Antenna Types and Antenna Characteristics

Antenna Types and Antenna Characteristics, antennas radiate uniformly across all azimuth, antennas for gsm, side lobes, radiation pattern, reference antenna.

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.
Article Topics :

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