyperflex cable between the BAMATECH dualband feed (DJ7GP) and the LNB. Compared to a LNB directly at the focus of the antenna (without the dualband feed) I loose 3 dB of SNR. So this is the compromise I accept for this architecture.

    The CW beacon generate S7 at the FT857D and the SDRplay (running SDRuno) shows -105 dBm with a noise level at -135 dBm (7.5 Hz resolution bandwidth). The difference to the WebSDR is very small. Only the very weak signals which you hardly could recognize at the WebSDR too, are difficuilt to copy.

    The transverter and PA from SG Lab. are in an outdoor cabinet (Outdoor Box 1). Some relais and attenuators are built in to switch between high and low power. Considering the attenuation of the cable between the outdoor box 1 and the feed, delivers 200 mW (for digital modes) or 10 W to the feed. 10 W generates a signal similar to the BPSK beacon level.

    On the RX path a variable attenuator brings the noise level to S0 at the FT857D. The SDRplay is fed via a splitter and another variable attenuator. This is for the visual reception. Of course, the SDR can only be used to a limited extent in transmit mode because of the crosstalk directly into the SDR on 70 cm. For monitoring my own signal I use one of the WebSDRs.

    Power supplies for 24 V (PA), 12 V (transverter and LNB) and 5 V (GPSDO) are within the outdoor box 1.

    Just to get an idea regarding the effort. The overall expenses were about 900 Euro, not considering the already existing FT857D and some parts like relais, plugs, cables and so on. It took me 3 months to set this up.

    CU on QO-100,

    73 de Matt, DF6LO

  • "Compared to a LNB directly at the focus of the antenna (without the dualband feed) I loose 3 dB of SNR."

    I managed to re-gain the 3 dB of decreased SNR by installing a low noise preamplifier from Kuhne electronic at the feed.

    Now I can observe the noise floor of the transponder with about 2 dB and even very low SSB signals are easy to copy. No difference compaired to the WebSDR anymore. :)

    (with a local spurious signal at 432 MHz)

    I observed a strong disturbance of the GPS reception with a lost of PLL lock of the GPSDO obviously caused by the much stronger IF signal due to the pre-amplifier and perhaps harmonics. The 75 Ohm match at the LNB and the components in the outdoor cabinet (splitter and DC feed) might be bad too which lead to standing waves and unwanted emission from the coaxial cables. I had to decouple the GPS antenna cables from the LNB cables and I installed a low pass so called LTE filter. The GPS reception is back to normal now.

    CU on QO-100

    73 de Matt, DF6LO

  • I remember almost being banned from Earth here using this approach.

    In fact the first NB hardware posting here was 'transceive' (or simplex), look here (and read the reactions (in another thread).

    (that posting was the incentive to add the rule "Full-Duplex operation is mandatory (you must be able to monitor your own downlink while transmitting!)"

    It's always the same: people who scream the hardest .. you don't hear them ...

  • I'm fully aware about the mandatory full-duplex operation requirement. Thats why I mentioned the following ... "For monitoring my own signal I use one of the WebSDRs." .. and thats what I'm doing infact. It's helpful to check the uplink signal strength.

    Transceive operation is much more convinient (It's annoying to listen to your own delayed signal and when you don't use headphones it causes echos) and it's a must when you want to use WSJT-X for digital modes with an external soundcard like the SignaLink USB together with the FT857 ... at least I didn't find an alternative solution.

    Well ... but digital modes might trigger the next clash8)