Wireless Antenna Terminology

The purpose of this page is to provide definitions of Wireless LAN / fixed-wireless antennas and their characteristics. The definitions in quotation marks are taken from IEEE Standard Definitions of Terms for Antennas, IEEE Std 145-1983.

Antenna
"That part of a transmitting or receiving system which is designed to radiate or to receive electromagnetic waves". An antenna can also be viewed as a transitional structure (transducer) between free-space and a transmission line (such as a coaxial line). An important property of an antenna is the ability to focus and shape the radiated power in space e.g.: it enhances the power in some wanted directions and suppresses the power in other directions.

Adaptive (smart) Antenna
"An antenna system having circuit elements associated with its radiating elements such that one or more of the antenna properties are controlled by the received signal".

Wireless Access Point Antenna
Wireless Access Point Antenna, which can be either an omni-directional antenna or multiple panel (directional) antennas mounted on a tall tower or building.

Antenna Polarization
"In a specified direction from a wireless antenna and at a point in its far field, is the polarization of the (locally) plane wave which is used to represent the radiated wave at that point". "At any point in the far-field of an antenna the radiated wave can be represented by a plane wave whose electric field strength is the same as that of the wave and whose direction of propagation is in the radial direction from the antenna. As the radial distance approaches infinity, the radius of curvature of the radiated wave's phase front also approaches infinity and thus in any specified direction the wave appears locally a plane wave". In practice, antenna polarization of the radiated energy varies with the direction from the center of the antenna so that different parts of the pattern and different side lobes sometimes have different polarization. The antenna polarization of a radiated wave can be linear or elliptical (with circular being a special case).

Bandwidth
See "Frequency Bandwidth"

Beamwidth
See "Half-Power Beamwidth"

CPE Antenna
Customer premises antenna, usually a small directional antenna which points to an access point.

Coaxial Cable
A type of cable which contains two conductors, one inside and the other outside around it, separated by an insulating layer. They share the same axis and are concentric. Coaxial cable "co-ax" is commonly used in cable-TV and Ham radio applications.

Coaxial Dipole Antenna
"An antenna comprised of a extension to the inner conductor of a coaxial line and a radiating sleeve which in effect is formed by folding back the outer conductor of the coaxial line".

Collinear Array Antenna
"A linear array of radiating elements, usually dipoles, with their axes lying in a straight line". Collinear arrays are usually found in omnidirectional antennas.

Co-Polarization
"That polarization which the antenna is intended to radiate".

Cross-Polarization
"In a specified plane containing the reference polarization ellipse, the polarization orthogonal to a specified reference polarization". The reference polarization is usually the co-polarization.

dBd
See "Antenna Gain"

dBi
See "Antenna Gain"

Directional Antenna
"An antenna having the property of radiating or receiving electromagnetic waves more effectively in some directions than others". A directional antenna is usually defined as uni-directional and not omni-directional.

E-Plane
"For a linearly polarized antenna, the plane containing the electric field vector and the direction of maximum radiation". For a vertically-polarized WLAN antenna, the E-plane usually coincides with the vertical/elevation plane.

Effective Radiated Power (ERP)
"In a given direction, the relative gain of a transmitting antenna with respect to the maximum directivity of a half-wave dipole multiplied by the net power accepted by the antenna from the connected transmitter". EIRP is the effective radiated power with respect to the directivity of an isotropic radiator.

Far-Field Region
"That region of the field of an antenna where the angular field distribution is essentially independent of the distance from a specified point in the antenna region". The radiation pattern is measured in the far field. The far-field minimum distance is defined as 2*D^2/lambda, where D is the largest dimension of the antenna and lambda is the free-space wavelength.

Frequency Bandwidth
"The range of frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard". VSWR of a wireless antenna is the main bandwidth limiting factor.

Front-to-Back (F/B) Ratio
"The ratio of the maximum directivity of a wireless antenna to its directivity in a specified rearward direction". Sometimes, the directivity in the rearward direction is taken as the average over an angular region.

Gain Pattern
Normalizing the power/field to that of a reference antenna yields a gain pattern. When the reference is an isotropic antenna, the gain is expressed in dBi. When the reference is a half-wave dipole in free space, the gain is expressed in dBd.

H-Plane
"For a linearly polarized antenna, the plane containing the magnetic field vector and the direction of maximum radiation". For a vertically-polarized WLAN antenna, the H-plane usually coincides with the horizontal/azimuth plane.

Half-Power Beamwidth (HPBW)
"In a radiation pattern cut containing the direction of the maximum of a lobe, the angle between the two directions in which the radiation intensity is one-half the maximum value". The Half-power beamwidth is also commonly referred to as the 3-dB beamwidth. Beamwidth typically decreases as antenna gain increases.

Half-Wave Dipole
"A wire antenna consisting of two straight collinear conductors of equal length, separated by a small feeding gap, with each conductor approximately a quarter-wave length long".

Input Impedance
"The impedance presented by an antenna at its terminals". The input impedance is a complex function of frequency with real and imaginary parts. The input impedance is graphically displayed using a Smith chart.

Isolation
"A measure of power transfer from one antenna to another". This is also the ratio of the power input to one antenna to the power received by the other antenna, expressed in decibels (dB). The same definition is applicable to two-port antennas such as dual-polarization or dual-band antennas.

Isotropic Radiator
"A hypothetical, lossless antenna having equal radiation intensity in all directions". For a WLAN antenna, the gain in dBi is referenced to that of an isotropic antenna (which is defined as 0 dBi).

Linear Array
A set of radiating elements (e.g. dipole or patch) arranged along a line. Radiating elements such as dipole and patch have dimensions comparable to a wavelength. A linear array has a higher gain, than a single radiator, and its radiation pattern can be synthesized to meet various antenna performance requirements such as upper side lobe suppression and null fill. It should be noted that the gain of any antenna is proportional to its size.

Log-Periodic Antenna
"Any one of a class of wireless antennas having a structural geometry such that its impedance and radiation characteristics repeat periodically as the logarithm of frequency". Log-periodic dipole arrays are the most common type, and have a very broad frequency range and uni-directional characteristics.

Microstrip Antenna
"A wireless antenna which consists of a thin metallic conductor bonded to a thin grounded dielectric substrate". An example of such antenna is the microstrip patch.

Major / Main Lobe
"The radiation lobe containing the direction of maximum radiation". For most practical directional antennas there is only one main beam.

Normalized Pattern
Normalizing the power / field with respect to its maximum value yields a normalized power / field pattern with a maximum value of unity (or 0 dB).

Null Filling
The process to fill the nulls in the antenna radiation pattern to avoid blind spots in a coverage area.

Omnidirectional Antenna
"An antenna having an essentially non-directional pattern in a given plane of the antenna and a directional pattern in any orthogonal plane". For Wireless LAN antennas, the omnidirectional plane is the horizontal plane.

Parabolic Reflector Antenna
Parabolic Reflector antenna consists of a parabolic metal surface (dish) with a feed antenna in front. The feed antenna consists of a directive antenna such as a dipole and reflector, log-periodic dipole array or horn antenna. Parabolic Reflector antenna is capable of producing extremely high gains, usually in the 20 - 30 dBi range.

Power Handling
Is the ability of a Wireless LAN antenna to handle high power without failure. High power in antenna can cause voltage breakdown and excessive heat (due to conductor and dielectric antenna losses) which would result in an antenna failure.

Radiation Efficiency
"The ratio of the total power radiated by a Wireless LAN antenna to the net power accepted by the antenna from the connected transmitter".

Side Lobe Level (SLL)
The ratio, in decibels (dB), of the amplitude at the peak of the main lobe to the amplitude at the peak of a side lobe.

Side Lobe Suppression
"Any process, action or adjustment to reduce the level of the side lobes or to reduce the degradation of the intended antenna system performance resulting from the presence of side lobes". For WLAN antennas, the first side lobe above the horizon is preferred to be low in order to reduce interference to adjacent sites. At the other hand, the side lobes below the horizon are preferred to be high for better coverage.

Voltage Standing Wave Ratio (VSWR)
The ratio of the maximum/minimum values of standing wave pattern along a transmission line to which a load is connected. VSWR value ranges from 1 (matched load) to infinity for a short or an open load. For most Wireless LAN antennas the maximum acceptable value of VSWR is 2.0. VSWR of 1.5 or less is excellent. This is approximately the same as a Return Loss of 14.5 dB. What this means is that most of the signal from the transmitter to the antenna is being radiated. (96% radiated and 4% reflected) A VSWR of 2.0 (return loss of 9.5 dB) means that 90% is radiated and 10% reflected.

Yagi or Yagi-Uda Antenna
Yagi Antenna is a directive antenna array usually consisting of a dipole, reflector and multiple director elements having gain in the 6 to 18 dBi range.