First of all, there is 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. There are a few possible definition of co polarization and cross polarization, check Ludwig's classical paper:

http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=1140406

If the main polarization is liniar (eg vertical, horizontal or oblique), cross- polarization is perpendicular (main polarization vertical => cross polarization horizontal). If main polarization is circular, cross polarization is inverse circular (eg. if main polarization is RHCP, cross polarization is LHCP).

Polarization is how the E-field is radiated by the antenna.

3 types.

i) linear : horizontal or vertical.

ii) elliptical

iii) circular : RHCP od LHCP

Co-polarization : the desired polarization of the wave to be radiated by the antenna.

Cross - polarization (X-pw) : the orthogonal radiation of the desired polarization of wave (the polarization of above wave).

Example:

-if the desired polarization is horizontal, the X-pw is vertical.

-it the desired polarization is RHCP, the X-pw is LHCP.

why is it necessary to know X-pw?

- to minimized the interference of the waves.

In the context of light scattering, co-polarization is the light scattered vertically or horizontally from a vertically or horizontally polarised incident light respectively (i.e IVV or IHH ) whilst cross polarization refers to light scattered vertically or horizontally from a horizontally or vertically polarised incident light respectively (i.e IVH or IHV).

Cross polarization (sometimes written X-pol, in antenna slang) is the polarization orthogonal to the 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)

to understand antenna basics and the diffrence between co- and cross-polarization I recommend you the following website: http://antenna-theory.com/

cross-pol with level < -135 dB, it is logic ??? thank you

When antenna radiation is horizontally polarised and at receiver end the amount of power received by vertically polarised antenna then ratio of horizontally polarised received power to vertically received power is called crossed polarisation.

Ideally nothing should be received in vertical polarised is zero, however due to various situations (ground condition, atmospheric condition etc) affects the polarisation and traveled signal polarisation is affected.

The horizontal/vertical is an example for linear polarisation. It could be vice versa or it could be CW & CCW in case of circular polarised signal

The Cross-Polarization determines the Polarization purity of the proposed antenna with respect to the desired polarization. An antenna can exhibit a perfect or desired polarization (let say, vertically polarization, hence the cross polarization level should be zero, negative infinite dB). But it may vary in the sidelobes or within the major lobe. So one has to calculate the ratio of the desired (co) polarization (E-field intensity) with cross polarization and dB of it (or with gains).

cross-polarization is always perpendicular to the desired polarization direction. If the cross-polarization of any antenna is higher in linear polarization, it shows less polarization purity and antenna's behavior is tending towards elliptical polarization.

Co-polarization is the antenna's radiation in your desired directions. Where as cross-polarization is the antenna's radiation in the unwanted directions, i.e the cross-polar is basically considered as a dissipation in antenna radiation.

So there must be single co pole direction and in any other directions it is cross pole... Is it Mr. Ehab?

A purely polarized antenna will have low cross polarization values.

cross polar is the undesired polarization. for Example in CST software if you want to plot gain of a RHCP antenna, x-pol is LHCP and you should plot each gain separately settings in POST-PROCESSING.

Cross polarization refers to the orthogonal component of the desired polarization.