II Array Antennas Array of Radiating Elements Higher

II. Array Antennas

Array of Radiating Elements -Higher Directivity: arrange the radiating elements in space Linear Array Planar Array Conformal Array -Phased Array for Electronically Scanning: change the phase of the exciting current of each element -Element Pattern: Pattern of each radiating element -Array Factor: Pattern of Arrayed isotropic elements

Array of 2 Isotropic Elements

Equally Spaced Uniform Array

Uniformly Excited, Equally Spaced Linear Array (UE, ESLA) Current amplitudes are all equal

1. As N increases, the main lobe narrows 2. As N increases, there are more side lobes: # of SL/period = 3. Width of minor lobe = half of that of main lobes 4. As N increases, the side lobe peak decreases 5. is even function

Ex) 4 -element UE, ESLA

Main Beam Scanning and Beam-width Main beam maximum direction can be controlled by changing the phase difference between adjacent elements Main Beam Scanning

Visible Region and Radiation pattern

Beam-width 1. BWFN (Beam Width between First Nulls) a) Broad Side:

b) End Fire:

2. HPBW (Half Power Beam Width) a) Near Broad Side: b) end-fire:

Endfire Array 1. The Ordinary Endfire Array One major lobe at direction Two major lobes at directions One main, and one grating lobe For only one main lobe

2. Hansen-Woodyard Endfire Array Visible region To obtain narrower beamwidth than the Ordinary case are not included in Visible Region Optimum phasing and spacing for maximum directivity

Pattern Multiplication Far-zone field of uniform array of identical elements is equal to the product of the field of a single element at a reference point and the array factor of that array Ex) collinear array of short dipoles

Ex) half wave length spaced collinear array of 2 short dipoles

Ex) half wave length spaced parallel array of 2 short dipoles Pattern of short dipole parallel to x-axis E-Plane Pattern H-Plane Pattern

half wave length spaced parallel array of 2 short dipoles E-Plane Pattern 3 -D Pattern H-Plane Pattern

Directivity of UE_ESLA Beam Solid Angle Radiated Power Directivity

Beam Solid Angle of UE_ESLA Max. in the visible region for Hansen-Woodyard

Broadside Case Another main lobes appear Directivity is decreased

Endfire Broadside Endfire

Nonuniformly Excited, Equally Spaced Linear Array (NE, ESLA) Array Factor is Z-Transform of Amplitude Distribution and has N-1 Zeros (Pattern Nulls) 1) Uniform Excited

By tapering the current amplitude Tapering toward the ends decrease the side lobe level increase HPBW Tapering toward the center increase the side lobe level decrease HPBW

2) Binomial Distributed 3) Triangular Distributed

4) Dolph-Chebyshev Distributed

Tapering toward the center (inverse taper)

Directivity of Nu. E, ESLA

Array Pattern Synthesis Fourier Series Method: -. Fourier series expansion of desired array pattern -. Least-mean-square error approximation to the desired pattern

Binomial Arrays: -. Binomial expansion of desired array pattern -. complete absence of side lobes but low directivity

Array Polynomial: 1. The array factor of an (N+1)-element array is the product of the array of N 2 -element arrays superimposed to produce nulls at the zeros of AF 2. Any polynomial of degree N can be interpreted as an (N+1)-element array 3. The product of 2 polynomials is the array factor for an array whose pattern is the product of patterns associated with each polynomial by itself

Multi-dimensional Array 2 -dimensional Planar Array If the currents separable

Uniform Excited

Principal plane pattern,

Circular Array

Phased Array Scan Principle Beam broadening: scanned off broadside Grating Lobe: if visible region is wider than main lobe For avoiding grating lobes the main beam widens , there are more than one

Feed Networks for Beam Scanning Beam Forming Network Parallel Feed (Corporate feed) Frequency Scanning Parallel-series Feed Electronically phase control

Butler Beam-forming Matrix

Impedance Effects of Mutual Coupling Mutual coupling between elements mth element in an array

2 -element Array for 2 identical elements

Mutual Impedance between 2 parallel dipoles Mutual Impedance between 2 collinear dipoles

Impedance Effects of Mutual Coupling -The strength of the coupling decreases as spacing increases, roughly as 1/d 2 : power density decreases as 1/d 2 -The far field pattern of each element predicts coupling strength. elements oriented to maximum directivity -> maximum coupling -Elements with electric field orientation (i. e. polarizations) that are parallel will couple more than when collinear. -Larger antenna elements with broadside patterns have lower coupling to neighboring elements: Broadside case: elements are along the line perpendicular to beam direction->weak coupling Endfire case : elements are along the line of beam direction->strong coupling Changing Scanning Angle->mutual coupling changed ->input impedance of each element changed

Changing Scanning Angle ->mutual coupling changed ->input impedance of each element changed Reflection Coefficient of each element at feed point changed Radiated Power of each element changed No power radiated -> Scan Blindness