Communication Avionics
Reliable communication equipment enables pilots to coordinate effectively with air traffic control and other aircraft. Understanding communication avionics helps pilots recognize how radios, audio panels, and related systems support clear transmission and reception during all phases of flight.
Introduction to Communication Avionics
Intercommunication Systems
- Intercommunication systems, or ICS, consists of a small control box within the aircraft which routes audio signals as selected as an intercom within the aircraft
- ICS provides as an intercom between crew members
- Provides for selection of radio to be used by routing the audio (voice) signal from the crewmember's microphone to the specific radio selected and by routing that radios received audio to the crewmember's headset
- Provides selections for monitoring additional audio sources such as other radios, navigation aids, warning tones, etc.
- These additional audio inputs may be selected/deselected individually as well as having separate volume control
Radio Frequencies
- Communication avionics will vary according to aircraft configuration
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Ultra-High Frequency (UHF) Communications:
- Allow for short range, line-of-sight, voice and data communications
- Frequency ranges from 225.0 to 399.95 MHz
- Some UHF radios have satellite communications (SATCOM) capability with the additional specialized antennas and power amplifiers
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Very-High Frequency (VHF) Communications:
- Allows for short to medium range, line-of-sight, voice and data communications
- The FCC assigns aviation VHF frequencies ranging from 118.000-136.975 MHz at 25 kHz spacing
- Only the emergency frequency 121.5 MHz will continue to have 100 kHz protection
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High-Frequency (HF) Communications:
- Allows transmission and reception of long-range voice and data
- Not commonly used
- Comparable to short-wave (HAM) radio
- Frequency ranges between 2.000 and 29.999 MHz
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Modulation:
- Modulation is the waveform associated with the frequency
- VHF use AM (amplitude modulation) and UHF use FM (frequency modulation)
Selective Calling
- The Selective Calling (SELCAL) is a communication system that permits the selective calling of individual aircraft over radio-telephone channels from the ground station to properly equipped aircraft, to eliminate the need for the flight crew to constantly monitor the frequency in use
Aircraft Antennas
- Aircraft antennas are arguably the most important piece of communications avionics
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UHF Antennas:
- UHF antennas on general avionics aircraft are usually for transponders and distance measuring equipment
- UHF is highly dependent upon line of site and are therefore mounted under the aircraft
- UHF antennas are generally located on the belly of the aircraft
- Two types of UHF antennas are the spike and blade
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Spike Antenna:
- The spike antenna is generally used for transponders, as it is tuned specifically for that frequency band
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Blade Antenna:
- The blade, or broadband antenna, is a four-inch long shark fin shaped antenna, tuned to a variety of frequencies (i.e., the variety of DME frequencies)
- Transponders may share a broadband antenna with DME equipment
- UHF antennas are located on the underside of aircraft to minimize line of sight limitations with the UHF band
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Communication Antennas:
- Each com transmitter has its own antenna, mostly for redundancy
- Installation positioning must account for shaddowing, whereby the fuselage of other aircraft structures may block signal transmissions
- Communication and GPS antennas are prone to interfere with one another, necessitating their placement on opposite sides of the aircraft
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Marker Beacon Antennas:
- Marker beacon signals are highly directional, which means you have to be almost directly over the transmitting ground station to receive them
- Marker beacon antennas therefore need to be on the bottom of the aircraft
- There are a few different types of marker antennas; the more common types look like little canoes about 10 inches long
- For some installations, flush antennas that appear to be flat plates are under the empennage
- An antenna that consists of a thick wire that protrudes straight down out of the empennage and then makes a turn toward the tail may also be used
- Marker beacon signals are highly directional, which means you have to be almost directly over the transmitting ground station to receive them
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Emergency Locator Transmitter Antennas:
- Emergency locator transmitter antennas are usually on the upper skin of the empennage and are made of a flexible material
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Antenna Preflight Considerations:
- The physical condition of the antenna plays an important role in its performance
- If the antenna is cracked, water may enter and cause delamination (a separation of the composite layers), which may render the antenna useless
- If the antenna base is not structurally strong, the antenna will vibrate from the slipstream and cause the skin to fatigue, eventually causing cracks
- The antenna must be electrically bonded (grounded) to the airframe so a good electrical connection is maintained
- If some corrosion gets underneath the antenna, this bond may be compromised and the antenna's efficiency may degrade
- Sealant around the base of the antenna helps to prevent this
- Antennas should never be painted over their original coatings; any paint buildup reduces the efficiency of an antenna
Cold Temperature Operation Considerations
Conclusion
- Note that because amplitude modulation regards the power a requency is sending, those transmitting with higher power (think strength) like ATC can override those with lower power
- During preflight, make sure the integrity of all antennas is inspected as this can impact functionality
- Dirty antennas do not perform as well
- Know where your antennas are and how shadowing may affect their range and coverage
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