Antenna Polarization

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Antenna polarization is a very important consideration when choosing and installing an antenna.
Most systems use either vertical, horizontal or circular polarization. Knowing the difference between polarizations and how to maximize their benefit is very important to users.
Polarization
An antenna is a transducer that converts radio frequency electric current to electromagnetic waves that are then radiated into space. The electric field plane determines the polarization or orientation of the radio wave. In general, most antennas radiate either linear or circular polarization.
A linear polarized antenna radiates wholly in one plane containing the direction of propagation. Where a circular polarized antenna, the plane of polarization rotates in a circle making one complete revolution during one period of the wave. If the rotation is clockwise looking in the direction of propagation, the sense is called right-hand-circular (RHC). If the rotation is counter clockwise, the sense is called left-hand-circular (LHC).
An antenna is said to be vertically polarized (linear) when its electric field is perpendicular to the Earth's surface. An example of a vertical antenna is a broadcast tower for AM radio or the "whip" antenna on an automobile. Horizontally polarized (linear) antennas have their electric field parallel to the Earth's surface. Television transmissions use horizontal polarization.

Circular polarized wave radiates energy in both the horizontal and vertical planes and all planes in between. The difference, if any, between the maximum and the minimum peaks as the antenna is rotated through all angles, is called the axial ratio or elliptically and is usually specified in decibels (dB).
If the axial ratio is near 0 dB, the antenna is said to be circular polarized, when using a Helix Antenna. If the axial ratio is greater than 1-2 dB, the polarization is often referred to as elliptical, when using a crossed Yagi.
Important Considerations
Polarization is an important design consideration, as each antenna in a system should be properly aligned for maximum signal strength between stations. When choosing an antenna, it is an important consideration as to whether the polarization is linear or elliptical. If the polarization is linear, is it vertical or horizontal? If circular, is it RHC or LHC?
This is becomes a greater concern in Wireless Lan devices as line-of-sight (LOS) paths are required due to the low power levels involved, consequently the polarization of the antennas at both ends of the path must use the same polarization.
In a linearly polarized system, a misalignment of polarization of 45 degrees will degrade the signal up to 3 dB and if misaligned and 90 degrees the attenuation can be more than 20 dB.
Likewise, in a circular polarized system, both antennas must have the same sense. If not, an additional loss of 20 dB or more will be incurred. 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 will mean a great deal.
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 are nearby.
Typical Applications
Vertical polarization is most commonly used when it is desired to radiate a radio signal in all directions over a short to medium range.
Horizontal polarization is used over longer distances to reduce interference by vertically polarized equipment radiating other radio noise, which is often predominantly vertically polarized.
Nevertheless both horizontal and vertical polarization may be deployed over long distance if a reflector is deployed to focus the energy being emitted.
So consequently the decision is using the polarization, which offers the best rejection of local unwanted signal.
Circular polarization is most often used in 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.
These Antennas make very good point-to-point long run connections due to a combination of linear noise rejection and high gain. The two most common a crossed yagi or helix
When setting up an exclusive communications link, it may be wise to first test the link with vertical and then horizontal polarization to see which yields the best performance (if any).
If there are any reflections in the area, especially from structures or towers, one polarization may outperform the other. Further, if there are other RF signals in an area, using a polarization in the opposite predominant high level signals will give some isolation as discussed earlier.
On another note, when radio waves strike a smooth reflective surface, they may incur a 180 degree phase shift, a phenomenon known as specula or mirror image reflection. The reflected signal may then destructively or constructively affect the direct LOS signal.
Circular polarization has been used to an advantage in these situations since the reflected wave would have a different sense than the direct wave and block the fading from these reflections.
Diversity Antennas
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). 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 with some kind of "voting" system to choose the busiest signal. This is commonly used in many 802.11 wireless network equipment.
However in theory for the best results when using external antennas they should be at least 20 wavelengths apart, so that the signals are no longer correlated, particularlly in medium and long-distance situations.