1 Basic Antenna Concepts

7/30/2019 1 Basic Antenna Concepts

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BASIC ANTENNA CONCEPTS

What is an antenna ?

It is a device which radiates or receives electromagnetic wave

An antenna is a system of elevated conductors which couples or matches the

transmitter or received to freespace.

A transmitting antenna connected to a transmitter by a transmission line, forces

electromagnetic wave into free space which travel in free space with velocity of

light, similarly, a receiving antenna connected to a radio receiver, receivers or

intercepts a portion of electromagnetic waves traveling through space.

So, antenna is defined as the structure associated with the region of transition

between a guided wave & free space wave or vice versa.

In this case three most important terms use shown below.

Transmission

Radiation

Propagation


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1. Transmission:

The high frequency electromagnetic wave is displaced along with transmittion

line known as transmittion & this wave is called guided wave.

2. Radiation:

When transmittion line one end is open so some energy are reflected back & some

energy are likage & this likage energy is called radiation.

Radiation is conversion of guided wave into free space wave

3. Propagation:

The electromagnetic wave is displaced in free space called propagation

= + j

= propagation constant

= attenuation constant

= phase constant

Various Parameters used in an Antenna design:

1. Radiation pattern:

Antenna radiates energy in particular direction this radiation is related to radiation

pattern.

An antenna radiate energy is not of the same strength in all directions. Instead, it

is more is one direction and less or zero is other direction.

The energy radiated in particular direction by an antenna is measured in terms of

FIELD STRENGTH at a point which is at a particular distance from the antenna.

For the calculation of field strength, the voltages at two points on electric lines of

force are taken and then it is divided by the distance between the two points.

Hence the unit of radiation pattern is volt meter.


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Radiation pattern of an antenna is nothing but a graph which shows the variation

in actual field strength of electromagnetic field at all points which are at equal

distance from the antenna.

Radiation pattern is the graphical representation of radiation intensity.

1. Field pattern:

If the radiation from the antenna is expressed in terms of field strength E. the

radiation pattern is called as the field strength pattern.

2. Power pattern:

The radiation in a given direction is expressed in terms of power per unit solid

angle, then the resulting pattern is power pattern.

The common type of radiation pattern can be classified as

Omni directional or broadcast type pattern.

Pencil beam pattern.

Fan beam pattern.

Shaped beam pattern.

Radiation intensity:

It is a power radiated by antenna per unit solid angle

There are two types of radiation pattern:

1. field pattern

2. power pattern


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Principal pattern:

Different parts of radiation pattern are referred to as lobes.

Major lobe: (main beam)

It defined as the radiation lobe contaning the direction of maximum radiation

Minor lobe:

All he lobe except the major lobes are minor lobe.

Side lobe:

It is a radiation lobe in any direction other than intended lobe.Back lobe:

Normally refers to aminor lobe that occupies the hemisphere in a direction

opposite to that of the major lobe.

Normalized field pattern:

N.F.P which is a dimensionless number with a max. value of unity.


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Normalized power pattern:

N.P.P. can be defined as ration of actual power per maximum power.

2. Beam area:

Maximum area of radiation pattern of omni direction:

Beam solid angle:


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3. Antenna gain:

Ability of antenna to radiate or to receive the energy from particular direction.

4. Directivity & directive gain:

Directive gain:

It is the radiation intensity per average radiation intensity

Directivity:

Max. Directive gain.

Directivity =

5. BAND WIDTH:

Antenna bandwidth is a width of freq. over which the antenna maintains certain

required characteristics like gain, front to back ration or s.w.r pattern , polarization &

impedance.

B.w. is represented by a ratio of highest to lowest operating freq.


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6. Beam width:

Beam width:

It is defined as the angular width (in degree) of the major lobe between the two

direction at which the radiated or received power in one half of the maximum power. It is

denoted by .

Half power beamwidth:

Antenna beam width is an angular width in degrees, measured on the radiation

pattern between points where the radiated power has fallen to half its max. value. This is

called as beam width between half power point or half power beam width. Also known

as 3 db bandwidth. Band width between two first nulls.

7. BEAM EFFICIENCY:

Beam efficiency is defined as ratio of power transmitted within cone angle Q1 to

power transmitted by the antenna.

And also defined as the ratio of the main beam area to the total beam area.


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8. STRAY FACTOR:

Ratio of minor beam area to the total beam area.

9. EFFECTIVE LENGTH OR EFFETIVE HEIGHT:

The term effective length of an antenna represents the effectiveness of an antenna

as radiator or collector of electromagnetic wave energy.

It is the ratio of induced voltage at the terminal of the receiving antenna under open

circuited condition to the incident electric field intensity E.

10. ANTENNA EFFICIENCY:

Antenna efficiency is defined as the ratio of output power to the input power


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11. POLARIZATION:

All electromagnetic, EM, waves, traveling in free space, have an electric field

component, E, and a magnetic field component, H, which are usually perpendicular to

each other and both components are perpendicular to the direction of propagation as

shown in figure. The orientation of the E vector is used to define the polarization of the

wave; if the E field is orientated vertically the wave is said to be vertically polarized.

Sometimes the E field rotates with time and it is said to be circularly polarized.

Polarization of the wave radiating from an antenna is an important concept when one is

concerned with the coupling between two antennas or the propagation of a radio wave.

A closely related parameter is the impedance of a wave; this is the ratio of E/H and for

free space is close to 377 ohms. This is not to be confused with the radiation resistance of

an antenna; its just that they have the same units. If a propagating radio wave encounters

a medium of a different impedance, part of the wave is reflected, much like the

reflections at a discontinuity in a transmission line. The remaining energy of the wave

that passes through the discontinuity is refracted in a different direction of propagation,

just like the distortion one sees as a light beam passes through water. The reflection and

refraction properties often depend upon the polarization of the EM wave.


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Types of polarization:

Linear

Elliptical

Circular

Cross

12. Front to back ratio:

It is the ratio of radiation intensity of desired direction to undesired direction. It is

inversely proposnal to efficiency.

13. Antenna aperture:

Effective aperture

It may be defined as the ratio of power received of the antenna load terminal to

the pointing vector in watt/m2 of the incident wave.

P: pointing vector of incident lane wave in watts/m2.

W: power received (watt)

A : effective aperture

14. Scattering aperture:

In scattering aperture energy is received and absorbed by radiation resistance

(Rr).

Scattering ratio:

It is the ration of the scattering aperture to the effective aperture denoted

by


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Loss aperture:

In loss aperture power is absorbed by loss resistance (RL).

Collecting aperture:

Collecting aperture is defined as the sum of all above apertures:

Ac = Ae + As + Al

Physical aperture:

Physical aperture is related to the actual physical size or physical structure

of antenna and is denoted by Ap.

1 Basic Antenna Concepts

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