Superdirective cascaded dipole array

Abstract

With a view to realizing a superdirective antenna, an endfire array consisting of parallel dipole elements is analyzed. As a feeding method, a cascaded connection of all elements by transmission lines is proposed. A possibility of a superdirectivity and a basic design guideline are found for the case where an element spacing and the characteristic impedance of the connecting transmission lines are set arbitrarily. As the element spacing is reduced, the antenna becomes smaller and the gain increases, although the feeding becomes more difficult. Hence, the case of a finite-element spacing is discussed. First, it is demonstrated that the maximum gain as a superdirective antenna is obtained by adjustment of the element lengths and the lengths of the connecting transmission line. Further, it is shown that the optimum element length for the maximum gain is near the half-wavelength even if all elements have an identical length for ease of design and that such a structure works as a superdirective antenna. Based on the findings, a quarter-wavelength spacing, three-element array is designed. The validity of the present method is demonstrated since the experimental results for the input impedance, gain, and directivity agree well with the theoretical prediction.