Antenna types

In mobile communications two main categories of antennas used are Omni and Directional antenna here I write about antenna characteristics and use.

ANTENNA TYPES Omni directional antenna  These antennas are mostly used in rural areas.  

In all horizontal direction these antennas radiate with equal power.

In the vertical plane these antennas radiate uniformly across all azimuth angles and have a main beam with upper and lower side lobes.

Directional antenna  These antennas are mostly used in mobile cellular systems to get higher gain compared to Omni directional antenna and to minimize interference effects in the network.  In the vertical plane these antennas radiate uniformly across all azimuth angles and have a main beam with upper and lower side lobes. 

In these types of antennas, the radiation is directed at a specific angle instead of uniformly across all azimuth angles in case of Omni antennas.

ANTENNA CHARACTERISTICS Radiation Pattern  The main characteristics of antenna are the radiation pattern. 

The antenna pattern is a graphical representation in three dimensions of the radiation of the antenna as a function of angular direction.

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Antenna radiation performance is usually measured and recorded in twoorthogonal principal planes (E-Plane and H-plane or vertical and horizontal planes).

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The pattern of most base station antennas contains a main lobe and several minor lobes, termed side lobes.

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A side lobe occurring in space in the direction opposite to the main lobe is called back lobe.

Antenna Gain  Antenna gain is a measure for antennas efficiency. 

Gain is the ratio of the maximum radiation in a given direction to that of a reference antenna for equal input power.

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Generally the reference antenna is an isotropic antenna.

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Gain is measured generally in “decibels above isotropic (dBi)” or “decibels above a dipole (dBd).

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An isotropic radiator is an ideal antenna which radiates power with unit gain uniformly in all directions. dBi = dBd + 2.15

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Antenna gain depends on the mechanical size, the effective aperture area, thefrequency band and the antenna configuration.

Antennas for GSM1800 can achieve some 5 to 6 dB more gain than antennas for GSM900 while maintaining the same mechanical size.

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Front-to-back ratio  It is the ratio of the maximum directivity of an antenna to its directivity in a specified rearward direction. 

Generally antenna with a high front-to-back ratio should be used.

First Null Beamwidth  The first null beamwidth (FNBW) is the angular span between the first pattern nulls adjacent to the main lobe. 

This term describes the angular coverage of the downtilted cells.

Antenna Lobes  Main lobe is the radiation lobe containing the direction of maximum radiation. 

Side lobes

Half-power beamwidth  The half power beamwidth (HPBW) is the angle between the points on the main lobe that are 3dB lower in gain compared to the maximum. 

Narrow angles mean good focusing of radiated power.

Polarization  Polarization is the propagation of the electric field vector. 

Antennas used in cellular communications are usually vertically polarized or cross polarized.

Frequency bandwidth  It is the range of frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard. 

VSWR of an antenna is the main bandwidth limiting factor.

Antenna impedance  Maximum power coupling into the antennas can be achieved when the antenna impedance matches the cables impedance. 

Typical value is 50 ohms.

Mechanical size  Mechanical size is related to achievable antenna gain. 

Large antennas provide higher gains but also need care in deployment and apply high torque to the antenna mast.