Antenna Arrays - EMPossible
Transcript of Antenna Arrays - EMPossible
Antenna Arrays
EE-4382/5306 - Antenna Engineering
Outlineโข Introductionโข Two Element Arrayโข Rectangular-to-Polar Graphical Solutionโข N-Element Linear Array: Uniform Spacing and
Amplitudeโ Theory of N-Element Linear Arrayโ Rectangular to Polar Graphical Solutionโ Broadside Arrayโ Ordinary End-Fire Arrayโ Phased Arrayโ Hansen-Woodyard End-Fire Array
โข N-Element Linear Array: Directivityโข Design Procedureโข Radio Observatory Antenna Arrays
2Linear Antenna Arrays
Introduction
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Antenna Arrays - Introduction
Slide 4
Antenna arrays are a configuration of multiple radiating elements in a geometrical order. Antenna arrays are an efficient way to freely change the pattern of an antenna, making it more directive and therefore increasing the gain. Electronically adjusting the excitation of individual elements leads to a phased (scanning) array, which enables greater degrees of freedom.
Linear Antenna Arrays
Antenna Arrays - Introduction
Slide 5
In an array of identical radiating elements, there are at least five factors that can be controlled to shape the overall pattern:
1. The geometrical configuration of the array (linear, circular, rectangular, elliptical, etc.)
2. The relative displacement between the elements3. The excitation amplitude of the individual elements4. The excitation phase of the individual elements5. The relative pattern of the individual elements
Linear Antenna Arrays
Two-Element Array
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Two-Element Array
Linear Antenna Arrays Slide 7
Two infinitesimal dipoles are placed along the z-axis. The total field radiated assuming no mutual coupling, is equal to the sum of the two elements. In the y-z plane:
๐ธ๐ก = ๐ธ1 + ๐ธ2
= เท๐๐๐๐๐๐ผ0๐
4๐
๐โ๐ ๐๐1โ
๐ฝ2
๐1cos ๐1 +
๐โ๐ ๐๐2โ
๐ฝ2
๐2cos ๐2
Where the ๐ฝ is the difference in the phase excitation between elements.Assuming far-field observations:
๐1 โ ๐2 โ ๐
๐1 โ ๐ โ๐
2cos(๐)
๐2 โ ๐ +๐
2cos ๐
๐1 โ ๐2 โ ๐
Two-Element Array
Linear Antenna Arrays Slide 8
Assuming far-field observations, the total field becomes
๐ธ๐ก = เท๐๐๐๐๐๐ผ0๐๐
โ๐๐๐
4๐๐cos ๐ ๐+๐ ๐๐ cos ๐ +๐ฝ /2 + ๐+๐ ๐๐ cos ๐ +๐ฝ /2
๐ธ๐ก = เท๐๐๐๐๐๐ผ0๐๐
โ๐๐๐
4๐๐cos ๐ 2 cos
1
2๐๐ cos ๐ + ๐ฝ
Field of single element Array Factor
AF = 2cos1
2(๐๐ cos ๐ + ๐ฝ
(AF)๐= cos1
2(๐๐ cos ๐ + ๐ฝ
๐ธ total = ๐ธ single element at ref. point ร [array factor]
Two-Element Array - Examples
Linear Antenna Arrays Slide 9
Given the array shown for two identical isotropic sources, find the total field when ๐ = ๐/2 and ๐ฝ = 0.
Two-Element Array - Examples
Linear Antenna Arrays Slide 10
Two-Element Array - Examples
Linear Antenna Arrays Slide 11
Given the array shown for two identical isotropic sources, find the normalized total field when ๐ = ๐/4 and ๐ฝ = โ90ยฐ.
Two-Element Array - Examples
Linear Antenna Arrays Slide 12
Two-Element Array - Examples
Linear Antenna Arrays Slide 13
Given the array shown for two identical isotropic sources, find the normalized total field when ๐ = ๐ and ๐ฝ = 0ยฐ.
Two-Element Array โ Examples
Linear Antenna Arrays Slide 14
Isotropic Point Sources โ Array Factor for two elements
Linear Antenna Arrays Slide 15
Isotropic Point Sources โ Array Factor for two elements
Linear Antenna Arrays Slide 16
Two-Element Array - Examples
Linear Antenna Arrays Slide 17
Given the array shown for two identical infinitesimal dipoles, find by the nulls of the total field when ๐ = ๐/4 and a. ๐ฝ = 0b. ๐ฝ = +๐/2c. ๐ฝ = โ๐/2
Two-Element Array - Examples
Linear Antenna Arrays Slide 18
Two-Element Array - Examples
Linear Antenna Arrays Slide 19
Two-Element Array - Examples
Linear Antenna Arrays Slide 20
Linear Antenna Arrays Slide 21
Antenna Array โ Scanning Array
N-Element Linear Array: Uniform Amplitude and Spacing
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Linear Array: Uniform Amplitude and Spacing
Linear Antenna Arrays Slide 23
An uniform array is an array of elements, all with identical magnitude, and each with a progressive phase.
Linear Array: Uniform Amplitude and Spacing
Linear Antenna Arrays Slide 24
Linear Array: Uniform Amplitude and Spacing
Linear Antenna Arrays Slide 25
๐ด๐น = 1 + ๐+๐(๐๐ cos ๐ +๐ฝ) + ๐+๐2(๐๐ cos ๐ +๐ฝ) +โฏ+ ๐+๐ ๐โ1 ๐๐ cos ๐ +๐ฝ
AF =
๐=1
๐
๐+๐ ๐โ1 ๐๐ cos ๐ +๐ฝ
AF =
๐=1
๐
๐+๐ ๐โ1 ฮจ
ฮจ = ๐๐ cos(๐) + ๐ฝ
Another useful expression is the closed form expression of the array factor.
๐ด๐น ๐๐ฮจ = ๐๐ฮจ + ๐๐2ฮจ + ๐๐3ฮจ +โฏ+ ๐๐๐ฮจ
๐ด๐น ๐๐ฮจ โ 1 = (โ1 + ๐๐๐ฮจ)
๐ด๐น =๐๐๐ฮจ
๐๐ฮจ โ 1= ๐
๐๐โ12 ฮจ ๐
๐๐2 ฮจ
โ ๐โ๐
๐2 ฮจ
๐๐12 ฮจ
โ ๐โ๐
12 ฮจ
= ๐๐๐โ12 ฮจ
sin๐2 ฮจ
sin12ฮจ
Multiply by ๐๐ฮจ
Subtract AF summation
Simplify
Linear Array: Uniform Amplitude and Spacing
Linear Antenna Arrays Slide 26
AF =sin
๐2 ฮจ
sin12ฮจ
โ sin
๐2 ฮจ
ฮจ2
๐ด๐น๐ =1
๐
sin๐2 ฮจ
ฮจ2
โ sin
๐2 ฮจ
๐2 ฮจ
for small values of ฮจ
ฮจ = ๐๐ cos(๐) + ๐ฝ
The nulls are given by setting the array factor to 0.
sin๐
2ฮจ = 0 > >
๐
2ฮจ แ
๐=๐๐= ยฑ๐๐ > > ๐๐ = cosโ1
๐
2๐๐โ๐ฝ ยฑ
2๐
๐๐
The number of nulls that can exist will be a function of the element separation ๐ and phase excitation difference ๐ฝ.
๐ = 1,2,3, โฆ (๐๐ข๐๐)๐ โ ๐, 2๐, 3๐,โฆ (๐๐๐ฅ๐๐๐ข๐)
Linear Array: Rectangular Plot
Linear Antenna Arrays Slide 27
First main maximum occurs when ๐
2= 0 > > ฮจ = 0
The principal maxima occurs when
๐๐ = cosโ1๐๐ฝ
2๐๐
Other main maxima occurs whenฮจ = ยฑ2๐๐, ๐ = 1,2,3,โฆ
Linear Array: Rectangular Plot
Linear Antenna Arrays Slide 28
Linear Array: Rectangular Plot
Linear Antenna Arrays Slide 29
Linear Array: Rectangular Plot
Linear Antenna Arrays Slide 30
Linear Array: Rectangular Plot
Linear Antenna Arrays Slide 31
Observations for rectangular plots of linear arrays with elements that are equally spaced, uniformly excited:
1. As ๐ increases, the main lobe narrows2. As ๐ increases, there are more side lobes in one period of ๐(ฮจ).
In fact, the number of full lobes (one main lobe and the side lobes) in one period of ๐(ฮจ) equals ๐ โ 1. There are ๐ โ 2 side lobes in each period.
3. The minor lobes are of width 2๐/๐ in the variable ฮจ and the major lobes are twice this width.
4. The side lobe peaks decrease with increasing ๐.5. ๐ ฮจ is symmetric about ๐.
Linear Antenna Arrays Slide 32
Rectangular to Polar Graphical Solution
Rectangular to Polar Graphical Solution
Linear Antenna Arrays Slide 33
In antenna theory, many solutions are of the form
๐ ๐ = ๐(๐ถ cos ๐พ + ๐ฟ)
Where ๐ถ and ๐ฟ are constants and ๐พ is a variable. The approximate array
factor of an N-element, uniform amplitude linear array is a sin ๐
๐where
๐ = ๐ถ cos(๐พ) + ๐ฟ =๐
2ฮจ =
๐
2๐๐ cos ๐ + ๐ฝ
๐ถ =๐
2๐๐
๐ฟ =๐
2๐ฝ
The ๐ ๐ function can be plotted in rectilinear coordinates, and transferred to a polar graph.
Rectangular to Polar Graphical Solution
Linear Antenna Arrays Slide 34
The procedure that must be followed in the construction of the polar graph is as follows:
1. Plot, using rectilinear coordinates, the function ๐ ๐ .2. a) Draw a circle with radius C and its center on the abscissa at ๐ = ๐ฟ
b) Draw vertical lines to the abscissa so that they will intersect the circle.
c) From the center of the circle, draw radial lines through the points of the circle intersected by the vertical lines.
d) Along radial lines, mark off corresponding magnitudes from the linear plot.
e) Connect all points to form a continuous graph.
Rectangular to Polar Graphical Solution
Linear Antenna Arrays Slide 35
Four element linear array -Example
Linear Antenna Arrays Slide 36
Find and plot the array factor of a four-element, uniformly excited, equally spaced array. The spacing is ๐/2 and 90ยฐ interelement phasing(i.e. ๐ฝ = ๐/2).
Four element linear array -Example
Linear Antenna Arrays Slide 37
Two element linear array -Examples
Linear Antenna Arrays Slide 38
Find and plot the array factor of a two-element, isotropic, equally spaced array with distance d = ๐/2 and uniform phase excitation ๐ผ =0ยฐ
Find and plot the same array factor of a two-element, isotropic, equally spaced array with distance d = ๐/2 but with phase excitation ๐ผ = 180ยฐ
Find and plot the same array factor of a two-element, isotropic, equally spaced array with distance d = ๐/4 but with phase excitation ๐ผ = โ90ยฐ
Two element linear array -Examples
Linear Antenna Arrays Slide 39
Five element Endfire Linear Array - Examples
Linear Antenna Arrays Slide 40
Find and plot the array factor of a five-element, isotropic, equally spaced array with distance d = 0.45๐ and uniform phase excitation ๐ผ = 0.9๐
Find and plot the array factor of a five-element, isotropic, equally spaced array with distance d = 0.5๐ and uniform phase excitation ๐ผ =๐
Five element linear array -Examples
Linear Antenna Arrays Slide 41
Broadside Array
Linear Antenna Arrays Slide 42
In many applications it is desirable to have the maximum radiation of an array directed normal to the axis of the array (๐ = 90ยฐ). To optimize this design, both the maxima of the single element and the array factor should be both directed toward ๐ = 90ยฐ. Recall the maximum of the array factor occurs when
ฮจ = ๐๐ cos(๐) + ๐ฝ = 0
Since it is desired to have the first maximum directed toward ๐ = 90ยฐ
ฮจ = ๐๐ cos(๐) + ๐ฝ แ๐=90ยฐ
= ๐ฝ = 0
To have the maximum of the array factor in an uniform linear array directed to the broadside to the axis, all elements need to have the same phase excitation.
Broadside Array
Linear Antenna Arrays Slide 43
To ensure that there are no other maxima in other directions (grating lobes), the separation between the elements should not be equal to multiples of a wavelength (๐ โ ๐๐, ๐ = 1,2,3,โฆ) when ๐ฝ = 0.If ๐ = ๐๐, ๐ = 1,2,3, and ๐ฝ = 0, then
ฮจ = ๐๐ cos ๐ + ๐ฝศ ๐=๐๐๐ฝ=0
๐=1,2,3,โฆ
= 2๐๐ cos ๐ ศ๐=0,๐ = ยฑ2๐๐ avoid this!
To avoid any grating lobes, the largest spacing between the elements should be less than one wavelength (๐ = ๐)
Broadside Array
Linear Antenna Arrays Slide 44
Broadside Array
Linear Antenna Arrays Slide 45
Broadside Array
Introduction to Antennas Slide 46
Broadside Array
Linear Antenna Arrays Slide 47
Ordinary End-Fire Array
Linear Antenna Arrays Slide 48
Instead of having the maximum radiation broadside to the axis of an array, it may be desirable to direct it along the axis of the array (end-fire). Sometimes it may be desirable that it radiates toward only one direction (๐ = 0ยฐ, 180ยฐ)
For the maximum toward ๐ = 0ยฐ:
ฮจ = ๐๐ cos(๐) + ๐ฝ แ๐=0ยฐ
= ๐๐ + ๐ฝ = 0 > > ๐ฝ = โ๐๐
For the maximum toward ๐ = 180ยฐ:
ฮจ = ๐๐ cos(๐) + ๐ฝ แ๐=180ยฐ
= โ๐๐ + ๐ฝ = 0 > > ๐ฝ = ๐๐
Ordinary End-Fire Array
Linear Antenna Arrays Slide 49
Ordinary End-Fire Array
Linear Antenna Arrays Slide 50
Ordinary End-Fire Array
Linear Antenna Arrays Slide 51
Ordinary End-Fire Array
Linear Antenna Arrays Slide 52
Linear Antenna Arrays Slide 53
Scanning/Phased Array
Linear Antenna Arrays Slide 54
Scanning/Phased Array
Linear Antenna Arrays Slide 55
Scanning/Phased Array
Linear Antenna Arrays Slide 56
We discussed the conditions to have an ordinary end-fire array in the previous sections.In order to enhance the directivity of an end-fire array without destroying any of the other characteristics, Hansen and Woodyard proposed in 1938 proposed that the required phase shift between closely spaced elements of a very long array should beFor the maximum toward ๐ = 0ยฐ:
๐ฝ = โ ๐๐ +2.92
๐โ โ ๐๐ +
๐
๐
For the maximum toward ๐ = 180ยฐ:
๐ฝ = โ ๐๐ +2.92
๐โ + ๐๐ +
๐
๐
For both directions, spacing should be
๐ =๐ โ 1
๐
๐
4โ ๐
4for large N
Hansen-Woodyard End-Fire Array
Linear Antenna Arrays Slide 57
Hansen-Woodyard End-Fire Array
Linear Antenna Arrays Slide 58
Hansen-Woodyard End-Fire Array
Linear Antenna Arrays Slide 59
Linear Arrays - Summary
Linear Antenna Arrays Slide 60
Linear Arrays - Summary
Linear Antenna Arrays Slide 61
Linear Antenna Arrays Slide 62
N-Element Linear Arrays: Directivity
Antenna Array Directivity
Linear Antenna Arrays Slide 63
For a linear antenna array, determine total length by
๐ฟ = ๐ โ 1 ๐
For a large broadside array (๐ฟ โซ ๐), directivity reduces to
๐ท0 โ 2๐๐
๐= 2 1 +
๐ฟ
๐
๐
๐โ 2
๐ฟ
๐
For a large ordinary end-fire array (๐ฟ โซ ๐), directivity reduces to
๐ท0 โ 4๐๐
๐= 4 1 +
๐ฟ
๐
๐
๐โ 4
๐ฟ
๐
For a Hansen-Woodyard end-fire array (๐ฟ โซ ๐), directivity reduces to
๐ท0 โ 1.805 4๐๐
๐= 1.805 4 1 +
๐ฟ
๐
๐
๐โ 1.805 4
๐ฟ
๐
Linear Antenna Arrays Slide 64
Linear Arrays: Design Procedure
Linear Arrays: Design Procedure
Linear Antenna Arrays Slide 65
๐ =๐ฟ + ๐
๐
๐ฟ = ๐ โ 1 ๐
Linear Arrays: Design Procedure Example
Linear Antenna Arrays Slide 66
Design an uniform linear scanning array whose maximum array
factor is 30ยฐ from the axis of the array ๐ = 30ยฐ . The desired half-
power beamwidth is 2ยฐ while the spacing of the elements is ๐/4. Determine the phase excitation of the elements, length of the array (in wavelengths), number of the elements, and directivity (in dB).
Linear Antenna Arrays Slide 67
Radio Observatory Antenna Arrays
Karl G. Jansky Very Large Array (VLA)
Linear Antenna Arrays Slide 68
โข Itโs a cm-wavelength radio astronomy observatory located 50 miles west of Socorro, NM
โข The radio telescope comprises 27 independent antennae, each of which has a dish diameter of 25 meters and weighs 209 metric tons.
โข The antennae are distributed along the three arms of a track, shaped in a wye-configuration, (each of which measures 21 km).
โข The frequency coverage is 74 MHz to 50 GHz (400 to 0.7 cm)
Very Long Baseline Array
Linear Antenna Arrays Slide 69http://www.vlba.nrao.edu/sites/
Very Long Baseline Array (VLBA) and High Sensitivity Array (HSA)
Linear Antenna Arrays Slide 70
โข VLBA is an interferometer consisting of 10 identical antennas on transcontinental baselines up to 8000 km (Mauna Kea, Hawaii to St. Croix, Virgin Islands).
โข The VLBA is controlled remotely from the Science Operations Center in Socorro, New Mexico.
โข The VLBA observes at wavelengths of 28 cm to 3 mm (1.2 GHz to 96 GHz)โข It is part of the High Sensitivity Array (HSA), which comprises the VLBA, phased
Very Large Array (VLA), Green Bank Telescope (GBT), Effelsberg, and Arecibo telescopes, and subsets thereof. This array spans around 12,000 km in length.
https://science.lbo.us/facilities/vlba