Array factor analysis for untra-wide-angle scanning performance of planar phased arrays

Abstract

At present, researchers generally believe that the change of maximum directivity of planar phased arrays in beam scanning process conforms with the cosine law, so that the wide-range beam scanning from broadside to end-fire is impossible to realize. There are three main factors that affect the scanning range of the planar phased array antenna: 1) array factor; 2) element pattern; 3) port impedance matching. Scholars have done a lot of researches on the element pattern and the port impedance matching. About the array factor, Elliott, a famous phased array scholar, made a theoretical study on the directivity of the linear array and the planar array in 1963 and 1964, concluding that the variation of directivity of planar phased array conforms with the cosine law. However, the above conclusion came from an approximate formula under certain conditions: 1) the element pattern in the half-space is omnidirectional; 2) the beam of a large array is narrow enough; 3) the beam cannot scan a range of several beamwidths close to end-fire. To make clear the scanning beam’s change law of planar arrays, the law of directivity changing with scanning angle of linear array and planar phased array with different sizes are analyzed by using the strict formula of array factor, and the directivity’s change law derived from strict formula of array factor is compared with the cosine law. The results show that 1) the variation law of the directivity of a linear array depends on the spacing between elements. When the spacing between elements is equal to half a wavelength, the directivity of the array does not change with the scanning angle. When the spacing between elements is less than half a wavelength, the directivity of the array increases with scanning angle increasing. 2) The directivity of a finite array decreases with scanning angle increasing. However, the scanning beam’s maximum directivity of a finite planar phased array does not satisfy the cosine law and it is not zero in the end-fire range. Based on the above conclusions, the idea that the change of array factor’s directivity is compensated for by element beamforming is further proposed, which points out a direction for realizing the ultra-wide-angle-range beam scanning of planar phased arrays.