PDF of this tutorialThe level of the polarization purity for the polarization diversity antenna should be pushed as high as possible for peak diversity gain performance. The system diversity gain relies on the combining method used by the base station equipment, which in turn relies on decorrelation of the two receive ports to be quite high. This decorrelation is achieved by ensuring that the receive and receive diversity ports of a diversity antenna are highly orthogonal to one another. The orthogonality measurement requires both magnitude and phase measurements to be stored during the testing process. A more simplified approach to ensure orthogonality is to measure the cross-polar discrimination for highly linearly polarized antennas.
The same set of antenna patterns in Figure 1 and Figure 2 are also used quite often to obtain the polarization purity and the cross-polar discrimination (XPD) of the slant-45 dual-polarized antenna. For this example, the boresite XPD is obtained by comparing A in Figure 1 to C in Figure 2, yielding a value of 19 dB. The XPD at 30 degrees is about 35 dB, whereas at the sector border (60 degrees), the XPD is back down to 8dB. Measuring the cross-polar discrimination off-boresite is also known as the polarization quality ratio (PQR). As the source and antenna-under-test are neither copolarized nor cross-polarized off-boresite, the use of an alternative set of orthogonal source positions may yield more insight into the polarization purity of the slant-45 antenna off-boresite and, more importantly, at the sector border.
