# What is Cross Polarization?

What is cross polarization in an antenna?

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. 