An investigation of concentric ring antennas with low sidelobes

Abstract

An analysis is made of circular antenna arrays with diameters being 2 to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9\lambda</tex> (the minimum inner circle diameter being <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4\lambda</tex> ) containing 3 to 18 concentric circles. For the purpose of computation of the array factor the elements of the array are assumed to be isotropic radiators. The elements of each circle have equal current amplitudes and are phased so that the contributions of all the elements add in phase in the direction of the main beam. The Chebyshev radiation pattern function is approximated by a truncated Fourier-Bessel series, from which the current amplitude of each circle is obtained. From these current amplitudes a method for computing the current amplitude to excite a new distribution of fewer circles is shown. Also, an empirical method is given for improving the sidelobe level of the radiation pattern by adding an element to the center of the array. A number of circles in the array sufficient to avoid pattern deterioration is found to be the integer nearest to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5d/4\lambda</tex> for -20 and -30 dB sidelobe level and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4d/3\lambda</tex> for -40 dB, where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</tex> is the diameter of the array. This represents a large reduction in the number of circles needed over the Fourier-Bessel series representation in the case of large antennas. Experimental verification of the computed pattern is made for an array of two concentric circles with diameters of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8\lambda</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.6</tex> at a frequency of 90 Mc/s. The elements of the array were vertical monopoles.