Antenna Arrays and Principles

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Questions and Answers

What is the expected effect on the radiation pattern when two isotropic point sources are spaced one-half wavelength apart and have identical amplitude and phase currents?

A uniform radiation pattern in all directions

A pattern with destructive interference at certain angles (correct)

An enhanced directive pattern towards one side (correct)

A pattern with a null in the direction between the two sources

When two isotropic point sources are spaced one-half wavelength apart and have opposite phase currents, what type of radiation pattern can be expected?

A symmetric pattern with maximum radiation away from the sources

A uniform pattern with no preferred direction

A radiation pattern with multiple nulls and lobes (correct)

A single peak in the direction of the sources

How does spacing affect the radiation pattern of isotropic sources with in-phase quadrature currents at one-half wavelength apart?

It results in an elliptical radiation pattern

The pattern will if anything be less directive

The pattern will display a rotating radiation pattern

It creates an increased intensity in the main lobe direction (correct)

For an array of two horizontal dipoles located along the z-axis, what would you expect if the excitation magnitudes and phases are identical?

A dipole pattern that is symmetrical about the z-axis Signup and view all the answers

What determines the nulls in the total fields of a two-point source antenna array where the sources are spaced at $d = \frac{\lambda}{4}$?

The phase difference caused by the spacing and the wavelength Signup and view all the answers

What is one way to achieve highly directive antennas for long-distance communication?

By forming an assembly of radiating elements Signup and view all the answers

What is the purpose of controlling the excitation phase of individual elements in an antenna array?

To shape the overall radiation pattern Signup and view all the answers

Which of the following is NOT a configuration of antenna arrays?

Spherical (3D) Signup and view all the answers

How can the directivity of an antenna be improved?

By increasing the electrical size of the antenna Signup and view all the answers

What happens to the fields from antenna elements in a well-designed array?

They interfere constructively in the desired direction Signup and view all the answers

What is the primary advantage of using an antenna array compared to a single-element antenna?

Higher gain and directivity Signup and view all the answers

What is the function of the relative displacement between elements in an antenna array?

To control the radiation pattern Signup and view all the answers

What is an essential characteristic of a low-directivity antenna element?

Wide radiation pattern Signup and view all the answers

What is the total electric field from an antenna array based on?

The algebraic sum of the electric fields Signup and view all the answers

What does the term 'spacing b/w two antennas' refer to in the phase difference equation?

The separation distance between two antennas in the array Signup and view all the answers

What is the phase difference in an antenna array defined by?

The linear combination of spacing, initial phase, and operating wavelength Signup and view all the answers

What might occur due to the interference of fields from an antenna array?

Constructive interference in desired directions, minor side lobes Signup and view all the answers

Which of the following accurately describes the components of total phase difference?

Total phase difference includes spacing and operating wavelength Signup and view all the answers

How can the arrangement of an antenna array affect radiation characteristics?

By establishing specific amplitude and phase relations Signup and view all the answers

What does each term $E_n e^{j heta_n}$ represent in the total electric field equation?

The magnitude and phase of a single antenna's electric field Signup and view all the answers

What happens to the fields from different elements of an antenna array?

They can lead to side lobes due to some remaining interference. Signup and view all the answers

Which variable represents the operating wavelength in the phase difference equation?

λ Signup and view all the answers

In what form is the total electric field of an antenna array expressed?

As a vector sum of magnitudes and phases Signup and view all the answers

What does Half Power Beam Width (HPBW) measure in an antenna?

The angle within which the power is at least half of its maximum value Signup and view all the answers

In terms of directivity, how does the broadside array compare to the end-fire array?

End-fire array has larger directivity than broadside array Signup and view all the answers

What is the formula for calculating the HPBW in an end-fire array?

∅HPBW = 2λ/dN Signup and view all the answers

What does First Null Beam Width (FNBW) approximate in relation to HPBW?

FNBW is approximately twice the HPBW Signup and view all the answers

For a broadside array, what is the relative relationship between HPBW and FNBW?

FNBW is greater than HPBW Signup and view all the answers

What role does the parameter 'N' play in the context of HPBW and FNBW calculations?

It is the number of elements in an antenna array Signup and view all the answers

What angle is generally associated with maximum directivity in an end-fire array?

One hundred eighty degrees Signup and view all the answers

Which of the following equations represents the relationship between wavelength ($ ext{λ}$) and element spacing in antennas?

dN = 2λ Signup and view all the answers

In the context of beam width terms, what does 'rad' signify?

It denotes angular distance measures Signup and view all the answers

What is the mathematical relationship between ∅2 and ∅1 as presented in the equations?

∅2 - ∅1 ≅ ∅n1 + - ∅n1 -. Signup and view all the answers

Which expression correctly represents the calculation of cos∅n1 + based on the maximum angle?

cos∅n1 + = cos∅mmax ± λ/dN Signup and view all the answers

How does the Half Power Beam Width (HPBW) relate to ∅mmax?

∅HPBW decreases when ∅mmax increases. Signup and view all the answers

Which component in the equation expresses a relationship between sin and cos functions?

sin A + B = sin A cos B + cos A sin B Signup and view all the answers

What condition is applied for N ≫ 1 regarding HPBW?

∅HPBW significantly decreases. Signup and view all the answers

When considering the visible nulls for positive angles, what term is factored into the equation?

λ/dN Signup and view all the answers

Which approximation can be made when x is very small regarding sin and cos functions?

sin x ≈ x and cos x ≈ 1 Signup and view all the answers

What does the equation ∅HPBW ≅ 2 sin ∅max represent?

The relationship with wavelength. Signup and view all the answers

Which of these concepts is least related to the HPBW calculation?

Frequency of operation Signup and view all the answers

What factor is doubled in the expression for ∅HPBW given in relation to ∅mmax?

Wavelength Signup and view all the answers

In the context of the HPBW, what does the λ/2 factor indicate?

Beamwidth relation to frequency Signup and view all the answers

What is represented by dN in the equations provided?

Distance between nulls Signup and view all the answers

The equation involving 2 sin A + 2 sin B expresses what relationship?

Difference in angles at maximum. Signup and view all the answers

Which of the following statements is true about the calculation of HPBW?

HPBW depends on angle measurement and signal characteristics. Signup and view all the answers

Study Notes

Antenna Arrays

Antenna arrays are used to achieve highly directive antennas for long-distance communication.
They can be configured as linear (1D), planar (2D), or conformal (3D).
Examples include Yagi-Uda arrays and log-periodic arrays.

Array Principles

Arrays leverage the pattern multiplication principle to enhance directivity.
This involves arranging multiple identical antenna elements with specific spacing, excitation phase, and amplitude.
The array's overall radiation pattern is controlled by these parameters.

Basic Principles

The total field of an antenna array is determined by the vector addition of the individual element fields.
Total phase difference (𝜓) between elements is influenced by their spacing (d), initial phase (𝛼), and operating wavelength (𝜆).
The total phase difference can be calculated using the formula: 2𝜋/𝜆 * (Total Path Difference).
The fields from the elements constructively interfere in desired directions, enhancing signal strength.
Conversely, they destructively interfere in other directions, minimizing unwanted radiation.
However, complete cancellation isn't always achieved, leading to some side lobes and back lobes.

Two-Element Antenna Arrays

An arrangement of two antennas with specific spacing, excitation magnitude, and phase, can be used to achieve specific radiation characteristics.
The array's radiation pattern can be controlled to favour certain directions.
For example, a two-element array with identical currents spaced a half-wavelength apart that have an in-phase current will result in radiation in the broadside direction.

Pattern Multiplication

Pattern multiplication helps visualize the radiation pattern of arrays with multiple elements.
The pattern of an entire array can be obtained by multiplying the individual element patterns of the array for a given direction.
For example, the pattern of an array of four isotropic sources is obtained by multiplying the patterns of four isotropic sources placed at specific distances.
Increasing the number of elements in an array will narrow the main lobe.

Half-Power Beam Width (HPBW)

HPBW is the angular width between the directions where the radiation power is half of the maximum power.
For a broadside array, the HPBW is halved from the total angle span of two directions where power is half of the maximum power.
The HPBW for end-fire arrays is approximately: 2𝜆/LN, where L is the total array length.
The HPBW of the broadside array is lower than that of the end-fire array.

First Null Beam Width (FNBW)

FNBW is the angular width between the first nulls in the radiation pattern.
It is approximately twice the HPBW.
FNBW is used to measure the sharpness of the main beam.

Understanding Nulls

Nulls are directions where the radiation power is zero.
The directions of nulls can be calculated using the formula: 𝑐𝑜𝑠∅𝑁𝑢𝑙𝑙𝑠 = 𝑐𝑜𝑠∅𝑚𝑎𝑥 ± 𝜆/𝑑𝑁, where ∅𝑚𝑎𝑥 is the direction of the main beam and 𝑑𝑁 is the spacing between elements.
This formula helps determine the location of nulls for a specific array configuration.
Nulls are important for suppressing unwanted radiation and improving the directivity of the antenna.

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Description

This quiz covers the fundamentals of antenna arrays, including their configuration types such as linear, planar, and conformal. Discover how the pattern multiplication principle enhances directivity and learn to calculate the total field using individual element fields. Dive into the details of radiation patterns and constructive interference to master antenna array concepts.