VERY HIGH FREQUENCY (VHF)

DESCRIPTION: The VHF communication system provides short-range (Line Of Sight) voice communication between:
• The Aircraft and Ground Stations.
• The Aircraft and other Aircraft.

VHF communication system transmits data in the form of codes. The frequency range allocated for commercial aviation VHF communication is 118.00 to 136.975 MHz in 25 KHz increments. The VHF communication system receives RF energy via the antenna, processes the RF and sends resulting audio to the Audio Integrating System. The VHF communication system also sends an audio signal to the SELCAL system to alert the flight crew of an incoming call. During transmission, the VHF communication system and RF energy transmitted via the antenna process the microphone audio from the Audio Integrating System.
There are two VHF communication systems installed and wiring provisions for a third, and each system contains the following components:
1. CONTROL PANEL: The control panels are located on the AFT electronic panel.
2. TRANSCEIVERS: The transceivers are located on the E2-1 shelf in the electronic equipment compartment.
3. ANTENNA: The number-1 antenna is located on the top of fuselage and the number-2 antenna on the bottom. The VHF communication system requires a primary input voltage of 27.5V DC at 1 ampere in receive mode and 27.5V DC at 6 ampere (7.5 ampere maximum) in transmit mode.

OPERATION:
• The pilot selects the communication frequency of the Airport or Air Traffic Control that is responsible for the airspace in which the aircraft is flying.
• The pilot actuated the transmitter section of the RT by pressing the microphone key.
• The pilot's voice is transformed into an electrical signal, amplitude modulated and mixed to produced a signal that is radiated from the transmit antenna.
• The receiver section of the ground station RT receives the radiated signal, filters and mixes it and then detects the modulated signal.
• A speaker or headset then transforms the modulated signal into an audio signal.
• The Airport or Air Traffic Controller replies using the same frequency.