Understanding Antenna Polarisation
The following article serves to build an understanding about the antenna parameter known as Polarisation. Polarisation is perhaps the most fundamental characteristic of an antenna which describes the plane in which its electric field radiates. It is an important parameter to understand as wireless communication requires transmit and receive antennas to operate on the same polarity. Misaligned antennas can result in significant signal losses, potentially as high as 20 to 30 dB depending on polar discrimination (more on this later).
Most transmissions are vertically polarised due to the simple construction of vertical polarised antennas and superior propagation through weather. A key exception tends to be UHF television transmissions, which historically have been horizontally polarised to provide some level of interference protection from overwhelmingly vertically polarised transmissions.
Modern day cellular, multipoint, and microwave communications utilise both polarisations simultaneously in the form of Dual Linear Slant polarisation. With improvement in technological design of both antennas and radios, dual polarised transmissions allow a near doubling of data rates. Slant transmissions also provide 3 dB protection from vertical/horizontal transmissions, as well as improved performance in multipath environments where polarisation state can be altered from a reflection surface.
|Linear Vertically Polarised Transmission||Linear Horizontally Polarised Transmission||Dual Linear Slant Polarised Transmission|
Another common type is circular or eliptical polaristion, whereby the tip of the electric field vector is seen to trace out a helix or corkscrew as it travels away from the antenna. The helix will either travel with a left or right-hand rotation, aptly named Left Hand Circular Polarisation (LHCP), or Right Hand Circular Polarisation (RHCP) respectively. Technically circular polarisation is a combination of vertical and horizontal waves with a 90° phase offset, which allows their transmission by relatively simple antenna designs. Circular polarisation is necessary for communications systems that are subject to antenna rotation, the most obvious of which is satellite where the orientation of transmit and receive antennas are not known.
Definition of Polarisation
The polarisation or polarisations of the electric field radiated by the antenna evaluated in the far field.
When disclosing polarisation on test reports and datasheets, Halberd Bastion follow the BASTA reporting recommendations. BASTA define the reporting of polarisation per the following (adapted) specification.
- The nominal value as a type and direction for the reference polarisation of the antenna.
- Linear polarisation is typically defined as H, V.
- Slant linear polarisation is typically defined as +45 / -45.
- Slant value, specified in degrees, is measured using the positive vertical axis as the zero reference.
- Multiple polarisation naming follows standard metric prefixes, excepting dual (two) to avoid confusion between di-pol and dipole.
- Circular polarisation is typically defined as left-handed (LHCP), right-handed (RHCP).
Testing & Reporting
Polarisations are defined per antenna port based on the orientation of the internal radiating elements. For more complex planar/patch antenna designs, polarisation is determined through electrical engineering simulation.
The measure and reporting to describe how polar an antenna is, is known as Cross Polar Discrimination, or XPD. XPD measures how well an antenna rejects transmissions of an orthogonal polarity. A highly polar antenna is necessary for successful MIMO and diversity communications, and essential for a high performing dual/tri/tetra polarised antenna.
While there is no limit to combination of polarisation types as reported on an antenna datasheet, it is however important that a system be adhered to. BASTA specification definition has been expanded to permit the disclosing of slant angles other than 45°. For example, a tetra-slant antenna might have additional ±30° and ±60° slants, positioned 30 and 60 degrees from the positive vertical axis respectively. The number of polarisations is disclosed using standard metric prefixes, excepting for dual polarised, as the metric prefix di- could easily lead to confusion between di-pol antenna and dipole antenna.
Example values include:
- Linear Vertical (V)
- Linear Horizontal (H)
- Slant Linear +45°
- Slant Linear -45°
- Dual Linear V, H
- Dual Slant Linear ±45°
- Tri-Linear V, ±45°
- Tetra-Linear V, H, ±45°
- Left Hand Circular Polarised (LHCP)
- Right Hand Circular Polarised (RHCP)
- NGMN, "Recommendations on Base Station Antenna Standards", NGMN Alliance, N-P-BASTA v9.6, Jan. 2013.