Communications Flashcards
What does the International Telecommunications Union do?
Coordinates the shared global use of the radio spectrum.
What is the Australian Communications and Media Authority responsible for?
The regulation of:
- broadcasting
- the internet
- radio communications
- telecommunications
- implementation of ICU policy and procedures
What kind of wave does sound travel by?
Pressure waves
What is the speed of sound?
Approx 340m/s or 1224 km/hr or 60 knots.
This speed will vary depending on air temperature.
What are radio waves?
Electromagnetic waves.
They can be transmitted through any medium.
Can be used for communication.
What is the speed of light?
300 million m/second
What are examples of electromagnetic waves?
- Radio and television signals
- Microwaves
- Radar signals
- Lamp light
- Fire heat
- Radiation from the sun and radioactive materials
What are the three main elements of wave motions?
- Amplitude
- Frequency
- Wavelength
What is the amplitude of a wave?
The distance from one extremity of the oscillation (or vibration) to the middle point or neutral value.
What is the frequency of a wave?
The number of complete waves (or cycles) passing a point per second.
Measured in Hertz (Hz).
One complete wave cycle past a given point = 1 Hz.
What is wavelength?
The length of one single wave.
Can also be expressed as the distance travelled by the wave during the transmission of one cycle.
Wavelength = Velocity/Frequency
What frequency and wavelength is HF?
3 - 30MHz
100m - 10m
What frequency and wavelength is VHF?
30 - 300MHz
10m - 1m
What frequency and wavelength is UHF?
300 - 3000MHz
1m - 100mm
At what rate does increasing transmitter power increase range?
The square root factor.
e.g. doubling power will increase range by 1.414
What is interference?
The reinforcement or cancellation that occurs when two waves overlap.
What are the advantages of the HF communications?
Allows communication with aircraft in:
- remote areas
- over the horizon
- out to sea beyond line-of-sight
What are the limitations of HF communications?
- Very susceptible to interference
- Fading due to interaction of ground and sky waves, particularly at night
- Coverage unreliable - affected by the Ionosphere
- Difficult to silence receivers during ‘no signal’ periods (annoying ‘hash’ when strong interference is present)
- HF antenna arrays are very large
What are the advantages of VHF communication?
- Principal VHF advantage is the signal quality. Static and other interference are almost non-existent.
- Waves are ‘direct’, and not refracted or reflected, so fading and garbling are also minimal.
- Requires small antenna systems, saving in space and weight in airborne equipment.
- Low transmitter power suits aircraft systems.
True or False. UHF has the same advantages and limitations as VHF.
True
What is UHF used for?
Military applications
What are VHF frequencies used for?
- Air Traffic Control
- General aviation
- Radio navigation aids
What is refraction?
The bending of radio waves across the boundary between media of differing density.
The velocity varies with density of the medium and layers can bend certain radio waves so they are effectively reflected back to Earth.
What are three types of refraction that radio waves may be affected by?
Ionospheric: The Ionosphere is a high altitude layers of ionised gases (approx 30 to 300 nm). HF band waves are susceptible but this depends on the frequency and the variable state of the Ionosphere. Reflected strength can vary from hour to hour, and minute to minute.
Atmospheric: Signal paths may be bent as a result of slight variations in atmospheric density, which arise from differing temperatures, pressures, and humidity, especially in the lower levels of the atmosphere.
Coastal: Radio wave path changes as it crosses a coastline. The degree to which a signal is affected is primarily dependent upon its frequency.
