PDF of this tutorialAnother way to approach the polarization purity of dual-polarized slant-45 antennas is to take two sets of antenna patterns with right angle source positions and record the magnitude and phase of the receive power at each azimuth angle. In this way, the previously presented antenna patterns in Figures 3 and 4 with horizontal and vertical source positions can be used to compute the orthogonality. To investigate the orthogonality for the dipole and microstrip patch antennas, the orthogonality of the dual-polarized slant-45 antennas is computed by using a formula submitted for publication by Lindmark and Nilsson. The results of the orthogonality computations are revealed in Figure 5, in which the solid line is for the microstrip patch-based antenna and the dashed line is for the dipole-based antenna.
Figure 5. Results of Orthogonality Computation
This computation shows that the orthogonality is perfect at boresite (zero degrees) for both the dipole and the microstrip patch-based antennas. At 78 degrees, well into the hand-over region, the orthogonality is good again for the microstrip patch-based antenna, but is quite poor for the dipole-based antenna. If Figure 5 is compared to Figures 3 and 4, the results of the orthogonality computation show that the best values for the orthogonality are located where the magnitudes of the horizontal and vertical source position patterns have the same signal strength.
Reviewing Figures 3 and 4 reveals that the dipole-based antenna has the same gain for the patterns taken with the horizontal and vertical source positions only at boresite. On the other hand, the patch antenna has the same gain at boresite and then again well into the system hand-over area at 78 degrees for the patterns taken with the horizontal and vertical source positions. Hence, when the dual-polarized base-station antenna yields quite similar antenna patterns for the horizontal and vertical source positions, antenna performance for system diversity gain is favorable. Based on the results presented, the system diversity gain will be higher for microstrip patch based antennas compared to dipole-based antennas. This is due to the fact that antenna polarizations from the two receive diversity ports must remain cross-polarized to one another over the 120-degree sector face and into the hand-over area to obtain peak system performance. However, the gain of the two polarization diversity ports must be the same over the intended coverage area as well.
