Tutorial Questions Flashcards
(30 cards)
What are the four main aims of military communication?
- World Wide coverage
- 24 hour availability
- Speed
- Security
- What is the difference between Strategic and Tactical communications.
Provide one Naval example of each.
Strategic communications involve long-range transmissions and are typically used for global command and control. An example is using satellite communications to transmit strategic orders from a command center to a naval fleet across the world.
Tactical Communications:
Tactical communications focus on short-range transmissions and are used in battlefield or immediate operational areas. An example is ship-to-ship communication within a naval task group during a tactical maneuver.
Naval Examples:
Strategic Example: Using a satellite communication system to relay strategic commands from a central naval command to a fleet in a different ocean.
Tactical Example: Communicating between two ships in a task group using VHF radio during a coordinated naval exercise.
- List the frequency bands used in radio communications; specify the limits of each band, the propagation(s) involved and the principle uses.
Very Low Frequency (VLF)
Limits: 3-30 kHz
Propagation: Ground waves
Uses: Secure military communication, government time stations, submarine communication (penetrates saltwater) .
Low Frequency (LF)
Limits: 30-300 kHz
Propagation: Ground waves
Uses: Aircraft beacon, navigation (LORAN), long-distance communication .
Medium Frequency (MF)
Limits: 0.3-3 MHz
Propagation: Ground waves and sky waves
Uses: AM radio broadcasting, maritime communication, navigational beacons .
High Frequency (HF)
Limits: 3-30 MHz
Propagation: Sky waves
Uses: Shortwave broadcasting, aviation communication, amateur radio, and marine communications .
Very High Frequency (VHF)
Limits: 30-300 MHz
Propagation: Direct waves and space waves
Uses: FM radio broadcasting, television broadcasting, emergency services, and long-range data communication .
Ultra High Frequency (UHF)
Limits: 0.3-3 GHz
Propagation: Direct waves and space waves
Uses: Digital broadcasts, military air band, satellite communications, mobile phones, and radar .
Super High Frequency (SHF)
Limits: 3-30 GHz
Propagation: Direct waves and space waves
Uses: Radar transmitters, wireless internet, satellite communication .
Extremely High Frequency (EHF)
Limits: 30-300 GHz
Propagation: Direct waves and space waves
Uses: Advanced communication systems, experimental purposes (not typically covered in the document, but included for completeness) .
These bands are used in various applications based on their propagation characteristics and frequency limits.
What happens to the ionosphere at night?
At night, the following changes occur within the layers of the ionosphere:
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 and follow the curvature of the Earth’s surface. They are composed of direct waves, reflected signals from the Earth’s surface, and surface waves that follow the Earth’s curvature, enabling coverage beyond the horizon. This propagation is not significantly affected by atmospheric changes and can maintain long-range communications, particularly at VLF and LF frequencies .
Sky Wave Propagation:
Sky waves propagate by being refracted back to Earth by the ionized layers of the ionosphere. This allows the waves to travel long distances beyond the horizon. The effectiveness of sky wave propagation is influenced by the frequency used and the state of the ionosphere, which changes between day and night .
Typical Skip Distance:
The typical skip distance, which is the minimum distance between the transmitter and the point where the sky wave first returns to Earth, can range from hundreds to thousands of kilometers, depending on the frequency and ionospheric conditions .
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? Conversely what happens if the Frequency decreases?
Diagram Description
Imagine a diagram showing the Earth’s surface, the ionosphere, and the propagation paths of the radio waves. The key elements would include:
Earth’s Surface: Represented by a curved line at the bottom.
Ionosphere: A curved layer above the Earth’s surface, indicating the ionized layers (D, E, and F layers).
Ground Wave: A wave traveling along the Earth’s surface.
Sky Wave: A wave traveling upwards from the transmitter, being refracted back to Earth by the ionosphere.
Transmitter and Receiver: Points on the Earth’s surface where the waves originate and are received.
Effects of Frequency Changes
Range of Ground Wave:
Increased Frequency: The range of the ground wave decreases. Higher frequencies suffer more attenuation and do not follow the Earth’s curvature as effectively.
Decreased Frequency: The range of the ground wave increases. Lower frequencies can travel further along the Earth’s surface with less attenuation.
Skip Distance:
Increased Frequency: The skip distance increases. Higher frequencies tend to pass through the lower layers of the ionosphere (D and E) and are refracted back to Earth from higher layers (F2), resulting in a longer skip distance.
Decreased Frequency: The skip distance decreases. Lower frequencies are more readily refracted by the lower layers of the ionosphere (D and E), resulting in a shorter skip distance.
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
1. Diurnal Variation:
- Hight and density of ionospheric layers very with time of day.
- Incident sunlight simulates the formation of free electrons and ionised molecules.
- Heats the ionospheric layers, increasing its density.
- Height and free electron density are at a maximum at around 1200.
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Seasonal Variation:
- As the incident intensity of sunlight is greater in the summer, the intensity of ionisation is greater in the summer.
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11-year Sunspot Cycle:
- As sunspots are closely associated with solar flares and are a source of high intensity UV radiation, they lead to the increased formation of free electrons and ionised gases.
Abnormal Variations
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Dellinger Fade outs:
- Sudden bursts of solar radiation.
- Sudden increases in ionization, particularly increasing absorption in the D layer.
- Short-term disruption in HF communications due to increased D layer absorption.
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Sporadic E:
- Irregular patches of high ionization in the E layer.
- Enhanced VHF propagation and potential blockage of HF signals.
- Can lead to long-distance VHF communication, often thousands of kilometers, while potentially blocking HF signals from reaching the F layers.
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Geomagnetic Storms:
- Disturbances caused by interactions between the Earth’s magnetosphere and solar wind.
- Disruptions in ionospheric layers, leading to rapid changes in propagation conditions.
- HF communication can be severely degraded or enhanced unpredictably.
These variations significantly impact the propagation characteristics of sky waves, influencing the effectiveness and range of HF communication systems.
What is NVIS and what can it be used for?
Near Vertical Incidence Skywave (NVIS):
Description:
Near Vertical Incidence Skywave (NVIS) is a method of ionospheric radio-wave propagation that involves transmitting signals at a high angle of radiation. The signals are refracted back to Earth by the ionosphere, enabling communication over a limited area, particularly useful in challenging terrains.
Uses:
- NVIS is particularly useful in hilly or forested regions where ground wave propagation is limited.
- It is commonly used for short to medium-range communication, typically within a few hundred kilometers.
- NVIS is employed in military and emergency communication systems to maintain connectivity in areas with difficult geography.
What part of the atmosphere will the space wave propagate through?
The space wave propagates through the troposphere, which is the lowest layer of the Earth’s atmosphere. This layer extends up to an altitude of about 8-15 kilometers (5-9 miles) above the Earth’s surface.
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Draw a fully labelled diagram of the propagation path for a 700 MHz transmission and explain the effect that ‘multipath’ has on the reception of a space wave. What methods can be used to overcome this effect.
Below is a textual representation of the diagram showing the propagation path for a 700 MHz transmission (UHF):
| Ionosphere | |-------------------------------------------| | | | | | | | | | Troposphere | |-------------------------------------------| | | | Direct Wave | | /\ | | / \ | | / \ | | / \ | |-------------------------------------------| | Earth's Surface | |-------------------------------------------| | | | Transmitter Reflected Wave | | /\ / | | / \ / | | / \ / | | / \ / | |-------------------------------------------| | Receiver | |-------------------------------------------|
- Direct Wave: The primary wave traveling directly from the transmitter to the receiver.
- Reflected Wave: The wave reflecting off the ground or other obstacles before reaching the receiver.
- Troposphere: The atmospheric layer through which the space wave propagates.
Multipath: This occurs when multiple copies of the transmitted signal arrive at the receiver at slightly different times due to reflections off buildings, hills, or other objects. This can cause the signals to interfere with each other, leading to constructive or destructive interference, which can result in signal fading, distortion, or loss of signal clarity.
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Diversity Reception:
- Spatial Diversity: Using multiple antennas placed at different locations to receive the signal. The probability that all antennas will suffer the same multipath interference is low.
- Frequency Diversity: Transmitting the same signal over different frequencies to ensure that even if one frequency experiences multipath interference, others may not.
- Polarization Diversity: Using antennas with different polarizations (horizontal and vertical) to reduce the effect of multipath.
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Equalization:
- Applying signal processing techniques to counteract the distortion caused by multipath. An equalizer can adjust the amplitude and phase of the received signals to mitigate the effects of delayed signals.
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Orthogonal Frequency Division Multiplexing (OFDM):
- A method used in modern communication systems where the signal is split into multiple narrowband channels at different frequencies. This reduces the impact of multipath interference because the delays affect only a small portion of the signal.
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Directional Antennas:
- Using antennas that focus the transmission and reception in a specific direction to reduce the chance of receiving reflected signals.
These methods help to mitigate the adverse effects of multipath interference, improving the reliability and clarity of the received signal.
Two ships have antennas at 32ft. What is the range of VHF comms? What happens if the height is increased to 50ft?
The formula for calculating the radio horizon distance is:
[ \text{Distance} (\text{nm}) = 1.23 \times \sqrt{\text{Height of Antenna 1 (feet)}} + 1.23 \times \sqrt{\text{Height of Antenna 2 (feet)}} ]
- Height of each antenna: 32 feet
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Calculation:
[ \text{Distance} (\text{nm}) = 1.23 \times \sqrt{32} + 1.23 \times \sqrt{32} ]
[ \text{Distance} (\text{nm}) = 1.23 \times 5.66 + 1.23 \times 5.66 ]
[ \text{Distance} (\text{nm}) = 1.23 \times 11.32 ]
[ \text{Distance} (\text{nm}) \approx 13.92 ]
- Height of each antenna: 50 feet
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Calculation:
[ \text{Distance} (\text{nm}) = 1.23 \times \sqrt{50} + 1.23 \times \sqrt{50} ]
[ \text{Distance} (\text{nm}) = 1.23 \times 7.07 + 1.23 \times 7.07 ]
[ \text{Distance} (\text{nm}) = 1.23 \times 14.14 ]
[ \text{Distance} (\text{nm}) \approx 17.38 ]
- Range with antennas at 32 feet: Approximately 13.92 nautical miles
- Range with antennas at 50 feet: Approximately 17.38 nautical miles
Increasing the height of the antennas from 32 feet to 50 feet increases the communication range by approximately 3.46 nautical miles.
What atmospheric condition causes extended ranges at UHF. What causes a reduction in range?
Extended Range:
- Tropospheric Ducting: Caused by temperature inversions, allowing signals to travel farther than normal.
Reduced Range:
- Precipitation: Rain, fog, and snow cause scattering and absorption, reducing the effective range.
Contrast and compare AMDSB and DSBSC in the frequency domain.
Amplitude Modulated Double Sideband (AMDSB):
- Components: Contains the carrier frequency, upper sideband (USB), and lower sideband (LSB).
- Carrier Frequency: Present at the center frequency.
- Sidebands: Symmetrical sidebands around the carrier frequency, each containing the same information.
- Bandwidth: Total bandwidth is twice the bandwidth of the modulating signal.
- Power Distribution: Significant power in the carrier, which does not contain useful information.
Double Sideband Suppressed Carrier (DSBSC):
- Components: Only the upper sideband (USB) and lower sideband (LSB) are present; the carrier is suppressed.
- Carrier Frequency: Not present.
- Sidebands: Symmetrical sidebands around the suppressed carrier frequency, each containing the same information.
- Bandwidth: Total bandwidth is still twice the bandwidth of the modulating signal, same as AMDSB.
- Power Distribution: No power wasted on the carrier, resulting in more efficient use of power.
- Carrier Presence: AMDSB has a carrier; DSBSC suppresses the carrier.
- Efficiency: DSBSC is more power-efficient because it does not waste power on the carrier.
- Bandwidth: Both have the same bandwidth, but DSBSC uses the available power more effectively for the sidebands.
Compare the bandwidth of AMDSB and AMISB if both sidebands are the same bandwidth.
AMDSB (Amplitude Modulated Double Sideband):
- Bandwidth: The total bandwidth is twice the bandwidth of the modulating signal.
- Calculation: If each sideband has a bandwidth ( B ), the total bandwidth for AMDSB is ( 2B ).
AMISB (Amplitude Modulated Independent Sideband):
- Bandwidth: Each sideband can carry different information and is treated independently, effectively halving the required bandwidth for each channel of information.
- Calculation: The total bandwidth for AMISB, carrying the same amount of information as AMDSB, remains ( 2B ) for the two independent sidebands together. However, if only one sideband is used for communication, the bandwidth required is ( B ).
- AMDSB: Bandwidth = ( 2B )
- AMISB: Bandwidth for a single sideband = ( B ), for two independent sidebands = ( 2B )
In essence, AMISB allows more efficient use of bandwidth when considering single sideband operations, effectively halving the bandwidth per sideband compared to AMDSB, but overall, if both sidebands are used independently, the total bandwidth remains the same .
Why do we use a pilot carrier in AM?
Purpose of a Pilot Carrier:
A pilot carrier is used in amplitude modulation (AM) primarily for the purpose of carrier re-insertion at the receiver. It provides a reference signal for demodulation, helping to improve signal quality and reducing distortion during the demodulation process.
Benefits:
- Synchronization: Ensures that the receiver can accurately demodulate the signal by providing a reference for the carrier wave.
- Reduction of Distortion: Helps in reducing phase and frequency distortion by allowing the receiver to lock onto the carrier frequency precisely.
In essence, a pilot carrier is critical for maintaining the integrity and quality of the received AM signal .
What functions accurately measure the bandwidth of an FM signal?
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Carson’s Rule:
- Formula: ( BW = 2(\Delta f + f_m) )
- Use: Practical approximation for FM signal bandwidth.
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Fast Fourier Transform (FFT):
- Use: Analyzes the signal’s spectrum to measure actual bandwidth.
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Spectrum Analyzer:
- Use: Provides a visual representation of the frequency domain for accurate bandwidth measurement.
Which demodulator will only demodulate AMDSB and why?
Envelope Detector:
- Use: The envelope detector is specifically designed to demodulate AMDSB signals.
- Why: It works by rectifying the signal to extract the variations in amplitude, which correspond to the original modulating signal. Since AMDSB has a carrier and both sidebands, the envelope detector can effectively track the amplitude changes, making it ideal for this type of modulation.
What will demodulate all AM signals?
Product Detector (Synchronous Detector):
- Use: A product detector can demodulate all types of AM signals, including AMDSB, AMSSB (Single Sideband), and AMISB (Independent Sideband).
- Why: It mixes the incoming AM signal with a locally generated carrier signal that is synchronized in frequency and phase with the carrier of the incoming signal. This process recovers the original modulating signal from any AM signal type, making the product detector versatile for all AM demodulation.
What can be used to demodulate FM signals?
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Frequency Discriminator:
- Use: Converts frequency variations in the FM signal to amplitude variations.
- Types: Foster-Seeley discriminator, Ratio detector.
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Phase-Locked Loop (PLL):
- Use: Locks onto the frequency of the incoming FM signal and tracks its variations to demodulate the signal accurately.
- Advantages: High accuracy and stability.
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Quadrature Detector:
- Use: Uses two signals 90 degrees out of phase to demodulate the FM signal.
- Advantages: Simple and effective for wideband FM signals.
These methods effectively convert the frequency variations of FM signals into the original baseband signal.
Compare and contrast the Dipole and Monopole antennas.
- Structure: Consists of two equal-length conductive elements.
- Feed Point: Center-fed, balanced.
- Radiation Pattern: Bidirectional, figure-eight pattern in the horizontal plane.
- Ground Plane: Does not require a ground plane.
- Applications: Commonly used in television antennas, shortwave radio, and ham radio.
- Structure: Consists of a single conductive element.
- Feed Point: Base-fed, unbalanced.
- Radiation Pattern: Omnidirectional in the horizontal plane.
- Ground Plane: Requires a ground plane (can be the Earth or a conductive surface).
- Applications: Commonly used in AM radio, mobile phones, and vehicle antennas.
- Radiation Pattern: Dipole has a figure-eight pattern, while monopole has an omnidirectional pattern.
- Ground Plane: Monopole requires a ground plane, while dipole does not.
- Usage: Dipole is typically used where bidirectional radiation is needed, and monopole is used for omnidirectional coverage.
Why is base tuner used?
Purpose:
A base tuner is used to match the impedance of the antenna system to the transmitter or receiver, ensuring maximum power transfer and efficient operation.
Functions:
- Impedance Matching: Adjusts the antenna impedance to match the transmission line and equipment, minimizing signal reflection and loss.
- Frequency Tuning: Tunes the antenna system to the desired operating frequency, enhancing performance and signal quality.
- Bandwidth Adjustment: Optimizes the antenna’s bandwidth, improving reception and transmission over a range of frequencies.
- Efficiency: Ensures efficient power transfer, reducing energy loss.
- Performance: Enhances signal strength and clarity.
- Versatility: Allows the use of a single antenna for multiple frequencies.
Which type of antenna would you use to propagate SHF. How does SHF propagate?
Type of Antenna:
- Parabolic Dish Antenna: Commonly used for Super High Frequency (SHF) propagation due to its high gain and directional capabilities.
- Horn Antenna: Also used for SHF, particularly in applications like radar and microwave communications.
Propagation Characteristics:
- Line-of-Sight: SHF signals propagate primarily through line-of-sight, requiring a clear path between the transmitter and receiver.
- Limited Range: SHF signals have a shorter range due to higher attenuation and are more affected by obstacles like buildings and terrain.
- Applications: Used in satellite communication, radar, and point-to-point microwave links.
Methods:
- Direct Waves: Travel directly from the transmitter to the receiver.
- Reflection: Can reflect off surfaces like buildings and the ground, but with significant signal loss.
- Tropospheric Scatter: Can use tropospheric scatter for longer distances, although with reduced signal strength.
Antenna: Parabolic dish or horn antenna.
Propagation: Line-of-sight, limited range, affected by obstacles.
Briefly explain the difference between the two error control coding protocols - ARQ and FEC.
- Method: Error detection and retransmission.
- Process: The receiver checks for errors and requests retransmission if errors are found.
- Use: Suitable for environments where retransmissions are possible and latency is not critical.
- Example: Used in TCP/IP for reliable data transfer.
- Method: Error detection and correction.
- Process: The sender adds redundant data to the message, allowing the receiver to detect and correct errors without needing retransmission.
- Use: Suitable for environments where retransmissions are not feasible or latency is critical.
- Example: Used in satellite communications and streaming applications.
- ARQ: Detects errors and requests retransmissions.
- FEC: Detects and corrects errors using redundant data.
Which error control code could be used on a broadcast system? Which code requires a duplex (two way) link?
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Forward Error Correction (FEC):
- Use: Suitable for broadcast systems.
- Reason: It allows the receiver to detect and correct errors without requiring retransmission, which is ideal for one-way communication systems like broadcasting where a return path is not available.
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Automatic Repeat reQuest (ARQ):
- Use: Requires a duplex (two-way) link.
- Reason: It relies on the receiver to send back acknowledgments and requests for retransmissions if errors are detected, necessitating two-way communication.
