ANTENNA ARRAY S A Short R eview

1. ANTENNA ARRAYSA Short Review

2. Array Factor (1)

3. Uniform, Linear Array • Equally spaced elements along the z-axis Different radiation patterns can be obtained bychanging d and 

4. 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.

5. 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:

6. 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)

7. 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”

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

9. 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.

10. 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. Mutual Coupling (cont.) may imply simultaneous solution of thousands of equations

11. 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

12. 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