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Antenna Theory: Basic Concepts

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Antenna Characteristics and Important Definitions:

In Antenna Radiation pattern term refers to the directional (angular) dependence of the strength of the radio waves from the antenna or other source. Basically it is the relative distribution of radiated power as a function of distance in space.

The power radiated (i.e., emitted, reflected, transmitted or received) from an antenna per unit solid angle is called the radiation intensity. In SI units it is measured as watts per steradian (W/sr). The radiation intensity is independent of distance.

Beam Efficiency:

The total beam area (WA) consists of the main beam area (WM) plus the minor lobe area (Wm).

Thus WA = WM+ Wm.

The ratio of the main beam area to the total beam area is called beam efficiency.

Beam efficiency (SM) = WM / WA.

Directivity:

The directivity of an antenna is equal to the ratio of the maximum power density P (q,f)max to its average value over a sphere as observed in the far field of an antenna.

D = P (q,f)max / P(q,f)av. Directivity from Pattern.

D = 4p / WA. Directivity from beam area (WA).

Aperture Efficiency:

The ratio of the effective aperture to the physical aperture is the aperture efficiency. i.e Aperture efficiency = hap = Ae / Ap (dimensionless).

Effective Height:

The effective height h of an antenna is the parameter related to the aperture. It may be defined as the ratio of the induced voltage to the incident field.i.e H= V / E.

Field Zones:

The fields around an antenna may be divided into two principal regions.

Near field zone (Fresnel zone)
Far field zone (Fraunhofer zone)

Polarization:

The polarization of the radio wave can be defined by direction in which the electric vector E is aligned during the passage of atleast one full cycle.Also polarization can also be defined the physical orientation of the radiated electromagnetic waves in space. The polarization are three types. They are Elliptical polarization ,circular polarization and linear polarization.

Antenna Efficiency:

The efficiency of an antenna is defined as the ratio of power radiated to the total input power supplied to the antenna. Antenna efficiency = Power radiated / Total input Power.

The antenna is a radiating device in which power is radiated into space in the form of electromagnetic wave. W’ = I2 R Rr = W’/ I2 Where Rr is a fictitious resistance called called as radiation resistance.

Antenna Beam Width:

Antenna beamwidth is a measure of directivity of an antenna. Antenna beam width is an angular width in degrees, measured on the radiation pattern (major lobe) between points where the radiated power has fallen to half its maximum value. This is called as “beam width” between half power points or half power beam width.(HPBW).

A isotropic radiator is a fictitious radiator and is defined as a radiator which radiates fields uniformly in all directions. It is also called as isotropic source or omni-directional radiator or simply unipole.

Gain:

The ratio of maximum radiation intensity in given direction to the maximum radiation intensity from a reference antenna produced in the same direction with same input power. i.e Maximum radiation intensity from test antenna (G)= Maximum radiation intensity from the reference antenna with same input power.

Self Impedance:

Self impedance of an antenna is defined as its input impedance with all other antennas are completely removed i.e away from it.

Mutual Impedance:

The presence of near by antenna no.2 induces a current in the antenna no.1 indicates that presence of antenna no.2 changes the impedance of the antenna no.1. This effect is called mutual coupling and results in mutual impedance.

Cross Field:

Normally the electric field E is perpendicular to the direction of wave propagation. In some situation the electric field E is parallel to the wave propagation that condition is called Cross field.

Axial Ratio:

The ratio of the major to the minor axes of the polarization ellipse is called the Axial Ratio (AR).

Duality Of Antenna:

It is defined as an antenna is a circuit device with a resistance and temperature on the one hand and the space device on the other with radiation patterns, beamangle, directivity gain and aperture.

Poynting Theorem:

It states that the vector product of electric field intensity vector E and the magnetic filed intensity vector H at any point is a measure of the rate of energy flow per unit area at that point. The direction of power flow is perpendicular to both the electric field and magnetic field components.

Beam Width Of Major Lobe:

It is defined the angle between the first nulls in other words it can be defined as twice the angle between the first null and the major lobe maximum direction.

Side Lobe Ratio:

Side Lobe Ratio is defined as the ratio of power density in the principal or main lobe to the power density of the longest minor lobe.