Comms Flashcards
(18 cards)
List the four aims of military communications
World wide coverage
24 hr availability
Speed
Security
What is the difference between Strategic and Tactical Communications.
Provide one example each.
Strategic Communications - Communication between command and all other units, it includes both voice and data information. (Long Range)
Tactical Communications - Communication between local units primary using UHF/VHF (ship/ship, ship/air, local nets, tug nets).
List the frequency bands used in radio communications; specifically the limits of each band, the propagations involved and the principle uses.
List the frequency bands used in radio communications; specifically the limits of each band, the propagations involved and the principle uses.
What happens to the ionosphere at night?
The D Layer disappears.
The E Layer remains weakly ionized but has little effect on frequencies above LF.
The F1 and F2 Layers combine to form a single F Layer, which is critical for HF propagation .
How does the ground wave propagate and how does the sky wave propagate? what is a typical skip distance in km?
Ground Wave Propagation - Ground waves travel along the Earth’s surface, following its curvature. They can cover hundreds of kilometers, especially at very low and low frequencies. (Longer wavelength = more diffraction).
Sky Wave Propagation - Sky waves bounce off the ionosphere and return to Earth, allowing long-distance communication. Effectiveness depends on frequency, ionospheric refraction, and time of day.
Typical Skip Distance - Skip distance typically ranges from 500 to 2,000 kilometers, depending on ionospheric conditions and frequency.
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Draw a fully labelled diagram of the propagation path for a 10 MHz transmission.
- What happens to the range of the ground wave if the frequency is increased?
- What happens to the range of the ground wave if the frequency is decreased?
- What happens to the skip distance if the frequency of transmission is increased?
- What happens to the skip distance if the frequency of transmission is decreased?
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List and describe the abnormal and regular variations in the ionosphere.
In each case, which layers are affected and what happens to the ranges / skip distance of the sky wave?
Regular Variations in the Ionosphere
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Daily Cycle (Diurnal Variation)
- Affected Layers: D, E, F1, F2
- Effect on Ranges/Skip Distance: Higher frequencies work better during the day; lower frequencies are better at night.
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Seasonal Changes
- Affected Layers: Mainly F1 and F2
- Effect on Ranges/Skip Distance: Longer skip distances in summer.
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Sunspot Cycle
- Affected Layers: All layers, especially F layer
- Effect on Ranges/Skip Distance: Longer skip distances during high sunspot activity.
Abnormal Variations in the Ionosphere
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Sporadic E
- Affected Layers: E layer
- Effect on Ranges/Skip Distance: Causes short skip distances, affects VHF, and blocks HF signals.
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Solar Flares
- Affected Layers: All layers, especially D layer
- Effect on Ranges/Skip Distance: Shortwave fadeouts, HF disruptions.
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Geomagnetic Storms
- Affected Layers: Mainly F layer
- Effect on Ranges/Skip Distance: Degrades HF communication, alters propagation paths.
What is NVIS and what can it be used for?
NVIS (Near Vertical Incidence Skywave)
Definition: NVIS is a radio wave propagation technique that uses high-angle skywave signals to achieve short to medium-range communication.
Uses:
- Effective in hilly or forested areas where ground wave propagation is difficult.
- Provides reliable communication in regions with challenging terrain by bouncing signals almost vertically off the ionosphere.
- Commonly used by military, emergency services, and amateur radio operators for local to regional communication.
What part of the atmosphere will the space wave propagate through?
Space waves primarily propagate through the troposphere, which extends from the Earth’s surface up to about 8-15 kilometers in altitude.
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Draw a fully labelled diagram of the propagation path for a 700 MHz transmission.
- Explain the effect that multipath has on reception of a space wave.
- What methods can be used to overcome this effect?
Two ships have antennas at 32ft.
- What is the range of VHF comms?
- What happens if the height is increased to 50ft?
Two ships have antennas at 32ft.
- What is the range of VHF comms?
- What happens if the height is increased to 50ft?
Range (nm) = √2 x Tx height (ft) + √2 x Rx height (ft)
Range at 32ft = √2 x 32 + √2 x 32 = 16nm
Range at 50ft = √2 x 50 + √2 x 50 = 20nm
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What atmospheric condition causes extended ranges at UHF.
What causes a reduction in range?
- Super Refraction: Significantly extends UHF range by bending radio waves further, allowing them to travel hundreds of kilometers. (Cool sea and hot air)
- Cause: Tropospheric absorption
- Effect: Reduces UHF range due to signal weakening by moisture and particles in the atmosphere.
What is the wavelength of 300MHz?
What is the frequency of a wave that is 2m long?
Formula - Wavelength = Speed of light (C) / Frequency (f)
For 300MHZ -
Wavelength = 3 x 10^8 / 300 x 10^6 = 1m
For 2m wavelength -
Frequency = 3 x 10^8 / 2 = 1.5 x 10^8 Hz = 150 MHz
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Sketch an AMDSB signal in the time and frequency domains.
Label the main components of the signal.
Compare and contrast AMDSB and DSBSC in the frequency domain.
In the frequency domain, AMDSB (Amplitude Modulated Double Side Band) includes a carrier frequency with two identical sidebands (upper and lower), requiring twice the highest frequency of the modulating signal for its bandwidth. It has lower efficiency because most transmitted power is used by the carrier.
In contrast, DSBSC (Double Side Band Suppressed Carrier) has no carrier frequency, only the sidebands are present, making it more power-efficient while maintaining the same bandwidth as AMDSB.
Compare the bandwidth of AMDSB and AMISB if both sidebands are the same bandwidth.
AMDSB (Amplitude Modulated Double Side Band):
- Bandwidth: The total bandwidth is double the highest frequency of the modulating signal. If one sideband is 3 kHz, then the total bandwidth is 6 kHz.
AMISB (Amplitude Modulated Independent Side Band):
- Bandwidth: Uses only one sideband, thus the bandwidth is equal to the highest frequency of the modulating signal. If one sideband is 3 kHz, then the total bandwidth is 3 kHz.
Comparison:
- AMDSB: 6 kHz (double the sideband bandwidth).
- AMISB: 3 kHz (single sideband bandwidth).
AMISB effectively reduces the required bandwidth by half compared to AMDSB while still transmitting the same information.
