# Antenna Types and Antenna Characteristics

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.
• Antenna radiation performance is usually measured and recorded in two orthogonal principal planes (E-Plane and H-plane or vertical and horizontal planes).
• The pattern of most base station antennas contains a main lobe and several minor lobes, termed side lobes.
• 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.
• Generally the reference antenna is an isotropic antenna.
• Gain is measured generally in “decibels above isotropic (dBi)” or “decibels above a dipole (dBd).
• An isotropic radiator is an ideal antenna which radiates power with unit gain uniformly in all directions. dBi = dBd + 2.15
• Antenna gain depends on the mechanical size, the effective aperture area, the frequency 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.

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.
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