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For reception of amateur radio satellites, turnstile antennas (consisting of two crossed dipoles fed out-of-phase) are often used.

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I've recently had an idea - what if two half-wavelength dipoles are made out of coax cable (so-called bazooka antennas), and then connected in a T-shape like this:

  ' 1/4 wl center
  '
  " 1/4 wl braid bent back
  "
  ."""""----
  # 1/4  1/4
  #
  #
  #
  # feed coax
  #

(the vertical and the horizontal antenna cables are connected together at the middle point to the feeder cable (denoted by #)

EDIT: The elements are connected by a phasing line of 1/4 wavelength for a 90-degree shift, not 180 as I originally described.

Would this antenna work like a turnstile?

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  • $\begingroup$ Do you mean 90° out of phase? $\endgroup$ – Mike Waters Oct 4 '17 at 19:02
  • $\begingroup$ @MikeWaters Yes, just like an RHCP turnstile. $\endgroup$ – Mark Oct 5 '17 at 19:32
  • $\begingroup$ Do you mean a coaxial dipole, with λ/4 of center conductor (with the dielectric) exposed, and λ/4 of braid folded back over the coax jacket? $\endgroup$ – Mike Waters Oct 5 '17 at 19:52
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    $\begingroup$ I'm a little uncertain about the exact antenna design you are proposing — mostly about whether you have two regular bazooka antennas ending at the dot or whether you are proposing to bend one halfway along its length, but it's possible there's something else too. I think this question would be greatly improved by a much more detailed diagram, explicitly marking the quarter/half wave lengths, the center point crossover wiring, and the location of the phasing device. $\endgroup$ – Kevin Reid AG6YO Oct 5 '17 at 19:53
  • $\begingroup$ @MikeWaters, yes - exactly. $\endgroup$ – Mark Oct 6 '17 at 8:11
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The purpose of a turnstyle antenna for satellite work is to generate a circularly polarized radiation pattern off of its axis. By adjusting the phasing of the feed to the two crossed dipoles, the circular polarization can be switched between left and right hand polarization.

In your diagram of the proposed antenna, one element is shown to meet at is end with the other element. This is essentially an inverted V antenna. Changing the phasing of the two legs will simply reduce the gain of the antenna. No circular polarization will be realized in this configuration. As a result, the antenna would not be effective as a satellite antenna.

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You specify that

the vertical and the horizontal antenna cables are connected together at the middle point (in parallel, center-braid braid-center and to the feeder cable)

Swapping the two conductors of a transmission line (in the cases where it makes sense at all) produces a 180° phase shift, not a 90° one. Therefore, this antenna design will not produce circular polarization.

If your antenna design is based on identical perpendicular elements, one of those elements must have a 90° phase shift applied such as by a delay line (1/4 wave extra length of coax).

If you did add a 90° phase shift to one element it seems like it could work but I'm not sure what the effect of having the two ends of differing voltage meet at a corner would be. I think at that point you would be better off using antenna modeling software to get a simulated answer.

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  • $\begingroup$ Yes, I didn't think of that initially. What would happen were it a 90 degree shift? (delay line, etc) $\endgroup$ – Mark Oct 6 '17 at 14:03
  • $\begingroup$ @Mark I have updated my answer. $\endgroup$ – Kevin Reid AG6YO Oct 6 '17 at 14:57
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There are 2 main ways to create the 180 degree phase shift required for circular polarization. A delay in the feed lint (1/4 wavelength longer to one than the other) or physical (antennas at right angles). It does not matter how the phase shift is achieved, the results are the same. So if the antenna matches one of these methods, circular polarization will be achieved. Even when minor errors are present the result is still "mostly" circular (think oval) polarization. If these conditions are not met you will get a different result.

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    $\begingroup$ "A delay in the feed line (1/4 wavelength longer to one than the other) or physical (antennas at right angles)." What did you mean? I thought that a second set of elements at right angles needed to be fed 90 degrees out of phase. $\endgroup$ – Mike Waters Oct 11 '17 at 1:39
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    $\begingroup$ Circular polarization can also be created with a helical antenna. $\endgroup$ – Mike Waters Oct 12 '17 at 17:42
  • $\begingroup$ Correct. I will fix my answer. I was thinking 90 degrees when the actual shift is a total of 180 degrees, usually done with 90 degrees mechanical and 90 degrees electrical (phasing) combining for 180 degrees. $\endgroup$ – Keith Martineau Oct 17 '17 at 0:07

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