The proportion of power in a wave that is transferred to an antenna depends on their relative polarisations.

For the general case (wave and antenna both elliptically polarised), power will only be transferred from the wave to the antenna if the 2 polarisation states have:
 - The same sense of rotation
 - Equal VARs and
 - Equal tilt angles

No power will be transferred if the 2 polarisation states have:
 - Opposite senses of rotation
 - Equal VARs and
 - The major axes of the ellipses are orthogonal

Usually try to produce a true circularly polarised wave.
Imperfections in the equipment often result in the wave being elliptically polarised

The polarisation state of any wave can be completely described by:

• The amplitudes of the major and minor axes of the polarisation ellipse, Emax and Emin 

• The tilt angle, T

• The sense of polarisation, i.e. left-handed or right-handed
  

Emax / Emin is the voltage axial ratio (VAR) of the wave, or AR = 20 log (Emax / Emin) dB

By using a dual-polarised feed an antenna can transmit two orthogonally polarised waves on the same frequency.

Another antenna can then receive the two orthogonally polarised waves and separate them by means of an electrically identical dual polarised feed.

In theory this can be done without any interference between the two signals. However, in practice there will always be some interference because the orthogonally polarised waves and/or the receiving antenna will not be perfectly orthogonal because of imperfections in the antenna and feeds or changes of polarisation which occur during transmission as a result of the signal passing through the atmosphere

A measure of the strength of a cross-polar transmitted signal that is received by an antenna as a ratio to the strength of the co-polar signal that is received.
i.e. 20 log (E11 / E21)

Cross-polar Discrimination (XPD):

A measure of the strength of a co-polar transmitted signal that is received cross-polar by an antenna as a ratio to the strength of the co-polar signal that is received.
i.e. 20 log (E11 / E12)

Note that to measure XPI, two highly orthogonal waves must be transmitted, to measure XPD only a single polarisation needs to be transmitted (see figures).