Radio Transmission and Reception in Aviation

Radio Transmission and Reception

• Intercoms and telephones allow for simultaneous talking and listening
• Audio signals can be transmitted over long distances using wiring in intercom systems or through radio systems using the atmosphere as the transmission medium

Electromagnetic Waves

• An electric current transmitted along a conductor develops an electromagnetic (EM) field
• The EM field induces a small current in receiving antennas, which is amplified to reproduce the originally transmitted signal
• Only AC produces an expanding and collapsing field that radiates EM waves
• Antenna length is critical and dependent on the wavelength of the frequency to be transmitted

Modulation

• Human voice frequencies range from 90 Hz to 10 kHz
• Frequency Modulation (FM) communications use the VHF or UHF band due to high bandwidth requirements
• FM carrier wave modulation is used for broadcasting

Frequency Bands

• A band is a small section of the radio communication frequencies spectrum
• The International Telecommunication Union (ITU) subdivides the radio spectrum into frequency bands

Modes of Propagation

• VLF, LF, and MF bands use antennas that maximize the production of ground waves
• These frequencies can travel thousands of kilometers and penetrate water to a depth of approximately 20 meters
• High power transmitters (hundreds of kilowatts) are used for these frequencies

Radio Communication in Aircraft

• Radio control can be performed via a Flight Management System (FMC) Control Display Unit (CDU)
• Frequency selections, tuning, and mode control can be performed through the FMC keypad and display
• The transceiver is installed in an equipment bay, and the antenna is mounted on the upper or lower surface of the fuselage
• "In-use" and standby frequencies are available for ease of switching between primary and secondary frequencies

Electromagnetic Radiation Hazards

• Overexposure to radiation can cause serious health problems
• Safe working distances must be maintained near operating antennas
• RF burns or electric shock can result from touching antennas or metallic objects in the radiation field

ACARS Components

• Airborne components include the Management Unit (MU), Control Unit (CU), and printer
• The ACARS management unit controls transmission and reception of air-to-ground and ground-to-air messages through the VHF transceiver
• The control unit allows changing of operation modes and flight crew to enter information into the data link system

Satellite Communications

• A communications satellite amplifies and relays radio telecommunications signals via a transponder
• SATCOM system uses a UHF radio installed in the aircraft to communicate with commercial satellites in orbit
• SATCOM can transmit voice and data from AIRCOM, ACARS, flight-crew, passenger telephone, telex, fax, and internet