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Definition of antenna related terms



Antenna efficiency

Fraction of the radiated power to the power put into antenna. Ideal perfectly matched lossless antenna has efficiency of 1 (or 100%).

Antenna Gain

Antenna gain is a measure of directivity properties and the efficiency of the antenna. It is defined as the ratio of the radiation intensity in the peak intensity direction to the intensity that would be obtained if the power accepted by the antenna were radiated isotropically. The difference between the antenna gain and the directivity is that the antenna efficiency is taken into account in the former parameter. Antenna gain is measured in dBi, i.e. decibels relative to isotropic antenna.

Antenna Impedance

Ratio of voltage to current at antenna input.

Antenna polarization

Polarization is the orientation of the electric field vector component of the electromagnetic field. In line-of-sight communications it is important that transmitting and receiving antennas have the same polarization (either horizontal, vertical or circular). In non-line-of-sight the received signal undergoes multiple reflections which change the wave polarization randomly.

Amplified antenna

Antenna equipped with the radio frequency amplifier to improve antenna sensitivity to weak signals.

Bandwidth

Antenna bandwidth is the frequency range within which the antenna performance meet specifications.

Beam width

Directional antennas have a radiation intensity peak in the particular direction. Beam width is the angular distance between the points on two opposite sides of the peak direction where the radiation intensity drops to the half of the peak intensity.

dBi

Decibels relative to an isotropic antenna. Units of antenna gain.

Directional antenna

Antenna having a preference for a particular direction and radiating (receiving) a signal more efficiently in (from) this direction than in other directions.

Directivity

Directivity is a measure of how strongly the antenna favors the particular direction of its maximum transmission (reception) sensitivity comparing to other directions. More specifically, it is defined as the ratio of the radiation intensity in the peak intensity direction to the averaged radiation intensity in all other directions.

Front-to-back (F/B) ratio

Another parameter measuring antenna directivity defined as the ratio of the peak radiation intensity in the "front" direction to the radiation intensity at 180 degrees behind the point ("back" direction).

Isotropic antenna

Antenna transmitting (receiving) equal radiation in (from) all directions. Isotropic antenna is a hypothetical idealized device that does not exist in reality. It is usually taken as a reference when measuring directivity of actual realizable antennas.

Noise figure

Signal-to-noise ratio degradation in antenna amplifier. Amplifier noise figure should be as low as possible.

Omnidirectional antenna

Antenna transmitting (receiving) equal radiation in (from) all horizontal directions. A typical example is a whip antenna. Whip antenna's radiation power is distributed equally in all directions in a plane perpendicular to the whip.

UHF

Ultra High Frequency (UHF) band. TV channels 14 to 69 are transmitted in UHF band in frequency range 470-806 Mhz.

VHF

Very High Frequency (VHF) band. TV channels 2 to 13 are transmitted in VHF band in frequency range 54-72 Mhz, 76-88 Mhz, 174-216 Mhz.

VSWR (or SWR)

Voltage Standing Wave Ratio is the ratio of the maximum to minimum voltage on the antenna feeding line. Standing wave pattern is created on the feeding line when the impedance match is not perfect and a fraction of the power put into antenna is reflected back and not radiated. For perfectly impedance matched antenna the VSWR is 1:1.