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Antenna Polarization

  1. Aug 4, 2008 #1
    What is it about an antenna that gives rise to a particular polarization? Can antennas have polarizers similar to the ones used in optical systems?
     
  2. jcsd
  3. Aug 4, 2008 #2
    The first part of your question is answered in a nutshell by saying that the orientation of the antenna elements determines the orientation of the transmitted emf field. Or conversely the orientation of the received emf field determines the physical orientation of the antenna elements. This is easy to see in the lower frequency ranges VHF and UHF because the orientation of the elements is quite obvious with just a visual inspection. If you see a long wire antenna strung horizontally, that is horizontal polarization because the electric field created by the antenna current is horizontal. The resulting magnetic field, which is at right angles to the e field is vertical. The polarization always refers to the orientation of the electric field. By the same reasoning, a physically orientated vertical mast antenna is vertically polarized. It gets slightly more complicated when we are talking about microwave antenna because the feed waveguide is a bit counter intuitive. When the waveguide appears to be vertical (long dimension vertical) the feed is actually horizontally polarized because the electric field is across the shorter dimension and is horizontal. When it comes to circular polarization, in most cases you cannot determine the polarization from a visual inspection of the feed and you need to know the orientation of the diplexer assembly internally.

    The second part of you question is what prompted me to respond in the first place, as it is the most interesting part. I will only talk about the microwave range here as it simply is not feasible to make an optical polarizer for a long wire antenna or even a short dipole, in my opinion. A lens of some sort is needed in order to achieve optical polarization, and since many microwave antennas use a radome covering, it is conceivable to incorporate optical polarization into the radome design. But optical polarization does come at considerable cost so it is necessary to weigh that cost against any benefits. Most fixed microwave antennas can achieve a very high level of polarization discrimination by careful orientation of the feed and there is really nothing to be gained by having an expensive optical polarizer built into the radome. However, radar antennas on aircraft, in particular are subject to interference from the sun reflected off of the wings and other parts of the aircraft and it has been found that optically polarized radomes are very useful in airborne applications.
     
  4. Sep 24, 2008 #3
    You can generate circular polarisation using a helix antenna OR feed right angle dipoles (which might be part of yagis) 90 deg out of phase by having the feeder to one dipole 1/4 longer than the other.
     
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