what is polarization of antenna

Polarization – AM radio antennas are generally some sort of vertical wire. This is because AM radio waves have vertical polarization. Other directions are possible. Most TV signals have horizontal polarization and thus require antennas with horizontal elements. The broadcaster chooses the polarization. That decision reflects what type of antenna is most convenient for that service and also how radio waves behave at that frequency.

If you hold an antenna perpendicular to the polarization of the signal, you will receive no signal. The received signal strength varies with the cosine of the angle of the antenna relative to the signal polarization. (Reflections off nearby objects commonly make this experiment imperfect.)

If a transmitter uses two antennas, one vertical and one horizontal, the result is most likely diagonal polarization, which the station could also produce with a diagonal antenna. Vertical, horizontal, and diagonal polarizations are examples of linear polarization. But if this station introduces a quarter-cycle delay into one of these two antennas, the result is circular polarization (or elliptical polarization if the two components are not equal).

Circular polarization can be visualized as a signal that spins as it moves through space. But it is equally valid to think of circular polarization as just two independent overlapping signals that have linear polarizations that are perpendicular and have a quarter-phase shift.

Circular polarization can be right circular or left circular. DBS satellites use circular polarization, with adjacent channels using opposite spin. A right circular antenna will ignore a left circular signal, and visa versa. DBS satellites alternate the polarization to make it easier for the receiver electronics to separate the channels.

A linear antenna can be used to receive a circularly polarized signal, but half of the transmitted power will be invisible to it. A circularly polarized antenna can be used to receive a linearly polarized signal, but half of the received signal will be retransmitted with the opposite linear polarization. Because of these problems, circular polarization has not become popular for terrestrial radio services other than FM broadcasting. A small percentage of TV broadcasters use circular polarization, which improves reception in a moving vehicle equipped with a vertical antenna.


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