Impedance characteristics of the normal mode helical antenna with a nearby conduction plate

Abstract

To improve the efficiency of a small-sized antenna, a normal mode helical antenna (NMHA) is promising with a high radiation resistance. However, its efficiency is subject to degradation due to a nearby human body or radio equipment. In this paper, we replaced the human body, etc., by a conductive plate as a basic problem to analyze the influence of the human body and calculated the impedance when NMHA is in proximity of a conductive plate. In our calculation method, the influence of the conductive plate was replaced by an electric image of NMHA. We obtained the current distribution of a two-element NMHA array which was placed in parallel by the improved circuit theory method which is similar to the moment method. As a result, we found that the real part of the mutual impedance between the array was positive, negative or zero depending on the value of the structural parameters of the antenna. The radiation resistance with a nearby conductive plate was the real part of subtracting the mutual impedance from the input impedance of a NMHA in a free space. In the shapes for zero or negative the real part of mutual impedance, the radiation resistance was equal to or greater than that of the unit antenna so that the conductive plate could be utilized effectively. We showed that the NMHA is a combined model of a dipole antenna and a loop antenna. For shapes with zero mutual impedances, we found that the far radiated field components of the dipole and those of the loop antenna were equal to the structural parameters which were almost equal to 2π2a2 = pλ (a: helical radius, and p: pitch).