Antenna Types and Evolution

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Questions and Answers

What is the primary factor influencing the radar range according to the formula?

The square root of $P_T$

The cube root of $P_T$

The fourth root of $P_T$ (correct)

The direct value of $P_T$

How is the transmitted power $P_T$ adjusted due to atmospheric attenuation in the given example?

Increased by 1.92 dBm

Increased by 1.556 times (correct)

Decreased by 1.556 times

Increased by 1.92 Watts

Which of the following values is correctly calculated for the receiver sensitivity in dBW?

-125 dBW

-105 dBW

-135 dBW (correct)

-115 dBW

What is the effective area $σ$ of the target as specified in the example?

10 m² Signup and view all the answers

What is the calculated transmitted power $P_T$ required to detect the target at the specified range?

6 kW Signup and view all the answers

Which antenna type is primarily used in TV broadcasting for the VHF and UHF bands?

Yagi Array Signup and view all the answers

What is a significant characteristic of helix and spiral antennas?

Broadband properties Signup and view all the answers

What distinguishes linear arrays from other array types?

They utilize dipoles or slotted waveguides as basic radiators Signup and view all the answers

Which antenna is characterized as having a composite structure for transmitting applications?

Rhombic Antenna Signup and view all the answers

Which array is known for its ability to be scanned over a region of space?

Phased Array Signup and view all the answers

What is a primary application of microwave horns?

Serving as feeds for parabolic reflectors Signup and view all the answers

Which antenna type is specifically mentioned for its broadband properties in HF communications?

Log-Periodic Array Signup and view all the answers

Which characteristic is primarily associated with both the rhombic antenna and the log-periodic array?

Broadband properties Signup and view all the answers

What is the purpose of adjusting the amplitude and phase excitation in a linear array used for satellite communications?

To produce a null in the direction of an interfering signal. Signup and view all the answers

How do electromagnetic waves propagate from a vertical dipole antenna?

As both ground waves and sky waves. Signup and view all the answers

What happens when the frequency at which the dipole is energized exceeds 16 kHz?

The charge distribution on the antenna reverses rapidly. Signup and view all the answers

What characterizes an isotropic radiator?

It radiates energy uniformly in all directions. Signup and view all the answers

What is the expression for the power density at a point Q from an isotropic radiator?

$P_a = P/4πr^2$ watts/m² Signup and view all the answers

What distinguishes the ground wave from the sky wave in electromagnetic wave propagation?

Sky waves are refracted back to earth by the ionosphere. Signup and view all the answers

What effect does the reversing field near a dipole antenna have on collapsing field lines?

It forces collapsing field lines to temporarily expand. Signup and view all the answers

In what scenarios are linear arrays particularly beneficial in satellite communication?

To enable signal processing capabilities. Signup and view all the answers

What does the equation $P_a = E * H$ signify in relation to power density?

It illustrates the relationship between electric and magnetic fields in terms of power density. Signup and view all the answers

In the context of radar systems, what factor does the power density $P_a$ depend on?

The distance from the radar to the target and the transmitted power. Signup and view all the answers

How is the electric field strength at point Q modified by the gain of a practical antenna?

It is multiplied by the square root of the gain. Signup and view all the answers

What is the form of the radar equation relating transmitted power and received power?

$P_R = P_TG_TG_RAe / (4π)^2r^4$ Signup and view all the answers

In which scenario does the minimum detectable signal-to-noise ratio (S/N) come into play within radar systems?

When calculating the received power versus the noise level. Signup and view all the answers

Study Notes

Antenna Types

Antenna designs have evolved into single and multiple radiators
Important types are listed
Monopole and dipole antennas are commonly used

Table 1.1

Single radiators: Monopole, Dipole, Wire, Helix, Spiral, Horn, Reflector
Multiple radiators: Yagi array, Rhombic array, Log-periodic array, Linear array, Planar array, Adaptive array

Figures 1.1, 1.2, and 1.3

Illustrate various antenna types, including Monopole, Dipole, Helix, Spiral, Horn, and Reflector

Antenna Evolution

Complex arrays (e.g., Yagi array) are used as components
Long wire antennas are more common in receiving applications
Rhombic antennas are used in transmitting applications
Helix and spiral antennas show broadband properties suitable for satellite communication
Microwave horns and parabolic reflectors are common in radar and satellite systems

Yagi Array

A popular TV antenna for VHF and UHF bands
Features convenient size, useful gain, and simplicity
Low cost

Log-Periodic Array

Recently used in HF communication
Attract due to broadband properties

Linear and Phased Arrays

Constructed using basic radiators (dipole or slotted waveguide)
One-dimensional structures called linear arrays
Used for satellite communications as adaptive arrays
Used to adjust amplitude and phase excitation to create nulls for signal cancellation

Wave Propagation

Typical antenna is vertical dipole
Electric field lines are vertical and magnetic are horizontal
Symmetrical about antenna
Charge on antenna reverses sign rapidly when the frequency is higher than 16kHz

Electromagnetic Waves

Consist of ground wave (used for medium distance communication) and sky wave (used for long distance communication, refracted by ionosphere)

Point Source Radiator (Isotrope)

Theoretical antenna
Radiates energy equally in all directions
Provides a basis for comparison of other antennas
Power density at a point is calculated using the equation Pa = P/(4πr²) watts/m² (where P is power and r is distance)

Radar Equation

Related to transmitting and receiving power in radar systems
The equation is used to calculate the range based on the minimum range and maximum range
The range equation depends on the fourth root of Pr(transmitted power)

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Description

Explore the evolution and classification of various antenna types including monopole, dipole, and complex arrays like Yagi and rhombic antennas. This quiz will test your knowledge on both single and multiple radiator designs, as well as their applications in communication and radar systems.