LINEAR WIRE ANTENNAS

Linear Wire Antennas

Infinitesimal dipole

Small Dipole / Short Dipole

Radiated Power from short dipole

Radiation resistance of short dipole

Infinitesimal dipole

Hertzian dipole

Hertzian dipole is an infinitesimal current element I dl which does not exist in real life.

OR

Hertzian dipole is a short linear antenna which, when radiating, is assumed to carry constant current along its length

For power to get to an antenna it must be connected to a transmission line.

To prevent standing waves from occurring within the line and for maximum power transfer, the resistance of the transmission line must be equal to the resistance of the antenna.

The antenna resistance is termed radiation resistance. This is defined as a fictitious resistance which would dissipate as much power as an antenna in question is radiating if it were connected to the same transmission line.

If an antenna is radiating 100 W when drawing a current of 2 A then its radiation resistance will be 25 ohm. (P=I2R).

Antenna resistance

Not all energy absorbed by an antenna is radiated.

Losses can occur within the antenna (imperfect dielectrics, eddy currents etc), as such antenna efficiency is defined

Rr is the resistance of the antenna

Rl is resistance due to losses

lr

r

input

dtransmitte

RR

R

P

P

Antenna resistance

Small Dipole

The length of a short dipole

1050

l

Small Dipole - Current Distribution:

Small Antenna: Vector Magnetic Potential

A of a small dipole is exactly half of A of an infinitesimal dipole.

Small Antenna: Electric & Magnetic Fields in Far Field

Directivity of short dipole is the same as those of the infinitesimal dipole, because the normalized patterns of both dipoles are the same.

Directivity of Infinitesimal Dipole and

Short Dipole:

ComparisonInfinitesimally Small Dipole

The radiated power

The radiation resistance

Short dipole

The radiated power is four times less than that of an infinitesimal dipole

the radiation resistance is also four times less compared to that of the infinitesimal dipole

2280

lRr

22

240 oT Il

P

FAR FIELD APPROXIMATION

FAR FIELD APPROXIMATION

FINITE LENGTH DIPOLECurrent Distribution:

Assumptions: Antenna is center fed. Current is min at the ends.

Current in a center-fed wire antenna has sinusoidal form with nulls at the endpoints

Io is constant

FINITE LENGTH DIPOLERadiated Field:

FINITE LENGTH DIPOLERadiated Field:

FINITE LENGTH DIPOLERadiated Field:

FINITE LENGTH DIPOLERadiated Field:

FINITE LENGTH DIPOLERadiated Field:

The full-wavelength dipole antenna is more directional than the shorter quarter-wavelength dipole antenna.

Larger antenna in general have increased directivity The 1.5-wavelength dipole pattern is also plotted in Figure 3. Note that

this pattern is maximum at approximately +45 and -45 degrees.

The dipole antenna is symmetric when viewed azimuthally; as a result the radiation pattern is not a function of the azimuthal angle ø.

Hence, the dipole antenna is an example of an omnidirectional antenna.

the E-field only has one vector component and consequently the fields are linearly polarized.

3D Pattern

Normalized 3d radiation pattern for the 1-wavelength dipole antenna

3D PatternNormalized 3D radiation pattern for the 1.5-wavelength dipole antenna