UPLINK - PATCH VS HELIX - what's better

  • The contraption, feeding Ron (DU3BC) preamp:



    Wondering now whether thinner wire might be better, if the problem is simply blockage. This helix uses 3mm wire. More to play with....

  • G3WDG I don't know what the f/D of the dish is, but you use the 'bare' waveguide as feed (which owns a poor match). As an experiment if you try to insert a dielectric lens the influence of the helix may decrease (?)

  • Hi Remco


    The 10GHz feed diameter is optimum for the 0.4 f/D of this dish. The match is -20dB return loss with a tuning screw (without the helix in place). Need to check return loss with the helix in place.


    73


    Charlie

  • Hello Charlie, which software are you using to make measurements? Does it work with RTL based SDRs? I am looking for such a software to get repeatable measurements...


    thank you

    73 de iz2eeq matteo

  • Hi Matteo


    Three different software packages.


    Spectravue for sun noise measurements, with homemade SDR based on Softrock. I'm not sure if this will work with RTL based SDR's. Hopefully someone else on the forum can advise.


    Minitournier for measuring MER of the WB beacon.


    RSP-SAS Spectrum Analyser in conjunction with RSP1A SDR for comparing the WB spectrum above noise.


    73


    Charlie

  • "UPLINK - PATCH VS HELIX - WHAT'S BETTER" aka how long is a piece of string? It depends - the patch is not optimum for a 0.6 f/D dish, though it isn't that bad. It is better for a dish 0.4 to 0.5. A helix, depending on the length may be better but it may also impact 10 GHz more. It needs simulating or measuring.


    The patch performance can be easily improved for longer dishes, but at a much higher level of complexity. You maybe gain a dB or two. For EME, or on RX that would really matter but for QO-100 TX you are in practice gaining very little, less than half an s-point or, for many just having to back off the PA a little less to avoid being too strong.


    Mike

  • 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 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 did - here's an image of the prototype - it ended up with much shorter helices and threaded plastic rods as mechanical support. Didn't work too bad, but I'm now using the G0MJW feed for it's beautiful simplicity.

  • I did - here's an image of the prototype - it ended up with much shorter helices and threaded plastic rods as mechanical support. Didn't work too bad, but I'm now using the G0MJW feed for it's beautiful simplicity.

    And how do you mount this "sunglass" at the dish ? It's very interesting to operate with only 1 dish, also portable.

  • Thomas, here is the same offset problem as with the multi-feed holder ...

    you can only operate one satellite from the focal point ... the two helix are out of focus and will be blasting past the satellite with the main lobe ...

  • Matthias,

    I do not think so. They will act as an array, but each helix has its own lobe of radiation.
    stacking antennas results in a doubling of the antenna area in the simplest case, thus also a gain of 3 dB. The distance between the antennas is chosen so that the 3 dB opening angle in the radiation pattern does not disturb anymore.
    As long as the signals are sent on their way in parallel, you will receive a correspondingly focuised 3 dB profit at the receiver.

    But, sent over a parabolic shaped reflector the lobes go apart and sprinkle ....


    For a common focus of the array on a parabolic reflector I lack the physical justification, but I like to learn ... (stacking 4 LNB´s and gain rise up 6dB ? :-) )

  • For a common focus of the array on a parabolic reflector I lack the physical justification, but I like to learn ... (stacking 4 LNB´s and gain rise up 6dB ? :-) )


    May I call you Schrödinger's Ham? ;) You are right and wrong at the same time.


    If we look at the near field, you are absolutely right, each helix of the array has it's own radiation lobe.


    But if we look at the far field, the individual lobes of an array combine into a single lobe.


    Getting far field conditions between the helices and the reflector isn't too hard at 13cm wavelength.


    Your idea with stacking four lnbs may even work in theory, but in the real world the stacking distance in relation to the wavelength might be too big to form a properly phased array.


    Have a look at this beautifully simulated example of a patch array: https://www.researchgate.net/p…h-antenna-array-in-3D.png


    Let's do the math. For a two antenna array I assume transition from near to far field at 2*(stacking distance^2)/lambda. For 13cm wavelength and 8cm stacking distance, this would be less than 10cm from the antenne (2*(0,08*0,08)/0,13 =~ 0.098). For the same stacking distance at 3cm this would already be almost 43cm (2*(0,08*0,08)/0,03 =~ 0,426).


    So if the distance between your "stacked LNBs" and the reflector is more than around 43cm, you should indeed observe stacking gain - but before you try, take into mind that you would need to phase the LNBs coherently with the same reference, otherwise phase difference would mingle with that gain.


    But that's only for theory. I tried the 13cm version and it worked not too bad. Now you go try the 3cm version and I will happily doubt your results afterwards.