Author: Wg Cdr Rajagopal

VHF Direction Finder Introduction to VDF VDF is a ground based equipment used in Air Traffic Control to find the bearing of aircrafts Bearing accuracy of VDF is up to half a degree however range information is not available VDF works in the VHF band with frequencies range between 118 to 137 MHz Loop antenna principle is used to find the direction of transmission where the transmission from an aircraft from a perpendicular direction to the loop ariel gives zero phase difference Rotating loop is replaced by electronically rotated fixed…

Antenna Transmitter and Receiver 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 Length of Antenna 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…

Modulation Modulation Modulation is the process of super imposing intelligence on the carrier wave Carrier wave is capable of travelling long distances due to its high frequency and energy Amplitude and frequency of a carrier wave remains constant Signal wave is a low energy wave containing voice or data intelligence Modulation is the process of attaching the signal wave on to carrier wave for transmission Keyed Modulation Keyed modulation is the the simplest form of modulation where intelligence is sent by switching on and off the carrier wave Morse code…