Radio Principles & Practice: Candidates will be required to have knowledge of Electrical Units such as Volt. Ampere, Ohm and Watt; Wavelength, frequency and their relationship; elementary knowledge of radio frequency propagation and night frequencies, skip distances, fading, ground shadow and its effect on communication, choice of frequencies to attain maximum efficiency in handling air ground HF communications. General knowledge of systems employed for air-ground communications including SELCAL operation inter-communication and announcing systems of aircraft; elementary knowledge of Radio-navigation Aids, operation of microphones and headphones, Squelch, AVC, Volume Control, Tuning of Transmitter, Simplex and duplex operation; Advantages and Disadvantages of Radio Telephone Communication; Limitations of range due to frequency Interference etc.