PDF of this tutorialAs discussed, the front-to-back ratio, the horizontal beamwidth, and the angle of the polarization of dual-polarized antennas can be determined by comparing antenna patterns obtained from four different source positions. From these measurements, the axial ratio of the antenna can, if necessary, be estimated. The axial ratio is also what is known as the XPD for antennas that are polarized in a near-linear fashion (i.e., vertical, horizontal, and slant-45). As another point of reference, the XPD of perfect circularly polarized antennas is zero, which gives rise to an axial ratio of unity. However, like their linearly polarized counterparts, a good pair of right-handed and left-handed circularly polarized antennas are orthogonal and have good receive diversity performance if they cophase to one another. For perfect orthogonality, the vector addition (i.e., dot product) of the radiation from the two ports from the dual-polarized base station antenna is zero. Thus, having a solid XPD over the entire sector face (i.e., PQR) is one way to ensure proper performance for linearly polarized antennas. A more exact method, however, is to inspect the orthogonality of the dual-polarized antennas over the entire coverage area for slant-45 antennas, as a result of polarization changes off-boresite. The orthogonality more accurately describes how well the two receive diversity ports are decorrelated to one another over the entire operating forward sector, but the orthogonality is more involved and tedious.
For antennas that are intended to be linearly polarized, the XPD or PQR is an easier and less complicated way to describe the polarization purity of the antenna. For reference, the XPD of vertically polarized base-station antennas that are used in the space diversity scheme is most often greater than 15 dB. In practice, linearly polarized antennas are never purely linearly polarized but maintain a reasonable XPD of better than 20 dB at boresite. The XPD of slant-45 antennas also tends to be reasonable at boresite and typically decreases in performance away from boresite. Having a reasonable XPD over the entire forward sector is another way of ensuring polarization purity (as opposed to using vertical and horizontal source positions, as previously discussed). Having reasonable XPD at boresite is wonderful in itself, but base station antennas are expected to perform over the entire sector and into the hand-over area. At the sector border, the PQR of some dual-polarized, slant-45 antennas tends to degrade. For these slant-45 antennas, performance near the sector boundaries ceases to be that of a linearly polarized, slant-45 antenna and begins to degenerate to that of poor-quality, vertically polarized antenna.
