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77 Cards in this Set
- Front
- Back
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3 principal paths for Radio waves to travel between antennas |
- surface wave - sky wave - space wave |
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3 main methods of propagation and what two make up the 4th? |
- Surface wave - Sky wave - Space wave Ground waves are made up of the surface wave and the space wave |
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4 factors that increase radio wave range |
- increasing power output - using a more efficient antenna - using a more sensitive receiver - Reducing the amount of noise (interference) |
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Surface waves follow the earths surface due to ... |
Diffraction |
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Diffraction: When a wave encounters an obstacle it can... (2) |
- Bend around the obstacle - Spread out past small openings in the obstacle |
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Space waves follow two distinct paths from a transmitting antenna to receiving antenna |
- Direct wave (through the air) - Ground reflected wave |
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Radio waves can be reflected by ... |
- the ground - hard, smooth objects |
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Radio waves can be reflected by ... |
- the ground - hard, smooth objects |
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Space waves suffer from XX when reflected waves meet direct waves |
Fading |
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Radio waves can be reflected by ... |
- the ground - hard, smooth objects |
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Space waves suffer from XX when reflected waves meet direct waves |
Fading |
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When reflected and direct waves are in phase |
The resulting wave is increased to the combined total of both waves |
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Radio waves can be reflected by ... |
- the ground - hard, smooth objects |
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Space waves suffer from XX when reflected waves meet direct waves |
Fading |
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When reflected and direct waves are in phase |
The resulting wave is increased to the combined total of both waves |
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When reflected and direct waves are out of phase |
The resulting wave is decreased to the larger wave minus the smaller wave |
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MEH |
Minimum Effective Height |
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Of antennas are too close to the ground: |
The received signals will be out of phase and fading will occur |
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Of antennas are too close to the ground: |
The received signals will be out of phase and fading will occur |
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What is the MEH of an antenna and what does this stand for |
MEH = Minimum Effective Height This is at least one wavelength above the ground |
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Radio waves are attenuated as they pass through obstacles such as buildings, this attenuation is called .... |
Absorption |
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Radio waves are attenuated as they pass through obstacles such as buildings, this attenuation is called .... |
Absorption |
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TWOL |
Two wire open line |
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Absorption is increased as frequency is ... |
Increased |
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In the XX Band absorption occurs due to precipitation such as rain |
SHF |
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Line of sight applies to XX waves |
Space waves |
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Line of sight applies to XX waves |
Space waves |
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Visible horizon is about XX km |
5km |
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The radio horizon is about XX more than the visible horizon. This can be increased by XX |
1/3 more Raisin g antenna height |
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Several methods are used to enable space wave communication... (3) |
- Radio Relay - Mobile communications - satellite communications |
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Several methods are used to enable space wave communication... (3) |
- Radio Relay - Mobile communications - satellite communications |
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Radio relay incorporates the use of ... |
Repeaters |
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Several methods are used to enable space wave communication... (3) |
- Radio Relay - Mobile communications - satellite communications |
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Radio relay incorporates the use of ... |
Repeaters |
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Space wave line of sight communications are used by military at 3 wave bands |
- VHF - UHF -SHF |
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Ground wave contains elements of both XX and XX |
Surface wave and Space wave |
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The dominant method of space and surface wave received from a ground wave is determined by .... (4) |
- Antennas - polarisation, position and height - terrain type - distance from transmitter - wave frequency |
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The dominant method of space and surface wave received from a ground wave is determined by .... (4) |
- Antennas - polarisation, position and height - terrain type - distance from transmitter - wave frequency |
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XX is the propagation of radio waves that are bent back to the earths surface by the ionosphere |
Sky wave |
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A single sky wave skip can be as large as ... |
4000km |
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Sky waves can be categorised into 3 ranges. Give range name and distances: |
Short range: 0-300km
Medium range: 300km - 1,500km Long range 1,500km - 4,000 km |
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The earths atmosphere can be broken down into 3 regions. Give region names and approx heights |
Troposphere: 0 - 14km Stratosphere: 14km - 40km Ionosphere: 40km - 400km |
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The ionosphere contains 4 layers of electrically charged glasses called: (4) |
D, E, F1 and F2 |
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The layers in the ionosphere are created by: |
- Energy from the sun entering the atmosphere - ionising waves create charge in the gasses |
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At night gasses in the ionosphere... |
Gradually lose their charge |
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The steepest angle that skywaves are bent back at any frequency is called the ... |
Critical angle |
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If the angle of a skywave is bigger than the critical angle... |
The wave escapes into space |
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Lower frequencies bend XX when they enter then ionosphere |
More |
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Useful frequencies for sky wave are in the XX band which is XXto XX |
HF 3MHz to 30MHz |
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Daily variations in the ionosphere can be countered by using a XX frequency at night |
Lower |
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MUF |
Maximum usable frequency |
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LUF |
Lowest usable frequency |
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OWF |
Optimum working frequency |
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XX distance is the minimum distance that sky wave can be received |
Skip |
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The size of the skywave skip distance depends on...(3) |
- The frequency of the wave - The angle of incidence - The density of the layer |
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In the ionosphere, the density of each charge is highest at the XX of each layer |
Centre |
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The XX is the point between where the surface wave becomes too weak and the sky wave is first returned to earth |
Skip Zone |
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The size of a skip zone depends on ... (2) |
- Surface wave distance - Skip distance |
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XX can occur when the sky wave returns to earth within the range of the ground wave |
Multipath fading |
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In XX the sky wave can cancel out or reinforce the ground wave depending on the phase difference |
Multipath Fading |
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Noise can come from a variety of sources (3) |
- Man made - Atmospheric - Interference |
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Man made noise mainly effects what bands? |
HF, VHF and UHF |
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Atmospheric noise mainly effects what band |
HF |
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Interference mainly effects what bands? |
HF and VHF |
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Sometimes layers of warm air are formed between layers of cool air, this is know as ... |
Temperature inversion |
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A tropospheric duct between the earth and a layer of warm air is known as a |
Ground based duct |
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Factors that cause variations in the ionosphere (4) |
- Daily Variations - Sun spots - Seasonal variations - Geographic variations |
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Tropospheric duct between 2 layers of warm air is called a ... |
Elevated duct |
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What are the EW implications of a HF or VHF transmission being propagated in a duct (2) |
They will go far beyond line of sight comma meaning: - Transmissions may be carried long distances into unknown regions - a lot of interference may be experienced from distant transmissions |
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The deeper a duct, the Lower The frequency that can be caught. What will a 5m and 500m duct support |
5m - 30GHz 500m - 30MHz |
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How many distinct layers in the ionosphere at day and at night |
Day - 4 Night - 2 |
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Radio waves bend in the ionosphere due to ... |
Refraction |
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The amount of bending (refraction) of a wave in a layer of the ionosphere depends on ... (3) |
- the angle the wave enters the layer - The frequency of the wave - the density of the layer |
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XX polarised antenna are used to project waves towards the sky |
Horizontally |
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Waves that enter the ionosphere from a shallower angle bend... |
Less |
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Waves that enter the ionosphere from a steeper angle bend ... |
More |
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Height of antenna to wavelength for short medium and high range skywaves |
Short range skywave 1/8 - 1/4 of wavelength Medium range skywave 1/4 of wavelength Long range skywave 1/2 of wavelength |
