Antenna Terms and Definitions

Antenna: An antenna is a device that transmits and/or receives electromagnetic waves. Electromagnetic waves are often referred to as radio waves. Most antennas are resonant devices, which operate efficiently over a relatively narrow frequency band. An antenna must be tuned to the same frequency band that the radio system to which it is connected operates in, otherwise reception and/or transmission will be impaired.

Antenna Factor: The Antenna Factor (AF) is defined as the ratio of the incident Electromagnetic Field to the output voltage from the antenna.

Aperture: An aperture is an opening or hole, for antennas typically this term refers to the opening of a horn antenna.

Balun: An acronym for Balanced/Unbalanced. Typically an RF transformer used to couple a balanced transmission line to an unbalanced antenna system.

Biconical Antenna: A broadband antennas made up of two cones/elements. This antenna type is often used for compliance testing; both immunity and emissions type testing.

Broadband antenna: An antenna capable of operation over a wide band of frequencies.

Directivity: It is a measure of how focused an antenna coverage pattern is in a given direction. A theoretical loss-less antenna element, referred to as a isotropic element, has 0 dBi directive gain equally distributed in all 3 dimensions. In order to achieve higher directive gain, antennas are normally designed to focus or concentrate the antenna pattern only in the direction of the radio link, thereby maximizing energy usage.

Dynamic Range: Is the range of power between the maximum signal and minimum signal that can be measured.

Electric Field (E-Field): An electric field or E-field is an effect produced by an electric charge that exerts a force on charged objects in its vicinity.

Far Field: The far field corresponds to an RF source-to-measurement antenna distance great enough that energy radiates from the source only in a radial direction. The E and H fields are mutually perpendicular to that direction and each other, and their ratio is 377 W, the impedance of free space.

Front-to-Back Ratio: This ratio is the main lobe gain divided by the rear lobe gain. Since antenna gains and Front-to-Back ratios are usually given in dB, you normally get the Front-to-Back figure by subtracting the rearward gain from the forward gain (both in dB).

Gain: The ratio of the signal, usually expressed in dB, received or transmitted by a given antenna as compared to an isotropic or dipole antenna. Antenna gain can only be achieved by making an antenna directional, that is, with better performance in one direction than in others.

Half-Power Beamwidth: The half-power beamwidth is the angular distance on ether side of the peak field, or main lobe, of the antenna that represents half of the peak field intensity. Half the power is equivalent to -3dB, so the half-power beamwidth is also sometimes referred to as the 3dB beamwidth.

Isotropic Radiator: A theoretical source point that radiates uniformly in all directions over a sphere centred on the source.

Magnetic Field (H-Field): The measured intensity of a magnetic field at a specific point. Usually expressed in amperes/meter.

Mismatch loss is a measurement describing the amount of power that will not be available at the load due to the reflected wave. It indicates the amount of power lost in the system due to the mismatched impedances.

Near Field: The close-in region of an antenna where the angular field distribution is dependent upon the distance from the antenna.

Polarization: Polarization is defined as the orientation of the electric field of an electromagnetic wave. Polarization is in general described by an ellipse. Two often used special cases of elliptical polarization are linear polarization and circular polarization. The initial polarization of a radio wave is determined by the antenna that launches the waves into space. The environment through which the radio wave passes on its way from the transmit antenna to the receive antenna may cause a change in polarization.

With linear polarization the electric field vector stays in the same plane. In circular polarization the electric field vector appears to be rotating with circular motion about the direction of propagation

Choice of polarization is one of the design choices available to the RF system designer. For example, low frequency (less than 1 MHz) vertically polarized radio waves propagate much more successfully near the earth than horizontally polarized radio waves, because horizontally polarized waves will be cancelled out by reflections from the earth. Mobile radio systems waves generally are vertically polarized. TV broadcasting has adopted horizontal polarization as a standard. This choice was made to maximize signal-to-noise ratios. At frequencies above 1 GHz, there is little basis for a choice of horizontal or vertical polarization, although in specific applications, there may be some possible advantage in one or the other. Circular polarization has also been found to be of advantage in some microwave radar applications to minimize the "clutter" echoes received from raindrops, in relation to the echoes from larger targets such as aircraft. Circular polarization can also be used to reduce multipath.

Radiation Pattern: A chart of relative radiation intensity (or power) versus direction. A plot of the radiated energy from an antenna. The variation of the field intensity of an antenna as an angular function with respect to the axis.

Reflection Coefficient indicates how much of an electromagnetic wave is reflected by an impedance discontinuity in the transmission medium. It is a ratio of the amplitude of the reflected wave to the wave incident at the junction. The magnitude of the reflection coefficient does not depend on the length of the line, only the load impedance and the impedance of the transmission line.

VSWR: As the electric wave travels through the different parts of the antenna system (radio, feed line, antenna, free space) it may encounter differences in impedance. At each interface, some fraction of the wave's energy will reflect back to the source, forming a standing wave in the feed line. The ratio of maximum power to minimum power in the wave can be measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface will reduce SWR and maximize power transfer through each part of the antenna system.

Wavelength: The wavelength is the distance between repeating units of a wave pattern. It is commonly designated by the Greek letter lambda (λ).

X-Band: A range of frequencies between 8GHz and 12GHz.