Antenna

GENERAL

An antenna is an electrical device which converts electric power into radio waves, and vice versa.

Broadly there can be 2 types of antenna,

1. Transmitter

2.Receiver

In transmission, a radio transmitter supplies an electric current oscillating at radio frequency (i.e. an alternating current) to the antenna’s terminals, and the antenna radiates the energy in the form of electromagnetic waves (radio waves) in the air. In reception, an antenna intercepts some of the power of an electromagnetic wave radiated in the air by the transmitter, in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified.

DIPOLE ANTENNA

A radiating wire (antenna) is most efficient when its length is equal to half(1/2) a wavelength.

This type of antennas are termed as Dipole Antenna and can be represented as

L = 1/2 λ

For example:

For a frequency of 100 MHz, the wire should be

(3 X 108)/(2 X 100 X 106) = 1.5m long

Which is practically impossible to be incorporated in the aircraft, and hence the Unipole Antenna

UNIPOLE ANTENNA

A radiating wire (antenna) where its length is equal to 1/4 of its wavelength, it is called an Unipole antenna

L = 1/4 λ

For V.H.F communication (30 -300 MHz), the length of the antenna required would beless than 60 cm.

Two Unipole antennas are sometimes connected back to back to work as a Dipole Antenna.

But for the frequencies within the range of10-100 kHz, the maximum dimension of even large aircraft is only a small fraction of a wavelength. At these frequencies, Capacitive type antennas maybe used.

CAPACITIVE ANTENNA

One plate of the capacitor is the airframe; the other a horizontal tube, vertical blade or a mesh (sometimes a solid plate). The aircraft causes the field to become intensified over a limited region near its surface.

The resulting comparatively strong oscillating electrical field between the capacitor’s plates causes a current to flow in twin feeder or coaxial cable connected across the antenna.

The airborne systems operating in the relevant frequency band are the receive-only systems

LOOP ANTENNA

An alternative to the capacitance antenna is the loop antenna which is basically a loop of wire which cuts the magnetic field component of the e.m. wave.

The field is intensified by use of a ferrite core on which several turns are wound.

Use of two loops mounted at right angles provides a means of ascertaining the direction of arrival (ambiguous) of an e.m. wave.

Such antennas are used for ADF(loop) and may also be used for Omega.

A PRACTICAL ANTENNA CANNOT BE OMNIDIRECTIONAL IN ALL PLANES, I.E. IN THREE DIMENSIONS.