Antennas Study Notes for Electronics and Communication Engineering for GATE EC Exam

Antenna

An antenna is a way for linking the electromagnetic waves from transmitter to receiver.

A resonant antenna is a transmission line whose length is exactly equal to the multiple of half wavelength, i.e.,

where, n = 1, 2, 3,….. positive real integer and λ is wavelength.

Basic Antenna Parameters: Antenna parameters are the factors that select the antenna for particular reason or application.

Directivity: Directivity is defined as the ratio of radiation intensity in particular direction to average radiation intensity i.e., 

• U(θ, φ) = radiation intensity in a direction
• U_av = average radiation intensity

Dipole Antenna

• A dipole antenna may be defined as a symmetrical antenna in which the two ends are at equal potential relative to mid-point.
• A physical dipole consists of two equal and opposite charges i.e., in the literal sense, two poles.
• The dipole magnetic field lines are shown in the figure.
• A half-wave dipole λ/2 antenna is the fundamental ratio antenna of the metal rod on a thin wire, which has a physical length of half wavelength in free space.

Note: A λ/2 antenna is also known as Hertz antenna or half-wave doublet.

Antenna’s Power and Field: Normalized power and field pattern can be obtained by dividing a power or field component by its maximum value respectively i.e.,

Normalized power

 

Normalized field pattern: