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?

<p>10 m² (D)</p> Signup and view all the answers

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

<p>6 kW (C)</p> Signup and view all the answers

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

<p>Yagi Array (C)</p> Signup and view all the answers

What is a significant characteristic of helix and spiral antennas?

<p>Broadband properties (C)</p> Signup and view all the answers

What distinguishes linear arrays from other array types?

<p>They utilize dipoles or slotted waveguides as basic radiators (A)</p> Signup and view all the answers

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

<p>Rhombic Antenna (C)</p> Signup and view all the answers

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

<p>Phased Array (C)</p> Signup and view all the answers

What is a primary application of microwave horns?

<p>Serving as feeds for parabolic reflectors (C)</p> Signup and view all the answers

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

<p>Log-Periodic Array (D)</p> Signup and view all the answers

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

<p>Broadband properties (C)</p> 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?

<p>To produce a null in the direction of an interfering signal. (C)</p> Signup and view all the answers

How do electromagnetic waves propagate from a vertical dipole antenna?

<p>As both ground waves and sky waves. (C)</p> Signup and view all the answers

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

<p>The charge distribution on the antenna reverses rapidly. (D)</p> Signup and view all the answers

What characterizes an isotropic radiator?

<p>It radiates energy uniformly in all directions. (A)</p> Signup and view all the answers

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

<p>$P_a = P/4πr^2$ watts/m² (D)</p> Signup and view all the answers

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

<p>Sky waves are refracted back to earth by the ionosphere. (D)</p> Signup and view all the answers

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

<p>It forces collapsing field lines to temporarily expand. (D)</p> Signup and view all the answers

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

<p>To enable signal processing capabilities. (D)</p> Signup and view all the answers

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

<p>It illustrates the relationship between electric and magnetic fields in terms of power density. (C)</p> Signup and view all the answers

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

<p>The distance from the radar to the target and the transmitted power. (C)</p> Signup and view all the answers

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

<p>It is multiplied by the square root of the gain. (B)</p> Signup and view all the answers

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

<p>$P_R = P_TG_TG_RAe / (4Ï€)^2r^4$ (B)</p> 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?

<p>When calculating the received power versus the noise level. (C)</p> Signup and view all the answers

Flashcards

Single-Radiator Antenna

An antenna with a single radiating element.

Multiple-Radiator Antenna

An antenna with multiple radiating elements, working together.

Monopole Antenna

A basic antenna with a single wire connected to ground.

Dipole Antenna

A simple antenna with two equal-length wires, carrying opposite currents.

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Yagi Array

An antenna made of several elements, with a driven element and reflector(s) that amplify signals.

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Yagi TV Antenna

A compact antenna used for VHF/UHF bands found in TV receivers.

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Rhombic Antenna

A type of antenna used in HF communication, known for its broadband properties.

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Log-Periodic Antenna

An antenna with broad frequency range that uses a series of elements for efficient transmission in HF communication.

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Radar Range

The maximum distance a radar system can detect a target, determined by the system's power, antenna gain, target size, and signal-to-noise ratio.

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Transmitted Power (PT)

The power output of a radar transmitter, crucial for determining radar range.

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Receiver Sensitivity

The ability of a radar system to receive weak signals, representing the minimum detectable signal.

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Atmospheric Attenuation

The loss of signal strength due to the atmosphere, affecting radar range.

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Pulse Length

The time duration of each radar pulse, indirectly affecting the minimum detectable range of a radar system.

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Adaptive Array Antenna

A linear array antenna that can adjust its amplitude and phase excitation to create nulls in specific directions, canceling interfering signals.

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Induction Field

The electromagnetic field that exists close to an antenna, primarily due to the immediate electric and magnetic field lines.

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Radiation Field

The electromagnetic field that travels outwards from an antenna, responsible for transmitting signals over distances.

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Ground Wave

A ground wave traveling along the Earth's surface, used for medium-distance communication at lower frequencies.

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Sky Wave

A sky wave traveling upwards and reflected back to Earth by the ionosphere, used for long-distance communication at higher frequencies.

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Isotrope (or Point Source Radiator)

A theoretical antenna that radiates energy equally in all directions. A helpful comparison point for other antenna types.

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Power Density (Pa)

The power flow per unit area, measured in watts per square meter, representing the power density of the electromagnetic field.

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Radiation Pattern

The pattern depicting the radiation intensity of an antenna in different directions. For the isotrope, this pattern is circular in any plane.

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Relationship between E and H

The relationship between the electric field strength (E) and magnetic field strength (H) in an electromagnetic wave is defined by the characteristic impedance of free space (120Ï€ ohms).

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Antenna Power Gain (G)

The power gain (G) of an antenna is the ratio of the power density in a specific direction to the power density that would be produced by an isotropic radiator (radiating equally in all directions).

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Radar Equation

The radar equation describes the relationship between the transmitted power (Pr), antenna gain (Gr), target area (σ), distance (r), and the received power (Pr). It explains how a radar system works by measuring the reflection of radio waves from a target.

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Minimum Detectable Signal-to-Noise Ratio (S/N)

The minimum detectable signal-to-noise ratio (S/N) represents the threshold at which a radar receiver can identify a signal amidst background noise. It involves factors like noise power (N) and sensitivity.

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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/(4Ï€r²) 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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