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.

Basic Cable Coding System

(1)           (2)               (3)       (4)       (5)                     (6)

 1         E         F          6          B         22              NMS   V

Position 1	Unit number, used where components have identical circuits.
Position 2	Circuit function letter and circuit designation letter which indicate circuit function and the associated system.
Position 3	Cable number, allocated to differentiate.
Position 4	Cable segment letter, which identifies the segment of cable between two terminals or connections, and differentiates between segments of the circuit when the same cable number is used throughout.  Segments are lettered in alphabetical sequence, excluding the letter I and O.  A different letter is used for each of the cable segments having a common terminal or connection.
Position  5	Cable size.
Position 6	Suffix data, used to indicate the type of cable and to identify its connection function.  For example, in the example code NMS V indicates nyvinmetsheath ungrounded cable in a single-phase system.

Air Data Computer-Anologue

The air data computer (ADC) is an additional and improved means of supplying the pilot and co-pilot with airspeed, altitude, vertical speed and mach number.  Previously, air pressure was supplied to the primary pitot/static flight instruments by long lengths of P/S pipelines.  To minimize these, a central unit was developed where the pitot and static pressures could be fed. This unit is known as ‘The Centralized Air Data Computer’.  The outputs from the CADC are in the form of synchro signals therefore, the CADC may be referred to as being an analogue device.  These are still widely used, although later generation aircraft have switched to digital units.

The simple pitot static system is still fitted in almost all aircraft and will be used as a safety/comparison factor against the CADC and vice versa.  Two independent CADC systems would be installed, the input to each would be from independent sources for safety reasons.

A modular arrangement is shown along with a typical transmission system.