Remon
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Another thought: I have seen people using the rtl-sdr blog active patch antenna pointing to a dish with great results. The polarization ain't mirrored since there were no modifications to the antenna itself.
Need help to better understand LHCP and RHCP
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(Keep in mind it may have been designed for a slightly different frequency), has a 85 x 85 mm patch, with the corners cit at 31 mm. Mine's feed is about 6 mm from the edge (and would have been half-way along that edge if the corner were not cut) and to the left of one of the cut corners. This is about 8 mm above the reflector. The "director plate" is 78 x 78 mm square centered above that at a distance of about 5 mm above that.
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Keep in mind that I bought this from a now non-existed source so my measurements, as accurately as I tried, may not be the actual design. I wonder if their is a significant correction for the materials being used. In mine, the top and bottom are aluminum and the middle is "yellowish" something.
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Remon
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Thank you dlwtrunked!
Could you measure and double check a couple of things for me please?
The director is 78 x 78 mm? (found 76 x 76 mm somewhere else)
I found this picture and the gaps between the elements look pretty similar. Not 5 mm and 8 mm, but this could be an optical illusion:
Did you measure the space between the upper part of the reflector and the bottom of the brass plate? Same for the upper part of the brass plate and bottom of director? Then this would be the same size as the spacers in that antenna.
Only variable for me would be if it's the distance between patch and director that matter or distance between reflector and director. Because I use different materials (single sided 1.5 mm PCB with 0.35 mm copper, total of 1.85 mm)
Last but not least: what is the size of the reflector?
Sorry I ask so much.
Could you measure and double check a couple of things for me please?
The director is 78 x 78 mm? (found 76 x 76 mm somewhere else)
I found this picture and the gaps between the elements look pretty similar. Not 5 mm and 8 mm, but this could be an optical illusion:
Did you measure the space between the upper part of the reflector and the bottom of the brass plate? Same for the upper part of the brass plate and bottom of director? Then this would be the same size as the spacers in that antenna.
Only variable for me would be if it's the distance between patch and director that matter or distance between reflector and director. Because I use different materials (single sided 1.5 mm PCB with 0.35 mm copper, total of 1.85 mm)
Last but not least: what is the size of the reflector?
Sorry I ask so much.
Remon
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Thanks again. We posted about the same time. So you answer overlapped my question.
Edit: corners are 35 mm instead of 31? In your last picture the remains of the patch are 50 mm. (85 - 50 = 35)
Edit: corners are 35 mm instead of 31? In your last picture the remains of the patch are 50 mm. (85 - 50 = 35)
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See second post. The middle is probably brass with the other two being aluminium. Distance between were surface to surface. That photo looks just like my antenna. Measurement were as accurate as I could make them--of course they may not have been what the maker intended. The director is very close to 78 mm rather than 76 mm in mine.
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I have seen that and it puzzled me. But I would not be surprised if the antenna responds somewhat to the reverse polarization and in addition am not sure what happens in regard to polarization if you are not at the right feed point. Antennas can be funny. (I remember once (over 50 years ago) building what was supposed to be a VLF converter. It did not work as such but worked great as a VHF converter as long as I did not add an antenna of any sort at all --the power feed must have been acting as the antenna for that,)
If you take a circular polarized antenna, a patch, a helix or whatever and stick it in front of a dish for more gain, it will swap the polarity of the antenna. So if you have a RHCP patch antenna and stick it in front of an old DirecTV reflector or similar you will have a higher gain but LHCP antenna that should not pick up RHCP. That's how it works and if its not working like that then the antenna (patch) is linear pol and not circular.
Its possible you don't believe me but I've been there, done this a hundred times. Here I'm testing a prototype circular pol antenna in a near field range and the little horn antennas are opposite polarity of what the overall dish is. Different frequency range than what is being discussed but the principals are exactly the same.
Its possible you don't believe me but I've been there, done this a hundred times. Here I'm testing a prototype circular pol antenna in a near field range and the little horn antennas are opposite polarity of what the overall dish is. Different frequency range than what is being discussed but the principals are exactly the same.
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I certainly agree and understand well why that happens but in this particular case, without such a nice chamber etc., I wonder if things might perhaps differ from the ideal. I have found that I can just about point my dish (not in an ideal environment) anywhere and pick up INMARSAT in the strangest directions (as mentioned earlier I do not know the polarization of that received signal when doing so) and sometimes almost as good as the correct direction. I suspect using the feed like we are talking about may be less than idea as far as right/left-handedness goes but that is based on no facts, just experiences of mine in the past. I cannot remove the feed from my dish to put one of the other antennas as a feed there (I even have one with the other polarization that was made for me)--I wish I could. I have only been in a chamber like that once, the one at AFRL at Wright-Patterson AFB when I was being given a tour.
Just because. Before finalizing my reflector/director spacing I left the center coax a little loose and used pieces of 9913 coax foam for a little bit of spring between the corner srcrews. Peaked the signal, measured the spacing, and cut nylon spacers to that length.
I change the antenna design to make it LHCP?
If any of above question doesn't apply: what is the point of rotating the antenna if the signals is not vertical or horizontal polarized but circular instead?
Do I have to mirror and change polarization when using a dish to point the antenna at?
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I use a 3 panel design with one SMA connector. I also made a reversed version for my friend to use with his dish. The hardware was all plastic and the spacers are very critical. A mm too long or short will mis-tune the antenna. The panel profiles and alignment are also critical. After I built
mine, I used a VNA with a 175mm SMA cable. The cable is part of the matching device and should connect to LNA. The pictures are two prototypes and the light grey is currently inside my radome with azimuth and zenith controls.
If any of above question doesn't apply: what is the point of rotating the antenna if the signals is not vertical or horizontal polarized but circular instead?
Do I have to mirror and change polarization when using a dish to point the antenna at?
[/QUOTE]
I use a 3 panel design with one SMA connector. I also made a reversed version for my friend to use with his dish. The hardware was all plastic and the spacers are very critical. A mm too long or short will mis-tune the antenna. The panel profiles and alignment are also critical. After I built
mine, I used a VNA with a 175mm SMA cable. The cable is part of the matching device and should connect to LNA. The pictures are two prototypes and the light grey is currently inside my radome with azimuth and zenith controls.
The spacer between the bottom plate (reflector) and the driven element should be 7mm high (no more, no less). The spacer between the driven element and top plate (director), should be 5mm high) The black plastic screws that I used are 8-32 at least 3/4" long from McMaster Carr. I ordered ones that were 1-1/2" and cut off the excess. I used 2 hole, flange mounted, SMA connectors that were purchased on eBay. There was a problem connected the SMA center pin to the driven element but I used brass tubing that I fabricated to fit the SMA. To accept the tubing O.D., I drilled a #51drill hole in the driven element.
VNA measurements are find and dandy, but how about using a known good pair of circular pol antennas and measure the cross pol rejection? That will tell you if the design is right and/or explain why the antenna can be peaked by rotation or if it works the same when using a LHCP or RHCP connection.
Another thing if the design is not actually circular pol or if its not built right, you can loose up to 3dB of gain using a liner pol antenna to receive a circular pol signal.
Another thing if the design is not actually circular pol or if its not built right, you can loose up to 3dB of gain using a liner pol antenna to receive a circular pol signal.
Already tested one I built for left hand polarization. No sat signals were received. I have two other commercially made antennas to compare with. The right hand version yield equal results. 9 to 12 dB above the noise floor.
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