antenna arrays - empossible
TRANSCRIPT
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Antenna Arrays
EE-4382/5306 - Antenna Engineering
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Outline• Introduction• Two Element Array• Rectangular-to-Polar Graphical Solution• N-Element Linear Array: Uniform Spacing and
Amplitude– Theory of N-Element Linear Array– Rectangular to Polar Graphical Solution– Broadside Array– Ordinary End-Fire Array– Phased Array– Hansen-Woodyard End-Fire Array
• N-Element Linear Array: Directivity• Design Procedure• Radio Observatory Antenna Arrays
2Linear Antenna Arrays
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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
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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
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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:
𝐸𝑡 = 𝐸1 + 𝐸2
= ෝ𝒂𝜃𝑗𝜂𝑘𝐼0𝑙
4𝜋
𝑒−𝑗 𝑘𝑟1−
𝛽2
𝑟1cos 𝜃1 +
𝑒−𝑗 𝑘𝑟2−
𝛽2
𝑟2cos 𝜃2
Where the 𝛽 is the difference in the phase excitation between elements.Assuming far-field observations:
𝜃1 ≅ 𝜃2 ≅ 𝜃
𝑟1 ≅ 𝑟 −𝑑
2cos(𝜃)
𝑟2 ≅ 𝑟 +𝑑
2cos 𝜃
𝑟1 ≅ 𝑟2 ≅ 𝑟
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Two-Element Array
Linear Antenna Arrays Slide 8
Assuming far-field observations, the total field becomes
𝐸𝑡 = ෝ𝒂𝜃𝑗𝜂𝑘𝐼0𝑙𝑒
−𝑗𝑘𝑟
4𝜋𝑟cos 𝜃 𝑒+𝑗 𝑘𝑑 cos 𝜃 +𝛽 /2 + 𝑒+𝑗 𝑘𝑑 cos 𝜃 +𝛽 /2
𝐸𝑡 = ෝ𝒂𝜃𝑗𝜂𝑘𝐼0𝑙𝑒
−𝑗𝑘𝑟
4𝜋𝑟cos 𝜃 2 cos
1
2𝑘𝑑 cos 𝜃 + 𝛽
Field of single element Array Factor
AF = 2cos1
2(𝑘𝑑 cos 𝜃 + 𝛽
(AF)𝑛= cos1
2(𝑘𝑑 cos 𝜃 + 𝛽
𝐸 total = 𝐸 single element at ref. point × [array factor]
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Two-Element Array - Examples
Linear Antenna Arrays Slide 9
Given the array shown for two identical isotropic sources, find the total field when 𝑑 = 𝜆/2 and 𝛽 = 0.
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Two-Element Array - Examples
Linear Antenna Arrays Slide 10
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Two-Element Array - Examples
Linear Antenna Arrays Slide 11
Given the array shown for two identical isotropic sources, find the normalized total field when 𝑑 = 𝜆/4 and 𝛽 = −90°.
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Two-Element Array - Examples
Linear Antenna Arrays Slide 12
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Two-Element Array - Examples
Linear Antenna Arrays Slide 13
Given the array shown for two identical isotropic sources, find the normalized total field when 𝑑 = 𝜆 and 𝛽 = 0°.
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Two-Element Array – Examples
Linear Antenna Arrays Slide 14
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Isotropic Point Sources – Array Factor for two elements
Linear Antenna Arrays Slide 15
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Isotropic Point Sources – Array Factor for two elements
Linear Antenna Arrays Slide 16
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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 𝑑 = 𝜆/4 and a. 𝛽 = 0b. 𝛽 = +𝜋/2c. 𝛽 = −𝜋/2
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Two-Element Array - Examples
Linear Antenna Arrays Slide 18
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Two-Element Array - Examples
Linear Antenna Arrays Slide 19
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Two-Element Array - Examples
Linear Antenna Arrays Slide 20
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Linear Antenna Arrays Slide 21
Antenna Array – Scanning Array
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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.
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Linear Array: Uniform Amplitude and Spacing
Linear Antenna Arrays Slide 24
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Linear Array: Uniform Amplitude and Spacing
Linear Antenna Arrays Slide 25
𝐴𝐹 = 1 + 𝑒+𝑗(𝑘𝑑 cos 𝜃 +𝛽) + 𝑒+𝑗2(𝑘𝑑 cos 𝜃 +𝛽) +⋯+ 𝑒+𝑗 𝑁−1 𝑘𝑑 cos 𝜃 +𝛽
AF =
𝑛=1
𝑁
𝑒+𝑗 𝑛−1 𝑘𝑑 cos 𝜃 +𝛽
AF =
𝑛=1
𝑁
𝑒+𝑗 𝑛−1 Ψ
Ψ = 𝑘𝑑 cos(𝜃) + 𝛽
Another useful expression is the closed form expression of the array factor.
𝐴𝐹 𝑒𝑗Ψ = 𝑒𝑗Ψ + 𝑒𝑗2Ψ + 𝑒𝑗3Ψ +⋯+ 𝑒𝑗𝑁Ψ
𝐴𝐹 𝑒𝑗Ψ − 1 = (−1 + 𝑒𝑗𝑁Ψ)
𝐴𝐹 =𝑒𝑗𝑁Ψ
𝑒𝑗Ψ − 1= 𝑒
𝑗𝑁−12 Ψ 𝑒
𝑗𝑁2 Ψ
− 𝑒−𝑗
𝑁2 Ψ
𝑒𝑗12 Ψ
− 𝑒−𝑗
12 Ψ
= 𝑒𝑗𝑁−12 Ψ
sin𝑁2 Ψ
sin12Ψ
Multiply by 𝑒𝑗Ψ
Subtract AF summation
Simplify
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Linear Array: Uniform Amplitude and Spacing
Linear Antenna Arrays Slide 26
AF =sin
𝑁2 Ψ
sin12Ψ
≅sin
𝑁2 Ψ
Ψ2
𝐴𝐹𝑛 =1
𝑁
sin𝑁2 Ψ
Ψ2
≅sin
𝑁2 Ψ
𝑁2 Ψ
for small values of Ψ
Ψ = 𝑘𝑑 cos(𝜃) + 𝛽
The nulls are given by setting the array factor to 0.
sin𝑁
2Ψ = 0 > >
𝑁
2Ψ ቚ
𝜃=𝜃𝑛= ±𝑛𝜋 > > 𝜃𝑛 = cos−1
𝜆
2𝜋𝑑−𝛽 ±
2𝑛
𝑁𝜋
The number of nulls that can exist will be a function of the element separation 𝑑 and phase excitation difference 𝛽.
𝑛 = 1,2,3, … (𝑛𝑢𝑙𝑙)𝑛 ≠ 𝑁, 2𝑁, 3𝑁,… (𝑚𝑎𝑥𝑖𝑚𝑢𝑚)
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Linear Array: Rectangular Plot
Linear Antenna Arrays Slide 27
First main maximum occurs when 𝜓
2= 0 > > Ψ = 0
The principal maxima occurs when
𝜃𝑚 = cos−1𝜆𝛽
2𝜋𝑑
Other main maxima occurs whenΨ = ±2𝑚𝜋, 𝑚 = 1,2,3,…
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Linear Array: Rectangular Plot
Linear Antenna Arrays Slide 28
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Linear Array: Rectangular Plot
Linear Antenna Arrays Slide 29
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Linear Array: Rectangular Plot
Linear Antenna Arrays Slide 30
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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 𝑁 increases, the main lobe narrows2. As 𝑁 increases, there are more side lobes in one period of 𝑓(Ψ).
In fact, the number of full lobes (one main lobe and the side lobes) in one period of 𝑓(Ψ) equals 𝑁 − 1. There are 𝑁 − 2 side lobes in each period.
3. The minor lobes are of width 2𝜋/𝑁 in the variable Ψ and the major lobes are twice this width.
4. The side lobe peaks decrease with increasing 𝑁.5. 𝑓 Ψ is symmetric about 𝜋.
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Linear Antenna Arrays Slide 32
Rectangular to Polar Graphical Solution
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Rectangular to Polar Graphical Solution
Linear Antenna Arrays Slide 33
In antenna theory, many solutions are of the form
𝑓 𝜁 = 𝑓(𝐶 cos 𝛾 + 𝛿)
Where 𝐶 and 𝛿 are constants and 𝛾 is a variable. The approximate array
factor of an N-element, uniform amplitude linear array is a sin 𝜁
𝜁where
𝜁 = 𝐶 cos(𝛾) + 𝛿 =𝑁
2Ψ =
𝑁
2𝑘𝑑 cos 𝜃 + 𝛽
𝐶 =𝑁
2𝑘𝑑
𝛿 =𝑁
2𝛽
The 𝑓 𝜁 function can be plotted in rectilinear coordinates, and transferred to a polar graph.
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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 𝑓 𝜁 .2. a) Draw a circle with radius C and its center on the abscissa at 𝜁 = 𝛿
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.
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Rectangular to Polar Graphical Solution
Linear Antenna Arrays Slide 35
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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 𝜆/2 and 90° interelement phasing(i.e. 𝛽 = 𝜋/2).
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Four element linear array -Example
Linear Antenna Arrays Slide 37
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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 = 𝜆/2 and uniform phase excitation 𝛼 =0°
Find and plot the same array factor of a two-element, isotropic, equally spaced array with distance d = 𝜆/2 but with phase excitation 𝛼 = 180°
Find and plot the same array factor of a two-element, isotropic, equally spaced array with distance d = 𝜆/4 but with phase excitation 𝛼 = −90°
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Two element linear array -Examples
Linear Antenna Arrays Slide 39
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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 = 0.45𝜆 and uniform phase excitation 𝛼 = 0.9𝜋
Find and plot the array factor of a five-element, isotropic, equally spaced array with distance d = 0.5𝜆 and uniform phase excitation 𝛼 =𝜋
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Five element linear array -Examples
Linear Antenna Arrays Slide 41
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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 (𝜃 = 90°). To optimize this design, both the maxima of the single element and the array factor should be both directed toward 𝜃 = 90°. Recall the maximum of the array factor occurs when
Ψ = 𝑘𝑑 cos(𝜃) + 𝛽 = 0
Since it is desired to have the first maximum directed toward 𝜃 = 90°
Ψ = 𝑘𝑑 cos(𝜃) + 𝛽 ቚ𝜃=90°
= 𝛽 = 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.
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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 (𝑑 ≠ 𝑛𝜆, 𝑛 = 1,2,3,…) when 𝛽 = 0.If 𝑑 = 𝑛𝜆, 𝑛 = 1,2,3, and 𝛽 = 0, then
Ψ = 𝑘𝑑 cos 𝜃 + 𝛽ȁ 𝑑=𝑛𝜆𝛽=0
𝑛=1,2,3,…
= 2𝜋𝑛 cos 𝜃 ȁ𝜃=0,𝜋 = ±2𝑛𝜋 avoid this!
To avoid any grating lobes, the largest spacing between the elements should be less than one wavelength (𝑑 = 𝜆)
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Broadside Array
Linear Antenna Arrays Slide 44
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Broadside Array
Linear Antenna Arrays Slide 45
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Broadside Array
Introduction to Antennas Slide 46
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Broadside Array
Linear Antenna Arrays Slide 47
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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 (𝜃 = 0°, 180°)
For the maximum toward 𝜃 = 0°:
Ψ = 𝑘𝑑 cos(𝜃) + 𝛽 ቚ𝜃=0°
= 𝑘𝑑 + 𝛽 = 0 > > 𝛽 = −𝑘𝑑
For the maximum toward 𝜃 = 180°:
Ψ = 𝑘𝑑 cos(𝜃) + 𝛽 ቚ𝜃=180°
= −𝑘𝑑 + 𝛽 = 0 > > 𝛽 = 𝑘𝑑
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Ordinary End-Fire Array
Linear Antenna Arrays Slide 49
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Ordinary End-Fire Array
Linear Antenna Arrays Slide 50
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Ordinary End-Fire Array
Linear Antenna Arrays Slide 51
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Ordinary End-Fire Array
Linear Antenna Arrays Slide 52
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Linear Antenna Arrays Slide 53
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Scanning/Phased Array
Linear Antenna Arrays Slide 54
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Scanning/Phased Array
Linear Antenna Arrays Slide 55
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Scanning/Phased Array
Linear Antenna Arrays Slide 56
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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 𝜃 = 0°:
𝛽 = − 𝑘𝑑 +2.92
𝑁≅ − 𝑘𝑑 +
𝜋
𝑁
For the maximum toward 𝜃 = 180°:
𝛽 = − 𝑘𝑑 +2.92
𝑁≅ + 𝑘𝑑 +
𝜋
𝑁
For both directions, spacing should be
𝑑 =𝑁 − 1
𝑁
𝜆
4≅𝜆
4for large N
Hansen-Woodyard End-Fire Array
Linear Antenna Arrays Slide 57
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Hansen-Woodyard End-Fire Array
Linear Antenna Arrays Slide 58
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Hansen-Woodyard End-Fire Array
Linear Antenna Arrays Slide 59
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Linear Arrays - Summary
Linear Antenna Arrays Slide 60
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Linear Arrays - Summary
Linear Antenna Arrays Slide 61
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Linear Antenna Arrays Slide 62
N-Element Linear Arrays: Directivity
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Antenna Array Directivity
Linear Antenna Arrays Slide 63
For a linear antenna array, determine total length by
𝐿 = 𝑁 − 1 𝑑
For a large broadside array (𝐿 ≫ 𝑑), directivity reduces to
𝐷0 ≅ 2𝑁𝑑
𝜆= 2 1 +
𝐿
𝑑
𝑑
𝜆≅ 2
𝐿
𝜆
For a large ordinary end-fire array (𝐿 ≫ 𝑑), directivity reduces to
𝐷0 ≅ 4𝑁𝑑
𝜆= 4 1 +
𝐿
𝑑
𝑑
𝜆≅ 4
𝐿
𝜆
For a Hansen-Woodyard end-fire array (𝐿 ≫ 𝑑), directivity reduces to
𝐷0 ≅ 1.805 4𝑁𝑑
𝜆= 1.805 4 1 +
𝐿
𝑑
𝑑
𝜆≅ 1.805 4
𝐿
𝜆
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Linear Antenna Arrays Slide 64
Linear Arrays: Design Procedure
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Linear Arrays: Design Procedure
Linear Antenna Arrays Slide 65
𝑁 =𝐿 + 𝑑
𝑑
𝐿 = 𝑁 − 1 𝑑
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Linear Arrays: Design Procedure Example
Linear Antenna Arrays Slide 66
Design an uniform linear scanning array whose maximum array
factor is 30° from the axis of the array 𝜃 = 30° . The desired half-
power beamwidth is 2° while the spacing of the elements is 𝜆/4. Determine the phase excitation of the elements, length of the array (in wavelengths), number of the elements, and directivity (in dB).
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Linear Antenna Arrays Slide 67
Radio Observatory Antenna Arrays
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Karl G. Jansky Very Large Array (VLA)
Linear Antenna Arrays Slide 68
• It’s a cm-wavelength radio astronomy observatory located 50 miles west of Socorro, NM
• The radio telescope comprises 27 independent antennae, each of which has a dish diameter of 25 meters and weighs 209 metric tons.
• The antennae are distributed along the three arms of a track, shaped in a wye-configuration, (each of which measures 21 km).
• The frequency coverage is 74 MHz to 50 GHz (400 to 0.7 cm)
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Very Long Baseline Array
Linear Antenna Arrays Slide 69http://www.vlba.nrao.edu/sites/
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Very Long Baseline Array (VLBA) and High Sensitivity Array (HSA)
Linear Antenna Arrays Slide 70
• VLBA is an interferometer consisting of 10 identical antennas on transcontinental baselines up to 8000 km (Mauna Kea, Hawaii to St. Croix, Virgin Islands).
• The VLBA is controlled remotely from the Science Operations Center in Socorro, New Mexico.
• The VLBA observes at wavelengths of 28 cm to 3 mm (1.2 GHz to 96 GHz)• 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