Antenna Arrays - NUS/ECE EE4101 Antenna Arrays 1...

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Hon Tat Hui Antenna Arrays NUS/ECE EE4101 1 Antenna Arrays 1 Introduction 1. They can provide the capability of a steerable beam (radiation direction change) as in smart antennas. 2. They can provide a high gain (array gain) by using simple antenna elements. 3. They provide a diversity gain in multipath signal reception. 4. They enable array signal processing. Antenna arrays are becoming increasingly important in wireless communications. Advantages of using antenna arrays:
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Hon Tat Hui Antenna Arrays NUS/ECE EE4101 2 An important characteristic of an array is the change of its radiation pattern in response to different excitations of its antenna elements. Unlike a single antenna whose radiation pattern is fixed, an antenna array’s radiation pattern, called the array pattern, can be changed upon exciting its elements with different currents (both current magnitudes and current phases). This gives us a freedom to choose (or design) a certain desired array pattern from an array, without changing its physical dimensions. Furthermore, by manipulating the received signals from the individual antenna elements in different ways, we can achieve many signal processing functions such as spatial filtering, interference suppression, gain enhancement, target tracking, etc.
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Hon Tat Hui Antenna Arrays NUS/ECE EE4101 3 2 Two Element Arrays Two Hertzian dipoles of length d separated by a distance d and excited by currents with an equal amplitude I but a phase difference β [0 ~ 2 π ). Far field observation point d r r 1 θ z x Dipole 2 Dipole 1 π = 0 , cos 1 d r r j Ie I = 2 I I = 1
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Hon Tat Hui Antenna Arrays NUS/ECE EE4101 4 11 1 1 1 2 2 2 sin cos 42 4 sin cos 4 jkr jkr jkr jkr kI d e kd e Ej θ j θ I rr kI d e kd e θ j θ I ηπ η ππ −− ⎛⎞ =+ = ⎜⎟ ⎝⎠ = AA Use the following far-field approximations: 0 ≤ θ ≤ π () 1 1 cos jk r d jkr ee θ = E 1 = far-zone electric field produced by antenna 1 = E 2 = far-zone electric field produced by antenna 2 = 1 ˆ E a 2 ˆ E a
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Hon Tat Hui Antenna Arrays NUS/ECE EE4101 5 The total E field is: () 12 cos cos 1 ˆ cos ˆ 4 cos 1 ˆ 4 cos AF ˆ 4 jkr jkd jkr jj k d jkr E E kd e j θ II e r kI d e j θ ee r kId e j θ r θ β η π =+ ⎛⎞ ⎡⎤ ⎜⎟ ⎣⎦ ⎝⎠ = Ea a a a A A A where
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Hon Tat Hui Antenna Arrays NUS/ECE EE4101 6 cos AF Array Factor 1 jj k d ee β θ == + [ ] cos 2 1 1 1 jkd e I I I + = The magnitude of the total E field is: () 1 cos cos 2 1 12 c o s c o s 2 jk d k d e k d βθ + ⎡⎤ += + ⎣⎦ ⎢⎥ cos ˆ 4 radiation pattern of a single Hertzian dipole jkr kId e j θ r η π ⎛⎞ = ⎜⎟ ⎝⎠ Ea A
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Hon Tat Hui Antenna Arrays NUS/ECE EE4101 7 Hence we see the total far-field radiation pattern | E | of the array (array pattern) consists of the original radiation pattern of a single Hertzian dipole multiplying with the magnitude of the array factor |AF|. This is a general property of antenna arrays and is called the principle of pattern multiplication .
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Antenna Arrays - NUS/ECE EE4101 Antenna Arrays 1...

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