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Helix antenna directivity

  1. Aug 3, 2016 #1
    Hello. I use a group of 4 helix antennas in the band of 1420 MHz. It works fine but by nature directivity is very poor ( around 50° at -3 dB). Is there a way to improve directivity using mechanical, electrical ... devices ?
     
  2. jcsd
  3. Aug 3, 2016 #2

    Baluncore

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    A helix is usually used when a broad beam is needed that can see a satellite from one horizon to the other.

    The beamwidth of a helix is decided by it's pitch diameter and length. When 4 are used together the combined array pattern should be narrower than one helix alone, but it will depend on separation and phasing of the elements.

    1. Is the antenna ever used for transmit?
    2. A parabolic reflector will reverse the CP hand. Are you receiving linear or circular polarisation?
    3. Is 1420MHz the 21cm hydrogen line, radio astronomy? or a satellite link?
    4. Why must you use the helix antenna array?
    5. How do you aim the antenna at the target?
     
  4. Aug 4, 2016 #3
    Thanks for reply. Antenna is only used to receive 21 cm HI line. Antenna automatically tracks target. I can't use dish antenna which is cumbersome.
     
  5. Aug 4, 2016 #4

    davenn

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    I presume your helix is in front of a reflector panel either mesh or solid ?

    where did you obtain the design ? got any photos of it ?

    have a look at this paper from Paul Wade ( he is an antenna guru)

    http://www.w1ghz.org/antbook/conf/Helical_feed_antennas.pdf


    cheers
    Dave
     
  6. Aug 4, 2016 #5

    Baluncore

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    A single helical antenna element has a wider beam than most other long antennas. The beam can be narrowed further by using an array of elements. You have 4 elements. You will be restricted by the area of your array. If the array is a square with side length s, it will have a beamwidth in radians of about s / λ. That will be true for generalised apertures such as arrays of dipoles, helices, dishes or horns. It is necessary to restrict s to prevent multiple side and forward lobes appearing.

    If a dish would be cumbersome then consider a smaller dish, which will have less directivity. In the end you will consider a dish about the same size as your helical array. It will have a similar directivity. But with a crossed dipole at the focus you can receive HP, VP, LCP & RCP, selectively or all at the same time.

    We know little about your application or the detail of your helical elements and their array spacing. It is hard to advise a better array spacing, or phasing harness configuration, for improved directivity without those details.
     
  7. Aug 5, 2016 #6
    Thanks for answers. Sorry but it's too difficult (URL !!) to join pics of my radiotelescope
     
  8. Aug 5, 2016 #7

    Baluncore

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    Drag your pictures.jpg onto the PF reply window and they will appear in your post.
     
  9. Aug 5, 2016 #8
    Seems to work ! Here is my "Amateur" radiotelescope. All data and command transfers are made through radio connection.
     

    Attached Files:

  10. Aug 5, 2016 #9

    tech99

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    A nice construction, congratulations.
    If you are doubtful about performance, is the measured beamwidth correct according to theory?
    Areas of doubt for me are:-
    1) Is it a known design? Are the ground planes correct size, i.e. big enough?
    2) Is the spacing of the helices correct in both planes? They look different.
    3) Is the phasing harness your own design or a known design?
    4) Is the plastic former for the helices as specified, it might be causing detuning or loss?
    5) Have you been able to measure VSWR to give a final health check?
     
  11. Aug 6, 2016 #10

    tech99

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    When I look closely, it does not look like a helix, but a series of loops.
    What are the boards alongside the helices? And any other pieces of metal near the area. The area needs to be absolutely clear of clutter.
    Not sure how the fed arrangement is constructed. You have a larger diameter section, not understood.
     
  12. Aug 6, 2016 #11
    All this is interesting, but very far from my original question. Some formulas give the gain and directivity for this kind of antenna. Based on these formulas directivity is necessary poor, but may be is there a trick for reducing beam width ...
     
  13. Aug 6, 2016 #12

    tech99

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    No tricks for reducing beamwidth I am afraid, other than using wide spaced antennas as an interferometer. I was thinking that you might not actually be achieving the theoretical beamwidth.
     
  14. Aug 6, 2016 #13

    Baluncore

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    You have selected “wide beam” elements, but will not increase the array dimension to reduce the beamwidth. The spacing of your array looks like a couple of wavelengths. That will have a more complex “array factor” pattern with significant side lobes and deep nulls.
    You are carefully aiming your phased array, but as an interferometer you should keep it fixed as the Earth rotates and your target passes through the deep nulls of your array.
    You could switch your feeds between sum and difference to generate a combined signal, but we have no idea how you have arranged your phasing harness, signal combiners, or the exact linear separation of your array elements which determine the complete pattern.
    All your helical elements need to be rotated so the connector on the reflector is in the same clock position. If not you will get messed up phase. Are all the elements precisely the same pitch?

    Have you detected an extraterrestrial signal yet? The Sun is quiet at the moment, maybe Venus would be a better target, but do you have the optimum CP for Venus? Do you have a nearby 1uW signal to check the antenna is working?

    The first helical antenna I built to look at the Sun, over 40 years ago did not work. Some of us have spent decades working on antenna design and at RA observatories, both amateur and professional. We will do everything we can to support amateur RA, which has a steep learning curve. There are still so many unknowns and unanswered questions in this thread that we cannot yet help you.
     
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