A Polarization Review An antenna is a transducer that converts radio frequency electric current to electromagnetic waves that are then radiated into space. The electric field or "E" plane determines the polarization or orientation of the radio wave. In general, most antennas radiate either linear or circular polarization.
Types of polarization Three types of polarization 1. Linear 2. Circular 3. Elliptical Vertical Linear Horizontal
Polarization
Circular
Elliptical
4.Classification of Polarization 5. Light in the form of a plane wave in space is said to be linearly polarized. Light is a transverse electromagnetic wave, but natural light is generally unpolarized, all planes of propagation being equally probable. If light is composed of two plane waves of equal amplitude by differing in phase by 90°, then the light is said to be circularly polarized. If two plane waves of differing amplitude are related in phase by 90°, or if the relative phase is other than 90° then the light is said to be elliptically polarized.
6.
Linear Polarization A plane electromagnetic wave is said to be linearly polarized. The transverse electric field wave is accompanied by a magnetic field wave as illustrated.
Index Polarization concepts
Compare with circular and elliptical polarization HyperPhysics***** Light and Vision
R Nave
Go Back
Circular Polarization Circularly polarized light consists of two perpendicular electromagnetic plane waves of equal amplitude and 90° difference in phase. The light illustrated is right- circularly polarized.
Index Polarization concepts
If light is composed of two plane waves of equal amplitude but differing in phase by 90°, then the light is said to be circularly polarized. If you could see the tip of the electric field vector, it would appear to be moving in a circle as it approached you. If while looking at the source, the electric vector of the light coming toward you appears to be rotating counterclockwise, the light is said to be right-circularly polarized. If clockwise, then left-circularly polarized light. The electric field vector makes one complete revolution as the light advances one wavelength toward you. Another way of saying it is that if the thumb of your right hand were pointing in the direction of propagation of the light, the electric vector would be rotating in the direction of your fingers. Circularly polarized light may be produced by passing linearly polarized light through a quarter-wave plate at an angle of 45° to the optic axis of the plate. Compare with linear and elliptical polarization Go Back
HyperPhysics***** Light and Vision
R Nave
Elliptical Polarization Elliptically polarized light consists of two perpendicular waves of unequal amplitude which differ in phase by 90°. The illustration shows right- elliptically polarized light.
If the thumb of your right hand were pointing in the direction of propagation of the light, the electric vector would be rotating in the direction of your fingers.
Cross polarization 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.
Typical 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. Diversity Reception Even if the polarizations are matched, other factors may affect the strength of the signal. The most common are long and short term fading. Long term fading results from changes in the weather (such as barometric pressure or precipitation) or when a mobile station moves behind hills or buildings. Short term fading is often referred to as "multipath" fading since it results from reflected signals interfering with the LOS signal. Some of these fading phenomenon can be decreased by the use of diversity reception. This type of system usually employs dual antennas and receivers with some kind of "voting" system to choose the busiest signal. However, for best results, the antennas should be at least 20 wavelengths apart so that the signals are no longer correlated. This would be 20-25 feet at 880 MHz, quite a structural problem. Nowadays we are inundated with mobile radios and cellular telephones. The polarization on handheld units is often random depending on how they are held by the user. This has led to new studies which have found that polarization diversity can be an advantage. The most important break through in this area is that the antennas at the base station do not have to be separated physically as described above. They can be collocated as long as they are orthogonal and well isolated from each other. Only time will tell if these systems are truly cost effective. Summary Polarization in an important parameter and consideration when selecting an antenna. It helps to have a good grasp of all the aspects of this subject. Hopefully the information contained in this paper will answer some of your questions. Other antenna considerations are discussed in reference 1. Ref. 1 "Antenna Selection and Specification Made Easy" by Joseph H. Reisert. Astron Antenna Co. technical application found on this website.
