What is Cross Polarization?

Cross Polarization is the polarization orthogonal to the desired polarization. For instance, if the fields from an antenna are meant to be horizontally polarized, the cross-polarization in this case is vertical polarization. If the polarization is Right Hand Circularly Polarized (RHCP), the cross-polarization is Left Hand Circularly Polarized (LHCP).

Most antennas have an intended polarization - the polarization in which antenna is supposed to work, at least in main direction of radiation. Cross-polarization stands for "perpendicular" or "rectangular" over intended polarization, which is called co-polarization.

Every polarization has an orthogonal counterpart, horizontal & vertical, ±45°(slant), and RHCP & LHCP. These terms are key parameters to measure of how purely polarized is an antenna. We can say, an antenna is never 100% polarized in a single mode such as linear or circular. Hence, mostly two radiation patterns of an antenna are presented. One of them is the co-polarization component and the cross-polarization radiation component.

Also, each polarization is constructed out of any two orthogonal polarizations. Linear polarizations can be constructed as 90º vertical, 0º horizontal, ± 45º slant. For a circularly polarized antenna, nature of movement of the electric field vector tip is the decides the same- clockwise or counter-clockwise (RHCP for clockwise, LHCP for counter-clockwise). For instance, if the fields from an antenna are meant to be horizontally polarized, the cross-polarization in this case is vertical polarization. If the polarization is Right Hand Circularly Polarized (RHCP), the cross-polarization is Left Hand Circularly Polarized (LHCP).

In a purely polarized antenna, the value of cross-polarization should be zero. However, in the real world, there is no such thing as a purely polarized antenna. Cross polarization is defined as the difference in decibels between the maximum radiation intensity of the required polarization (Co-Polarization) and cross polarizations. The cross polarization is specified for an antenna as a power level. The value is usually represented in negative dB. In practice, it indicates how many decibels (dB) the cross-polarization power level is below the desired polarization.

So if a vertically polarized antenna has a cross polarization of -30 dB, that means the value of the amplitude of the horizontally polarized signal that we will receive will be 30 dB lower than the desired/vertically polarized signal.