ICE5108084 Antenna Design Lecture 14 Linear Antenna Arrays

ICE-5108084 Antenna Design Lecture 14: Linear Antenna Arrays 14. 1 Introduction to Antenna Arrays 14. 2 Linear Antenna Arrays 14. 3 Theory of Linear Antenna Arrays 14. 4 Examples of Linear Antenna Arrays 1

14. 1 Introduction to Antenna Arrays § What is an antenna array? - Distribute many of a same radiating element over a line or a plane or a surface. - Combine array elements using a feed network for 1) high directivity (narrow beamwidth), 2) a scanned main beam, 3) multiple beams (a multi-beam antenna), 4) a shaped main beam 2

14. 2 Linear Antenna Arrays § Why linear antenna arrays? - To obtain a narrow beam in one direction and a broad beam in the other direction (= a fan beam). - Or to scann a fan beam in one direction. Example = a search antenna in azimuth plane 3

14. 3 Theory of Linear Antenna Arrays § Array factor: - Far-field approximation to the radiated field: parallel-ray approximation - Far field of a single element field; 4

- Far field of two elements: At x = 0, antenna 1 with I 1; at x = d, antenna #2 with current I 2 5

- Two-element array factor example: 6

- Principles of pattern multiplication: 7

- Array factor of a linear array: Number of elements: N Element spacing: d Element phase shift: α 8

- Beam scanning: The main beam is tilted toward the direction of decreasing phase. - Broadside array: main beam is normal to the array axis 0 = 90° - Scanned-beam array: main beam is tilted away from the array normal 0° < 0 < 90°, 90° < 0 < 180° - Endfire array: main beam is in the direction of the array axis 0 = 0°, 180° 9

- Array factor of a uniformly excited linear array: - Properties of AF of a uniformly excited linear array: Period: 2π Even function of ψ Symmetric about π Main-null (1 st null) distance: Δψ = 4π / N Sidelobe null distance: Δψ = 2π / N Even 10

- Graphical method for plotting the array factor: Example: N = 4, = π/2, d = /2 11

- Grating lobes (격자엽): additional main beams occurring when the element spacing is too large. 12

- Grating lobe example: N = 10, broadside 13

Example: N = 10, endfire 14

14. 4 Examples of Linear Antenna Arrays § WERA (Wav. E RAdar) ocean radar system - 16 -element monopole linear array - HF surface wave radar for oceanography - University of Hamburg 15

§ 4 -bay batwing antenna (super-turnstile antenna) array for FM radio broadcast - Super-turnstile antenna: Horizontally polarized Omnidirectional in the horizontal plane Wideband § 4 -dipole array for digital TV reception - Wire bowtie antenna = fan dipole antenna: wideband - Wire-grid ground plane: small weight, low wind resistance 16

§ Cylindrical reflector antenna feed: - University of Sydney - Molongo Observatory Synthesis Telescope (MOST) - 843 MHz, reflector 11. 6 × 778 m, feed 352 elements § Base station antenna for 4 G cellular communication - 12 -element linear array of crossed dipoles - Crossed dipoles in a baffle (a cavity) 17

ICE-5108084 Antenna Design Lecture 14: Linear Antenna Arrays 18