Posts by G4CDF


    The upside-down cheap food-box is made of Polypropylene PP5 and after being microwaved with >500W RF:) it stayed cool! (however, it has no microwave safe mark. This plastic is very sturdy, so sawing-drilling is easy. Only made RL-measurements with the "boxed" Helical feed in the feedpoint of a dish, measured abt 25dB at 2400MHz.

    Performance with my current 1.1m offset dish and abt. 3W at the Helix is very good, but could still be interesting to measure for any reduction on 2.4GHz and also for 10.5GHz using this type of weather-protection. I will come back on this.

    73 Ed PA1EW

    Thanks for the information Ed.

    As reported above I have taken your advice and purchased a PP5 box and hopefully can test it soon.

    73 Mike G4CDF

    I believe that every plastic box that has the microwave safe symbol is suitable for this purpose.

    Thanks Lucio, today I bought a cake box that is made from PP5 and is microwave safe. When I can I will test it on receive before I fit it.

    Unfortunately the power amplifier has died for some reason and I cannot transmit at present.

    Mike G4CDF


    When I tried this approach my main concern was the non ideal illumination even with my oval dish but I tried it anyway (see my previous posts). It seems to work quite well for me. The extra losses associated with combining 4 helix antennas seems to outweigh the advantage of better illumination and is mechanically difficult.

    73 Mike G4CDF

    Nice idea!. To add some weather protection the following construction could be used. Possibly adapting the size of the PP-box as required.

    73 Ed PA1EW


    Did you detect any reduction of Rx or Tx signal because of the PP box. This is a very good idea: there are many different sizes of box available.

    73 Mike G4CDF

    I would be interested to test you GNU Radio code. Can you share it ?

    The contains the development sources of a GNU Radio-based receiver for the Blockstream Satellite network. Very interesting.

    73 michel, hb9dug


    A zip file containing the GRC file is attached. Please note that you need to install limesdr.

    Its quite untidy at present but I am working on a hierarchical version.

    Let me know if you have any questions.

    73 Mike G4CDF

    Here are some pictures.

    I have done some rough modelling using MMANA-GAL.

    From an optimum illumination perspective four helices would be better but more complex and this seems to be a good compromise for my oval dish.

    Note that the matching stubs are air spaced so susceptible to snow and ice. The central brass rods of the helices seem to have negligible effect on modeled performance.

    No effect on receiver performance, either feed blockage or RF interference has been noticed.

    Does anyone know where I can find expected received signal level relative to the beacons for a given EIRP? I need this to determine whether this antenna is really working as expected.

    Mike G4CDF

    I have managed to lock my SDR receiver to the BPSK (upper) beacon using LimeSDR and GNU radio. It works quite well, but needs the operator to find the beacon first before it will lock. Thereafter it corrects for any temperature drift in the LNB

    I have now built the 2 phased helices above and below the Octagon LNB.

    With 10W at the feed and 85cm x 75 cm oval dish my CW test signal is about 1dB below the CW beacon so not too bad...

    The helix matching sections are currently airspaced and it is wet so could still be improved. The advantage of the helices is that they are wideband compared to patch feeds.

    I have found that a helix in front of the Octagon feed attenuates the received transponder noise by about 2dB. This is probably acceptable, but I am tempted to try a pair of phased helices either side of the LNB. Has anyone tried this?

    Mike G4CDF

    I am using an Octogon OSTLO LNB (unmodified) and an offset 85cm x 70cm dish orginally used on 20GHz. Receive is LimeSDR mini and Rigol spectrum analyser. Currently I have no transmit capability but planning to use the same dish if possible.

    NB transponder noise is 11dB above receiver noise. Largest signal received is 17 dB above transponder noise in 3kHz bandwidth but most are about 10dB above noise

    I am very impressed so far.

    Mike G4CDF (IO91NK)