Antenna Polarisation

Antenna polarisation (or polarization) is a very important consideration when choosing and installing an antenna and can mean as much as a 20db in signal loss if the receiver and transmitter antenna are not using the same polarisation. Most systems use either vertical, horizontal or circular polarisation. Knowing the difference between polarisations and how to maximise their benefit is very important to users.

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 polarisation or orientation of the radio wave. In general, most antennas radiate either linear or circular polarised.

A linear polarised antenna radiates wholly in one plane containing the direction of propagation. Where a circular polarised antenna, the plane of polarisation 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 polarised (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 auto-mobile. Horizontally polarised (linear) antennas have their electric field parallel to the Earth’s surface. Television transmissions use horizontal polarisation.

Circular polarised 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 polarised, when using a Helix Antenna. If the axial ratio is greater than 1-2 dB, the polarisation is often referred to as elliptical, when using a crossed Yagi.

Important Considerations
Polarisation 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 polarisation is linear or elliptical. If the polarisation 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 polarisation of the antennas at both ends of the path must use the same polarisation. In a linearly polarised system, a misalignment of polarisation 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 polarised 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 polarised 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 polarisation is another consideration. It happens when unwanted radiation is present from a polarisation, which is different from the polarisation in which the antenna was intended to radiate. For example, a vertical antenna may radiate some horizontal polarisation and vice versa. However, this is seldom a problem unless there is noise or strong signals are nearby.

Typical Applications
Vertical polarisation is most commonly used when it is desired to radiate a radio signal in all directions over a short to medium range.

Horizontal polarisation is used over longer distances to reduce interference by vertically polarised equipment radiating other radio noise, which is often predominantly vertically polarised. Nevertheless both horizontal and vertical polarisation may be deployed over long distance if a reflector is deployed to focus the energy being emitted. So consequently the decision is using the polarisation, which offers the best rejection of local unwanted signal.

Circular polarisation is most often used in satellite communications. This is particularly desired since the polarisation of a linear polarised 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 polarisation will keep the signal constant regardless of these anomalies. These circular polarised antennas also 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 (point-to-point link), it may be wise to first test the link with vertical and then horizontal polarisation to see which yields the best performance (if any). If there are any reflections in the area, especially from structures or towers, one polarisation may outperform the other. Further, if there are other RF signals in an area, using a polarisation 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 specular or mirror image reflection. The reflected signal may then destructively or constructively affect the direct LOS signal. Circular polarisation 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.