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I had a 100cm prime focus dish laying around and I thought of making a circularly polarized feed for it, since it seemed pretty straightforward from what I've been reading. Helical feeds are supposed to be very wide and forgiving. So I built this feed: enter image description here The problem I have is, even though the feed and signal are circularly polarized, if I rotate the feed or the antenna 90 degrees, the signal completely disappears. I was told I'm losing a lot of signal due to the fact that this antenna is for a linear feed and so it's turning my CP signal into LP, and then the feed is losing signal again due to the fact that it's CP again. What I tried was to cover my antenna with fine aluminium wire mesh (the kind you would use on a screen door), so the antenna ended up looking like this:

enter image description here

But again I have the problem that rotating either the antenna or feed make it lose signal. I don't have a VNA to analyze the feed (frequency of interest is 1.7GHz). So the best I can do is point it at a known satellite (in my case, GOES-16. I have a working system for it to which I can compare results, but it's a slightly larger antenna with a linear feed. GOES LRIT is linear).

  1. Does the wire mesh I added help with preventing the antenna from linearizing my circular polarized signal?
  2. What can be causing the problem of losing signal as I rotate the feed or antenna?
  3. If the signal is linear and my antenna and feed are circular, I'm supposed to get the same signal level regardless of rotation, right? I'll be losing 3dB due to polarization mismatch but it should be the same regardless.
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    $\begingroup$ Is the feed really CP - did you test it alone? Also did you account for the plastic? Though these helices are usually quite forgiving, if you designed right up to the maximum C/$\lambda$ it would stop working on plastic. $\endgroup$ – tomnexus Mar 8 '20 at 16:02
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    $\begingroup$ Second, the dish is definitely only for linear pol, unless covered with metallic mesh. Also, the feed looks quite close to the dish, and the dish is quite flat, is the groundplane really at the focus? Lots of simple calculators online for that. $\endgroup$ – tomnexus Mar 8 '20 at 16:15
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    $\begingroup$ Mesh should definitely be on the front, being behind the linear metal tubes won't be good for it at ~2 GHz $\endgroup$ – tomnexus Mar 8 '20 at 16:16
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    $\begingroup$ @tomnexus mesh is between the antenna and the feed. i made a new version with much less plastic, only two turns. but it still changes as i rotate it $\endgroup$ – hjf Mar 8 '20 at 20:05
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    $\begingroup$ @MikeWaters antenna was calculated on jcoppen's helical antenna calculator. it's only two turns. it is in the focal point, i believe, since that's about how long the original strut in this antenna was. also, i tuned the height to get the maximum signal, so it should be in the focal point. the mesh is in front of the grid (between grid and feed) $\endgroup$ – hjf Mar 8 '20 at 20:08

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