A wireless link between a transmitter and a receiver includes an antenna at the receiver for receiving a wireless signal from the transmitter. Generally, an assumption is made that the polarization of the field is optimum: that is, the characteristic polarization of the antenna and the signal being received are the same. If the signal is polarized differently from the antenna then the received signal power available at the antenna terminals will be less than maximum that is obtainable from the signal by the antenna. Losses resulting from polarization mismatch between the signal at the receiver and the antenna can have any value between infinity (in practice >40 dB) and zero. For example, where the antenna is vertical linear polarized (i.e. it expects to receive a vertical linear polarized signal) the losses due to polarization mismatch between the antenna and the actual signal can be 0 dB where the signal is vertical linear polarized, 3 dB where the signal is 45-degree slant linear polarized or left- or right-hand circular polarized, or up to infinity where the signal is horizontal linear polarized. Other polarizations of the antenna and/or signal may lead to other levels of losses due to polarization mismatch.
The polarization of the antenna of the receiver may also depend not only on the design of the receiver but also how the receiver or its antenna is physically aligned by a user. For example, the user may move the receiving apparatus in such a way that changes the polarization state of the receiving antenna.
The polarization state of the signal at the receiver can depend on the polarization of the antenna used by the transmitter to transmit the signal, and also may depend on properties of the propagation channel that may change the polarization state of the signal before it reaches the receiver. Generally, the signal once it reaches the receiver may have any arbitrary elliptical polarization state. Circular and linear polarization states are certain cases of elliptical polarization.