Radar and Antenna Systems Quiz

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Questions and Answers

What condition must be met for elliptical polarization to occur?

The time-phase difference must be an odd multiple of π/2 or not a multiple of π/2. (correct)

The time-phase difference must be an even multiple of π/2.

The components must have a constant phase difference.

The magnitudes of both components must be equal.

How is aperture efficiency of an antenna defined?

The total power output of the antenna compared to input.

The ratio of effective aperture area to the physical aperture area. (correct)

The ratio of effective aperture area to the gain of the antenna.

The ratio of physical area to the physical radius of the antenna.

What is the effective aperture area for a lossless dish antenna?

Equal to the physical cross-sectional area of the dish. (correct)

Always less than the physical area of the dish.

More than twice the physical area of the dish.

Independent of any external factors.

Which statement best describes the relationship between effective aperture area and wavelength?

Effective aperture area is related to the wavelength and solid angle subtended by the main beam. Signup and view all the answers

How can antennas be broadly categorized?

As single radiators and multiple radiators. Signup and view all the answers

What is the path loss encountered over a distance of 120 km in the radar system?

192 dB Signup and view all the answers

If the receiver sensitivity is -105 dBm, what is the equivalent power in watts?

10^-13.5 Watts Signup and view all the answers

What is the impact of atmospheric attentuation on the transmitted power for this radar system?

Increases the required power by 1.556 times Signup and view all the answers

How are the pulse length and pulse repetition frequency related to radar functionality?

They define the minimum and maximum range of the radar Signup and view all the answers

What is the significance of the aerial gain in the calculation of transmitter power?

It enhances the effective area of detection Signup and view all the answers

What is the primary purpose of varying the height of the rhombic antenna?

To adjust the angle of elevation for different ionospheric conditions Signup and view all the answers

What is the characteristic impedance of a rhombic antenna typically?

600—800 ohms Signup and view all the answers

Why does the rhombic antenna have a broad bandwidth?

Due to the ability to vary the frequency of operation over a 2:1 range Signup and view all the answers

What is a significant disadvantage of the rhombic antenna?

Presence of high sidelobes Signup and view all the answers

What angle does the lowest lobe of the rhombic antenna correspond to, based on height?

An angle of elevation a which varies with height h Signup and view all the answers

How is the efficiency of the rhombic antenna affected by its input power?

About half of the input power is dissipated in the matched termination Signup and view all the answers

What is the typical gain of a rhombic antenna?

15 dB Signup and view all the answers

What does the diameter of the ground area required for a rhombic antenna depend on?

Leg length L and height h of the antenna Signup and view all the answers

Which antenna is typically associated with receiving applications?

Rhombic antenna Signup and view all the answers

What characteristic makes the log-periodic array particularly appealing?

Broadband properties Signup and view all the answers

In which application are microwave horns commonly utilized?

Radar and satellite systems Signup and view all the answers

Which of the following is a type of multiple radiator antenna?

Yagi array Signup and view all the answers

What is a common feature of both frequency-scanned and phase-scanned linear arrays?

They scan over a region of space. Signup and view all the answers

Which antenna is described as a popular choice for VHF and UHF bands?

Yagi array Signup and view all the answers

Which radiator type is NOT mentioned as a basic radiator for constructing linear arrays?

Horn Signup and view all the answers

Which antenna is commonly associated with transmitting applications due to its composite structure?

Long wire antenna Signup and view all the answers

What is the primary characteristic of the helical antenna's radiating mode when its diameter is small compared to the wavelength?

It radiates in the normal mode perpendicular to its axis. Signup and view all the answers

Which equation represents the power gain of the helical antenna?

$G = \frac{12R_r}{4 \times 74}$ Signup and view all the answers

What is the bandwidth ratio that the helical antenna can operate over?

2:1 Signup and view all the answers

In which mode does a helical antenna radiate circularly polarized waves when its diameter is approximately equal to the wavelength?

Axial mode Signup and view all the answers

How is the helical antenna commonly fed?

Using a coaxial cable. Signup and view all the answers

What is a key feature of the helical antenna's construction?

It is simple to construct and convenient in size. Signup and view all the answers

What is one application where the helical antenna is often used?

Space applications for telemetry signals. Signup and view all the answers

Which parameters characterize the geometry of a helical antenna?

Diameter, turn spacing, pitch angle, uncoiled length, number of turns. Signup and view all the answers

What does the directivity of an antenna represent?

The ratio of radiation intensity in a given direction to that of an isotropic radiator Signup and view all the answers

Which statement about maximum directivity is correct?

Maximum directivity is always greater than or equal to 1 Signup and view all the answers

What components can contribute to the total efficiency of an antenna?

Conduction losses and dielectric losses Signup and view all the answers

How is the gain of an antenna defined?

The ratio of radiation intensity in a given direction to isotropic radiated power Signup and view all the answers

In an antenna with no losses, how does its gain relate to its directivity?

Gain equals directivity Signup and view all the answers

Which of the following factors does NOT affect the gain of an antenna?

Total radiated power Signup and view all the answers

Which of the following best describes the relationship between radiated power and input power in an antenna system?

Radiated power is always less than or equal to input power Signup and view all the answers

What does the reflection efficiency of an antenna account for?

Mismatch losses between transmission line and antenna Signup and view all the answers

Signup and view all the answers

Study Notes

Antenna Fundamentals

An antenna is a transducer that converts electrical power into electromagnetic waves and vice versa
It's used for both transmitting and receiving signals
Antennas are named after insect feelers, due to the long, thin, metallic conductor shape
They transition between transmission lines (e.g., waveguides) and free space
Their purpose is to efficiently convert guided waves to free-space waves (and vice versa)
Antenna design aims for a specific radiation pattern in space

Radiation Mechanism

Radiation occurs when there is a time-varying current or accelerating charge
Uniform velocity movement of charge does not produce radiation
Oscillating charges generate electromagnetic waves that spread into space
Electromagnetic waves are created due to varying electric and magnetic fields between conductors, which then propagate between them
Electromagnetic waves are initially sustained within the transmission line and antenna, but convert to free-space waves when they reach the open end of the transmission line

Maxwell's Equations

Maxwell's equations describe how electric charges and currents create electric and magnetic fields
They also illustrate how these fields generate each other

Basic Types of Antennas

Antennas are categorized based on their physical structure, frequency ranges, and application modes.
Physical Structure:
- Wire antennas (e.g., dipoles, monopoles, loops, helices)
- Aperture antennas (e.g., horns, waveguides)
- Reflector antennas (e.g., parabolic reflectors)
- Lens antennas (e.g., convex and concave lenses)
- Microstrip antennas
- Array antennas
Frequency of operation
- Very Low Frequency (VLF), Low Frequency (LF), Medium Frequency (MF), High Frequency (HF), Very High Frequency (VHF), Ultra High Frequency (UHF), Super High Frequency (SHF), Microwave, Radio wave.
Mode of applications:
- Point-to-point communications, Broadcasting applications, Radar communications, Satellite communications

Elementary Antennas

Half-Wave Dipole: A fundamental radiator with an input impedance of ~75Ω and sinusoidal current distribution
Half-wave dipole antennas are efficient and commonly used, providing a sinusoidal current distribution.
Properties of the antenna (directivity and efficiency) depend on its size and shape.
Helical antenna: suitable for providing circular polarization to the transmitted signal.
Log-Periodic antenna: An example of a frequency-independent antenna, efficient from very low to very high frequencies, capable of being scanned over a region of space.
Rhombic antenna: High gain antenna for HF used for long-distance communication.

Radiation Pattern

Radiation pattern is used to describe how an antenna radiates energy in space and its efficiency
Radiation patterns are plotted to represent the intensity of radiated energy over all directions
Field patterns are plotted as a function of electromagnetic fields (electric and magnetic)
Power patterns are plotted as a function of the magnitude of electromagnetic fields
Antennas are categorized as isotropic or directional in respect to the radiation pattern.

Antenna Field Regions

Near-field region: characterized by reactive fields
Fresnel region: radiation and reactive fields coexist, and the angular field distribution depends on the distance from the antenna
Far-field region: radiation pattern is independent of the distance from the antenna, called the Fraunhofer region.

Antenna Parameters

Directivity is the ratio of maximum radiation intensity to average radiation intensity
Efficiency is a ratio of radiated power compared with supplied power and factors of loss (e.g. reflections and conduction/dielectric losses)
Gain is a measure of directivity and efficiency (directivity corrected for antenna losses)

Polarization

Polarization describes the orientation of the electric field vector during radiation or reception
Polarization is categorized as linear, circular, or elliptical

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Description

This quiz covers essential concepts in radar and antenna systems, including elliptical polarization, effective aperture areas, and antenna categorization. Test your knowledge on topics such as path loss, atmospheric attenuation, and rhombic antennas.