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HF Com
Chapter 4
High Frequency Communications
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Textbook page 23
When an airplane leaves the coastline for a transoceanic flight, it moves into a polar region or
ventures over a remote area, it loses VHF communications.
VHF signals are line of sight and cannot curve over the horizon. For long-range flight, the airplane switches to HF (High frequency) communications.
Textbook page 23
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Radio Frequency (RF )
Very Low Frequency VLF 3 kHz -30 kHz
Low Frequency LF 30 kHz - 300 kHz
Medium Frequency MF 300 kHz - 3 MHz
High Frequency HF 3 MHz - 30 MHz
Very High Frequency VHF 30 MHz - 300 MHz
Ultra High Frequency UHF 300 MHz - 3 GHz
Super High Frequency SHF 3 GHz - 30 GHz
Extremely High Frequency EHF 30 GHz -300 GHz
HF isn’t very High
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HF Com
Works by “skipping” through the ionosphere.
Since the ionosphere is not constant, and changes between day and night and season to season, HF isn’t always reliable.
Will eventually be replaced by satellite communications (SAT COM).
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Antenna Tuning via Antenna Coupler
Uses ¼ wave antenna. However with varying frequencies this could be impractical as the lower frequencies could require an antenna > 100 feet long.
The Autotune system eliminates the need for long antennas by using tuning circuits in the antenna coupler to electrically match up, thus enabling the use of a short-fixed length antenna.
Textbook page 24 “Coupler”
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HF System
Textbook page 24
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Single Sideband (SSB)
HF radio transmits using AM (amplitude modulation).AM has 3 components: The RF carrier, and an upper (USB) and lower (LSB) sideband.
Originally, most of the power was lost in the carrier in AM.
Since voice audio only uses the sidebands, modern transmitters direct the transmitter power into one sideband thus not losing it in the carrier signal.
Textbook page 25
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ModulationSignalInformation
Carrier Wave
Modulated Carrier Wave
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HF System
Page 27
Page 26
Page 25
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Review Q&A Chapter 4 HF Com
4.1 Why are High Frequency communications not as reliable as VHF?Answer: Because HF depends on varying atmospheric conditions to work.
4.2 What is the advantage of “Autotune”?Answer: It allows for a short-fixed antenna and eliminates the need for the pilot to tune the radio.
4.3 Why is SSB (single sideband) more efficient than conventional AM radio?Answer: It directs the transmitter power to the sideband thus eliminating losses in the carrier signal.
4.4 What made HF datalink possible?Answer: Improved filtering technology.
4.5 What are 3 major components of an HF LRU?Answer: Antenna Coupler, Power Amplifier, and Receiver-Exciter.
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Review Q&A Chapter 4 HF Com
4.5 What are 3 major components of an HF LRU?Answer: Antenna Coupler, Power Amplifier, and Receiver-Exciter
4.6 Name 2 advantages of HF datalink.Answer: Lower pilot workload, Shorter Message Transmission time, Lower channel access time, Less Training for flight crew, Relieves congestion on voice frequencies, Automatic selection of frequency & data rates, Less vulnerability to human error, Automatically detects and corrects errors, can operate in noisier environments, Increases HF traffic capability, assured communications link, automatic air/ground HF linkage with lower acquisition cost compared to SATCOM, improved voice/data quality and datalink messages are not written or sensitive to verbal language.
4.7 What is the purpose of an HF antenna coupler?Answer: To tune the antenna
4.8 Where is the HF antenna mounted on many airliners?Answer: In the vertical stabilizer.
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SatCom
Chapter 5
Satellite Communications
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Textbook page 34
AES (Aircraft Earth Station)
Satellite
GES (Ground Earth Station)
SatCom (Satellite Communications)
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SatCom (Satellite Communications)
• http://orbitaldebris.jsc.nasa.gov/photogallery/beehives.html
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http://www.duncanaviation.aero/homepage.php
http://www.duncanaviation.aero/straighttalk/index.php
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Antenna MountingHF and Satcom
HF page 27
Satcom page 37
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VHF Comm Antennas
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Aircraft Structures
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Pitot-Static System
Some of the most important flight instruments derive their indications from measuring air pressure.
Gathering and distributing various air pressures for flight instrumentation
is the function of the pitot-static system.
FAA H 8083-31 Vol 2Page 10-12
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Pitot Tube
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Pitot Tube
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Pitot Tube
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Static Port
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AOA Sensor
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Pitot-Static System
Airspeed
Vertical SpeedIndicator
Altimeter
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6 Pack or Basic “T”
Textbook page 157
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ASI Air Speed IndicatorPitot Static System
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Altimeter
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VSI Vertical Speed Indicator
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Turn Coordinator
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ADI Attitude Director Indicator
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ADI Attitude Director Indicator
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ADI Attitude Director Indicator
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6 Pack or Basic “T”
Textbook page 157
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