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Table of Contents:

Definition and Overview

1. Copolar and Total Power Front-to-Back Measurement

2. Cross-Polar Discrimination

3. Polarization Purity

4. Microstrip Patch Antennas versus Dipole Antennas

5. Further Discussion of XPD

6. Orthogonality for Dipole and Microstrip Patch Antennas

7. Tracking of Antenna Beams

8. Summary

Self-Test

Glossary

PDF of this tutorial

It is interesting to explore the feasibility of using noncopolarized antenna pattern measurements to determine the polarization angle of linearly polarized slant-45 antennas that use dipole and microstrip patch antenna elements. Some numerical results, which have been submitted for publication in a technical journal, present the theoretical horizontal pattern performance of slant-45 dipole and microstrip patch-based antennas. The patterns for an antenna that uses dipole elements is shown in Figure 3 for copolarized slant-45 vertical and horizontal source positions.

Figure 3. Patterns for a Slant-45 Antenna Using Dipole Elements

As demonstrated in Figure 3, the solid line shows the antenna pattern as taken with a slant-45 source. The closely spaced dashed lines show the antenna patterns taken with a vertical source and the dash-dot-dash line for the horizontal source. At boresite, the patterns for the horizontal and vertical source positions show the same gain, which shows that the dipole antenna is indeed slant-45 polarized at boresite, assuming that the phase references for both polarizations are either equal or differ by 180 degrees.

Recognizing that the dipole-based antenna is slant-45 at boresite, Figure 3 demonstrates that the antenna is not slant-45 at the sector borders. Near 60 degrees on either side of boresite, the difference in the patterns between the vertical and horizontal source positions is quite large—approximately 6 dB. This large difference demonstrates that the signal is becoming more like that from a vertically polarized antenna and less like that from a slant-45 polarized antenna. This conclusion can also be reached by noting that the beamwidth of the antenna pattern for the vertically polarized source is the broadest of the three. The same type of analysis is performed on an antenna that uses microstrip patch elements, as demonstrated in Figure 4.

Figure 4. Patterns for a Slant-45 Antenna Using Microstrip Patch Elements

For the patch antenna, the difference between the horizontal and vertical sources is not as dramatic as seen in the dipoles; thus, the radiated signal from the antenna using patch elements remains far closer to a slant-45 polarization over the 120-degree sector than the antenna using dipole elements. Hence, the superior performance of the microstrip patch-based antenna should lead to improved system performance for the wireless network.