Posts by G3WDG

    I have been checking out today the receiver I intend to use for the NB transponder. This uses an EME quality preamp (ca 0.5dB noise figure) at the feed with coax feed to the downconverter at the back of the dish. The feed is a simple 22mm copper pipe squeezed down at one end to fit inside a WG16/WR90 waveguide flange.


    I have two preamps available, from DU3BC and F1OPA. They perform very similarly on EME, but I wanted to check first their gain response at 10.706GHz, since they both use internal bandpass filters. The results are given below. The DU3BC has slightly higher gain and was selected for initial tests.


    The downconverter is an old WDG002 design, which has a two stage preamp (tuned for 10368) and a pipe cap type image filter (tuned to 10488). Performance at 10706 is degraded compared to 10488, mainly due to the attenuation of the image filter at 10706. LO injection is at 2586MHz from an LMX2326 synthesiser, locked to 10MHz.


    Even though it is operating well above 10488, good signals were received from the 10706 beacon at an IF frequency of 362MHz - screen capture from HDSDR is attached.

    Thanks - you clearly have lots more experience with Octagons than I do I have only owned one since today!


    I have no plans to use it on the transponder - I have a fully GPS locked system ready for that, but it is unable to tune to the existing beacons owing to filters in the downconverter and preamps.


    73


    Charlie

    I've been experimenting today with an RSP1A as IF for an unmodified Octagon LNB.


    SDR software is HDSDR and I have that working well in conjunction with WSJT-X v2 so that WSJT-X controls the frequency of HDSDR via virtual serial connection. Audio is connected from HDSDR to WSJT-X using virtual audio cable.


    Receiving both 10705 and 11205 beacons well and audio sounds perfectly good enough for SSB. Some drift that could be accommodated with the occasional retune.


    For digital work it will be necessary to lock the Octagon (apart from JT65C which will probably cope with the drift).


    73 Charlie G3WDG

    Here is a waterfall plot of the 10706 beacon received using unmodified Octagon LNB and RSP1A SDR driven by HDSDR, audio fed via VAC to WSJT-X. Dish = 0.3f/D 3m, underilluminated.


    The straight trace is a GPS locked signal generator at the IF, showing that the RSP1A is stable. Looks like the unmodified Octagon would be adequate for SSB operation, with a little retuning from time to time! Assuming of course that the transponder LO is as stable as the beacon!