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Communications
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- 1What is the EM Spectrum in order of largest wavelength to shortest?Radiowaves
Microwaves
IR
Visible light
UV
X-rays
Gamma rays
Cosmic rays - 2State, in order of lowest wavelength to highest, the radiowave frequency bandsVery Low Frequency (VLF)
Low Frequency (LF)
Medium Frequency (MF)
High Frequency (HF)
Very High Frequency (VHF)
Ultra High Frequency (UHF)
Super High Frequency (SHF)
Extra High Frequency (EHF) - 3What are radiowaves higher than 1,000MHz (1GHz) called?Microwaves
- 4Which way do the electric and magnetic waves oscillate in a radio wave?Electric = Vertical
Magnetic = Horizontal - 5What are the principal paths that radio wave may follow from transmitter to receiver?Surface Wave
Sky Wave
Space Wave - 6Explain a surface waveSurface waves follow the Earth's contour.
Due to diffraction which is assisted by the Earth's attenuation of the radio energy, causes the wave front to lag at the surface and curve around the surface of the Earth.
Used for VLF, LF and MF
E.g. systems requiring consistency of signal over long distances, submarines - 7Explain Sky wavesWave is refracted by the ionosphere (change in direction due to change in velocity).
Possible to communicate with points on the other side of the globe at HF.
A combination of reflection and refraction is used to "bounce" sky waves between earth and the ionosphere a number of times. This 'multi-hop transmission' makes it possible to communicate with points on the other side of the globe. - 8What is skip distance?The distance between the transmitter and the point of reception of the first sky wave.
Only for SKY WAVES - 9How does changing frequency affect sky waves?Higher frequency = Sky wave penetrates deeper into the ionosphere before being returned to Earth. Too high and it passes through and out into space, too low and it may not refract enough and the signal could fall short.
Highest frequency at which refraction can take place for a given angle of incidence is the Maximum Useable Frequency (MUF). - 10Explain Space Waves.VHF and UHF transmissions normally require a combination of a direct path + an Earth reflected path to reach a receiver.
This combination forms a space wave. - 11What are the main characteristics of Space Waves?The radiation field is confined to the region above the radio horizon.
Within this field there may be marked variations in signal level.
Due to interference effects between the direct and Earth-reflected components, the radio horizon is 15% greater than the visual horizon. - 12Name some applications of Space Waves.Short-range communication
Television and FM sound broadcasting
Radio-relay
Radar
Navigational and approach aids
Communication by satellites - 13What is attenuation?Reduction of signal strength due to obstructions and absorption (water vapour/droplets, dust particles), interfering waves, transmitter and receiver antenna configuration (too close, LoS, antenna height, etc)
Limits the range of radio signals.
Aka fading. - 14What is diffraction?Waves appear to bend around obstacles + spread into the shadow zone behind the obstacles.
Can be used to send radio signals over a mountain range when there is no LoS available.
Amount of diffraction is related to the wavelength and the size of the gap through which the waves must pass. - 15What is refraction?The change is direction of the waves due to a change in velocity as they pass from one medium to another.
- 16What is the propagation method and some uses of High Frequency (HF) radio waves?Propagation = Sky wave
Uses:
Reliable CW (continuous wave) and RT
Point-to-point communications over long distances - 17What is the propagation method and some uses of Very High Frequency (VHF) radio waves?Propagation = Space wave
(within the radio horizon)
Uses:
Marker beacons
ILS localizer
Radio telephone
Navigation aids (VOR, PLB) - 18What is the propagation method and some uses of Ultra High Frequency (UHF) radio waves?Propagation = Space wave
Uses:
Radio telephone
ILS glide path
Pulse radio altimeters
TACAN, VORTAC (aka VOR)
Search and GCI radars - 19What is the frequency band of HF?3 to 30 MHz
- 20What is the frequency band of VHF?30 to 300MHz
- 21What is the frequency band of UHF?300 to 3000MHz
- 22What are some limitations of HF?Noise contamination
Static from the ionosphere
(static is caused by radio waves transiting the ionosphere) - 23What are some limitations of UHF/VHF?Operating range
Line of Sight (LoS) transmission - maximum theoretical range (approx. 100nm in normal operations, only a few miles at ground level)
Power has little effect on range - 24What is the spacing of VHF and UHF channels?VHF - 8.33KHz
UHF - 25KHz - 25What is a guided wave? Give 3 examples.A wave guided by a conductor (using cables)
Twisted pair cable, co-axial cable, fibre optic cable - 26What is an unguided wave? Give 3 examples.A wave which travels through air.
Radio waves, microwaves, IR - 27What are the two main methods of communicating via a 'channel'?Guided (cables) and unguided (wave travels through the air)
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- Fichas sobre Communications, creado por Joseph Tidswell el 27/06/2021.
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- Joseph Tidswell
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