Antenna Polarisation

By Jairam Sankar

63566
 

Also note that linearly polarized antennas will work with circularly polarized antennas and vice versa. However, there will be up to a 3 dB loss in signal strength. In weak signal situations, this loss of signal may impair communications.

Cross polarization is another consideration. It happens when unwanted radiation is present from a polarization which is different from the polarization in which the antenna was intended to radiate. For example, a vertical antenna may radiate some horizontal polarization and vice versa. However, this is seldom a problem unless there is noise or strong signals nearby.

Applications

Vertical polarization is most often used when it is desired to radiate a radio signal in all directions such as widely distributed mobile units. Vertical polarization also works well in the suburbs or out in the country, especially where hills are present. As a result, nowadays most two-way Earth to Earth communications in the frequency range above 30 MHz use vertical polarization.

Horizontal polarization is used to broadcast television in the USA. Some say that horizontal polarization was originally chosen because there was an advantage to not have TV reception interfered with by vertically polarized stations such as mobile radio. Also, man made radio noise is predominantly vertically polarized and the use of horizontal polarization would provide some discrimination against interference from noise.

In the early days of FM radio in the 88-108 MHz spectrum, the radio stations broadcasted horizontal polarization. However, in the 1960’s, FM radios became popular in automobiles which used vertical polarized receiving whip antennas. As a result, the FCC modified Part 73 of the rules and regulations to allow FM stations to broadcast RHC or elliptical polarization to improve reception to vertical receiving antennas as long as the horizontal component was dominant.

Circular polarization is most often use on satellite communications. This is particularly desired since the polarization of a linear polarized radio wave may be rotated as the signal passes through any anomalies (such as Faraday rotation) in the ionosphere. Furthermore, due to the position of the Earth with respect to the satellite, geometric differences may vary especially if the satellite appears to move with respect to the fixed Earth bound station. Circular polarization will keep the signal constant regardless of these anomalies.

Conclusion

Polarization is a critical characteristic of an antenna.

Simply, if the receiver antenna is not polarized correctly with respect to the transmit antenna, it will not receive all the available power of the signal. This is independent of distance or other parameters. This can seriously degrade the performance of any communication system and unnecessarily waste a lot of power from the transmitter.

If there is a complete polarization mismatch, such as transmitting with a vertically polarized dipole antenna and receiving with a horizontally polarized dipole antenna, the received signal will be nearly zero no matter how much power is transmitted.


The author is a final year btech ece student at RSET ,Cochin

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