Input Resistance of Electrically Short Not-Too-Closely Spaced Multielement Monopoles With Uniform Current Distribution

Abstract

In this letter, we derive an analytical expression for the mutual resistance of two electrically short not-too-closely spaced dipoles with uniform current distribution based on an induced electromotive forces method. We go on to apply the relations for folded dipoles for an investigation of the input resistance of a short multielement dipole arrangement. It is shown that the factor ranging from N1.77 to N1.97 takes into account the increase in input resistance arising from the mutual coupling of N={2,3,4} dipoles when they are spaced within the range from λ0/20 to λ0/5. Thus, even a larger distance between the dipoles than λ0/20 , which is usually considered if the term N2 is used, still leads to a significant improvement in radiation resistance. To verify the proposed concept, we compared the results provided by the electromagnetic (EM) simulation and by measurements on the prototype of an electrically small, winding-connected three-element monopole antenna, which occupies a cylindrical geometry with the diameter of the base equal to λ0/10 and the monopole height equal to λ0/20. The measured bandwidth for VSWR=2 corresponds to the quality factor, which is only 2.2 higher than the principal Gustafsson bound for cylindrical geometry with vertical polarization.