Radio Antenna – Ibex Aviation Academy

Antenna

Antennas are used to transmit and receive the radio waves
Antenna or Ariels should be made of good conductors like aluminium
Transmitters transmit modulated wave which are demodulated in the receiver to extract intelligence

Ideal length of an antenna has to be proportional to the wave length
Since it is difficult to construct antennas in kilometres for lower frequencies we use half or quarer wave dipole antennas
Size of a Half wave dipole antenna is made equal to half the wave length
Folded half wave dipoles are used for televisions in order to receive a broader band of frequencies
Size for quarter wave dipole antenna is a quarter of wavelength
An Ideal quarter wave antenna is called as Marconi antenna with a length of 95% of quarter of wavelength
Marconi antennas are fixed to metal surfaces of aircraft increases antenna size
Ariel Loading Unit enables utilization of the same antenna for a range of signals

Directional antennas help in focusing radio waves in a particular direction
For example, Instrument Landing System transmits waves in the runway direction
Parasitic elements like metallic reflectors with 5% longer length behind the antenna assists in directivity
Distance between ariel and reflector is a quarter of its wavelength
Yagi antenna use additional parasitic elements in front for directivity
Side lobes are unwanted signals transmitted by Yagi antennas in other than the desired direction

Loop antenna is a rotating loop shaped antenna used for direction finding
Rotating loop calculates the phase difference between its vertical elements
Phase difference is zero only when the loop is perpendicular to wave
Error signal is generated due to phase difference other directions inducing a current in the circuit
Automatic Direction Finder utilise loop ariel principle for direction measurement

Radio detection and ranging systems are used to detect objects using narrow and directional beams in SHF and EHF bands
A rotating antenna is used in radars to get a 360 deg picture
Radar antennas obtain power through wave guides which are vacuum filled flexible tubes of half wave length
Low levels of resistance in wave guides provide greater power supply to the Radar Transmitter Antenna

Radar systems use parabolic dish antenna to produce directional waves
Parabolic reflectors in radars produce parallel waves of equal path length and same phase
Side lobes from parabolic dish may affect the functioning of radar equipment

Flat plate or Slotted planar array antenna are used in Airborne weather radars
Many parallel beams are transmitted through slots in a metal plate producing a narrow beams of high energy
Flat plate antennas have lesser side lobes and require lesser power for transmission