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 Radiation Pattern

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.

Antenna Pattern Theoritical

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.