Author
Jim Syme
In the last “Back to Basics” blog post, Derren Oliver wrote about the ability of an antenna to focus the radio waves in the main beam, hence maximizing the signal level and reducing the interference to and from other links. Here, I’ll be explaining the microwave antenna’s ability to maintain radiated or received polarization purity between horizontally and vertically polarized signals. This is called cross-polar discrimination, or XPD.
In transmit mode, XPD is the proportion of signal that is transmitted in the orthogonal polarization to that which is required. In receive mode, it is the antenna’s ability to maintain the incident signal’s polarization purity.
For example, if a perfectly vertically polarized signal (containing no horizontal component) were incident upon a single polarized receive antenna, electrical and mechanical imperfections will introduce a small amount of ellipticity to the polarization of the signal. The signal can be thought of as having both vertical and horizontal components. The ratio of the consequential horizontal to vertical components is the XPD.
Here at CommScope we use the following formal definition of XPD: “Cross-Polarization Discrimination, in dB, is the difference between the peak of the co-polarized main beam, and the maximum cross-polarized signal over an angle twice the 3dB beamwidth of the co-polarized main beam.”
The angular region is included in the definition to account for any antenna movement caused by wind induced twist or sway.
We monitor these potential contributors to XPD in the factory by our Interport Isolation (IPI) measurement process. IPI has a relationship to XPD, and although it is not the only contributing factor, you can say that XPD will never be greater than IPI. As we have mentioned in previous blogs, to really understand and measure XPD you require a full range test, which CommScope performs in the design phase of all its antennas.
XPD is an important characteristic, particularly in dual-polarized systems, where cross-talk between polarizations can prevent the system’s quality objectives from being achieved. Radios can use cross-polar interference cancellers (XPICs) to isolate polarizations and compensate for any link or propagation induced coupling. However, good polarization purity from the antenna is important to allow the XPICs maximum flexibility to compensate for these dynamic variations.
Obviously, the subject of cross-polar discrimination is quite detailed. It is an important characteristic to understand when comparing microwave antennas. XPD can degrade network performance, meaning slower speeds, wasted spectrum and unhappy customers. We’ve got two more topics to introduce in the next blog post—return loss and voltage standing wave ratio (VSWR).
