PDF of this tutorialUsing a set of orthogonal source antenna orientations that are copolarized and cross-polarized at boresite may or may not provide easily interpreted results if the polarization of the antenna-under-test changes in polarization with respect to azimuth position. This type of phenomena is quite common with slant-45 antennas and is often the crux of any base station antenna design for slant-45 polarization diversity antennas. Thus, for certain types of antennas that are dual-polarized slant-45 antennas, using certain types of noncopolarized measurements are quite informative for quickly ascertaining the polarization-dependent beamwidth of the slant-45 polarization diversity antenna. Enormous insight into the polarization purity of the slant-45 antenna can be gleaned from using a set of source positions that are vertically and horizontally polarized.
Two additional pattern measurements per slant-45 antenna port with a horizontal and vertical source position add some additional insight into the performance of the dual-polarized base station antenna. By taking these two pattern cuts on the slant-45 antenna, with the source in the vertical and horizontal position, the amount of polarization loss in the pattern measurement is the same at boresite and directly behind the antenna. In other words, for antenna patterns taken with horizontal and vertical source positions, the amount of variation in the pattern as a result of polarization loss is less than using the slant-45 source position.
Antenna patterns taken with horizontal and vertical source positions demonstrate the stability of the polarization in the horizontal plane. Given a crisp slant-45 signal from the dual-polarized base station antenna, the beamwidth for both of these pattern cuts with vertical and horizontal source positions will be nearly the same. Thus, with well-behaved slant-45 antennas, the horizontal patterns for the vertical source position, the horizontal source position, and one slant-45 copolarized source position will all have approximately the same pattern shape over the forward 120-degree sector. Furthermore, the other slant-45 base station antenna port should have the same antenna patterns for the horizontal and vertical source position and the other slant-45 source position.
These noncopolarized source position measurements result in an easy method to determine the polarization purity for a slant-45 antenna. If the beamwidths obtained from the horizontal and vertical source position measurements are not the same, then the polarization that has the broadest horizontal beamwidth is the polarization that has the strongest signal, given that the polarization at boresite is truly slant-45. If the pattern cut for the vertical source position shows a broader beamwidth than the one taken for the horizontal source position, then the antenna polarization is less than slant 45, and the polarization of the base station antenna is in fact somewhere in between being vertical and slant-45. If there is any more than a 6dB difference in the gain at a particular azimuth angle between the vertical and horizontal source position measurements, this slant-45 antenna should no longer be considered a polarization diversity antenna but a rather poor vertically polarized antenna for that particular azimuth angle.
