Antenna Properties

Antenna gain:

To be meaningful, gain has to be related to some reference. The reference could be an isotropic antenna. An isotropic antenna is a theoretical reference, thought of as a point in mid air, radiating in all direction. Since it radiates in all direction it has unity gain. Gain related to an isotropic antenna is expressed in dBi.

Also a well-defined dipole design can be used as a reference. Gain related to dipole is expressed in dBd. The dipole reference is 2.14 dB gain compared to an isotropic reference. Thus:

(Gain in dBi)       =    (Gain in dBd)  + 2.14 dB

As an antenna is a passive element the only way to obtain a gain in one direction is to reduce power in other direction. This could be more accurately said as concentration of the signal i.e. higher the concentration, higher the gain will be.

Beam Width:

Antenna gain is actually defined by the Horizontal and Vertical beamwidth along with efficiency of the antenna and in general lesser the beam width higher the gain will be.

The beam width is defined as the opening angle between the two points on each sides of the main lobe direction where the radiated power is 3 dB lower than in main direction. Both Horizontal and vertical beamwidth are of prime importance in selecting an antenna system.

The conventional wisdom currently accepted in the GSM World is that 65⁰ and 90⁰  horizontal beam width antenna perform better in digital systems.

 By using 65⁰ or 90⁰ antenna excessive overlap is avoided as excessive overlap can cause higher bit error rate and can degrade quality because of lot of handovers between adjacent sectors. Please note that a better gain will also be achieved for a reduced beamwidth.

Besides horizontal beam width, vertical beamwidth is of great importance to RF Engineers as in combination with knowledge of both, overall gain of an antenna can be defined if antenna efficiency is known.

Polarization

Polarization can be defined as the direction of oscillation of the electrical field vector.

Mobile communications: vertical polarization

Broadcast systems: horizontal polarization

In most cases the propagation characteristic of an antenna can be described via elevations through the

horizontal and vertical radiation diagrams. In mobile communications this is defined by the magnetic

field components (H-plane) and the electrical field components (E-plane)

E-Plane

For a linearly-polarized antenna, this is the plane containing the electric field vector and the direction of maximum radiation. The electric field or "E" plane determines the polarization or orientation of the radio wave. For a vertically-polarized antenna, the E-plane usually coincides with the vertical/elevation plane. For a horizontally-polarized antenna, the E-Plane usually coincides with the horizontal/azimuth plane.

H-plane

In the case of the same linearly polarized antenna, this is the plane containing the magnetic field vector and the direction of maximum radiation. The magnetizing field or "H" plane lies at a right angle to the "E" plane. For a vertically polarized antenna, the H-plane usually coincides with the horizontal/azimuth plane. For a horizontally-polarized antenna, the H-plane usually coincides with the vertical/elevation plane

Source Wikipedia * Martin lee