HF Com

Chapter 4

High Frequency Communications

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

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

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).

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”

HF System

Textbook page 24

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

ModulationSignalInformation

Carrier Wave

Modulated Carrier Wave

HF System

Page 27

Page 26

Page 25

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.

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.

SatCom

Chapter 5

Satellite Communications

Textbook page 34

AES (Aircraft Earth Station)

Satellite

GES (Ground Earth Station)

SatCom (Satellite Communications)

SatCom (Satellite Communications)

• http://orbitaldebris.jsc.nasa.gov/photogallery/beehives.html

http://www.duncanaviation.aero/homepage.php

http://www.duncanaviation.aero/straighttalk/index.php

Antenna MountingHF and Satcom

HF page 27

Satcom page 37

VHF Comm Antennas

Aircraft Structures

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

Pitot Tube

Pitot Tube

Pitot Tube

Static Port

AOA Sensor

Pitot-Static System

Airspeed

Vertical SpeedIndicator

Altimeter

6 Pack or Basic “T”

Textbook page 157

ASI Air Speed IndicatorPitot Static System

Altimeter

VSI Vertical Speed Indicator

Turn Coordinator

ADI Attitude Director Indicator

ADI Attitude Director Indicator

ADI Attitude Director Indicator

6 Pack or Basic “T”

Textbook page 157