Study Material Aircraft-Maintenance-Engineering
Very High Frequency Communication VHF
Very High Frequency Communication is the standard civil short range communication facility using the band of frequencies between 118.000 MHz and 137 MHz. VHF is used by ground control facilities and aircraft or by aircraft and other aircraft on one of 760 possible frequency channels with 25 kHz spacing between channels. Another VHF service available is weather information transmitted from ground stations
Current International Civil Aviation Organisation (ICAO) regulations require a new channel spacing one third of the current spacing ie. 8.33 kHz apart. This will treble the number of available channels to 2280 (3760). Frequency selection is by means of 2 from 5 system converted to Binary Coded Decimal (BCD), from which control of phase-locked loops (PLL) determine transmitter Tx and receiver Rx frequencies.
VHF uses line of sight space wave transmissions with a theoretical range of 123 miles between an aircraft at a height of 10-,000 ft and a ground station at sea level. In practice, however, useable range will also vary depending upon factors such as transmitter power, receiver sensitivity, atmospheric and temperature conditions, ground station geographical situation etc. and any obstruction in between aircraft and ground station eg. mountains, hills and trees. As a general rule, satisfactory two-way communication can typically be maintained up to 200 miles, this range dependent on the aircraft height.
Speech amplitude modulation is used for radio telephony transmissions which are vertically polarised transmissions. A common aerial for transmit and receive purposes is used, this system being known as a single channel simplex system.