ANTENNA ARRAYS

Array Factor (1) . The field excited from each antenna element interferes with each other. This interaction depends on the phase and amplitude of the contributor and can be either constructive or destructive. In one direction interaction can be destructi ve causing nulls of the radiation pattern while in some other dire ction the fields may add up and produce the maxi mum of antenna pattern.

Phased Array Antennas Each antenna element can be controlled individually by phase or time delay. By changing the feeding it is possible to construct a directive beam that can be repositioned electronically. Amplitude control can be used for pattern shaping The beam can be pointed to new direction, narrowed or widened in microseconds. An array that has a main peak at a certain angle can also have other peak values depending on the spacing between the antenna elements.

Grating Lobes AF for uniform excitation: AF will have a maximum when exponent is a multiple of 2 grating lobes will occur at: to avoid grating lobes:

8 element array with /d=1 and for uo=0.5 (scan angle of 30o) uo=0.5 (scan angle of 30 degrees) uo=0 (broadside)

Mutual Coupling element pattern of the antenna changes from its free space (isolated) value when it is inserted into an array this coupling effect will be different for each element of the array. it may be necessary to use the concept of “active element pattern”

Element pattern of a dipole located as a center element of a 7X9 array

Analysis Including Mutual Coupling In a strong mutual couping environment array pattern = element pattern X array factor does not work ! Solving the problem using numerical methods is not practical. Therefore other effective methods are needed to account for mutual coupling effects.

Mutual Coupling (cont.) Finite Array Approach: Used for small and medium arrays. Active element pattern is calculated separately for each element in the array. these patterns are added up to obtain theoverall array pattern. may imply simultaneous solution of thousands of equations

Mutual Coupling (cont.) Infinite array assumption: For large arrays, the central elements that are far away from edges are affected less infinite array concept can then be used It is assumed that for all elements the currents are similar except for some complex constants. When this approach is used, it is sufficient to analyze only one element completely

Mutual Coupling (cont.) For medium size arrays, the exact AEP methods are difficult to use and average AEP method yields in errors in calculating the array pattern For these arrays the combination of the two methods are used to obtain more accurate results for the array pattern

Array Blindness Direct consequence of mutual coupling Can result in complete cancellation of the radiated beam at some scan angle Occurs when most of the central elements of the array have reflection coefficients close to unity

Array Performance Array Lattice Array Bandwidth Differences Between Single Element and Array Performances Amplitude Tapering For Sidelobe Level Control Wide-Angle Impedance Matching (WAIM)

Array Performance Array Lattice The position of the array elements describes the array lattice and there are basically three types for planar arrays

Array Performance Array Bandwidth The bandwidth of the array depends on the radiators, phase shifters, feeding networks etc. Phase shifters and feeding networks possess error transfer functions which grows with increasing bandwidth. The error analysis of the effect on the pattern will determines the bandwidth.

Array Performance Single Element and Array Performance Due to the mutual coupling effects in the array environment the single element performance and the array performance of most antennas are different

Array Performance Amplitude Tapering for Sidelobe Level Control The amplitude tapering in the excitation of the array elements determines the array sidelobe level, array gain and the beamwidth. Stronger tapering results in reduced sidelobe at the expense of increased beamwidth and reduced gain. - Powers of cosine - Taylor distributions - Modified Sin u/u taper of Taylor distributions - Dolph-Chebyshev distributions

Array Performance Modified Sinu/u taper of Taylor Distributions

Array Performance Dolph-Chebyshev Distributions Is the optimum distribution in the sense of narrowest beam for a given SLL Sidelobes do not decay in amplitude. The power of percentage in the main beam varies with the number of elements in the array for a given SL

Example of illumination coefficients and array pattern for a 20 dB taper applied to a 16 element array

Array Performance Wide-Angle Impedance Matching WAIM Scan impedance is the impedance of an element as a function of scan angle with all elements excited with proper amplitude and phase. For wide scan angles another mismatch due to the scan angle occurs. WAIM techniques are used to overcome this problem - Transmission line region techniques - Free space WAIM techniques

Array Performance Wide-Angle Impedance Matching WAIM Transmission Line Techniques Passsive circuits to control higher order modes in the aperture - separate interconnections between the elements - active tuning circuits Free Space Techniques - Reduced element spacing - Dielectric slabs or dielectric sheets