Characteristic mode-based design and analysis of an electrically small planar spiral antenna with omnidirectional pattern

The antenna under consideration is a five-turn Archimedean rectangular two-arm wire spiral antenna, which is 46 x 15.7cm in size is presented in this paper. It is placed in the x-y plane. There are two ports (one per arm) located at the center. Normally, a two-arm spiral antenna cannot practically radiate spiral mode 2, as it requires the two-ports at the center to be excited in phase (i.e. an unbalanced feed), but it can be simulated using a method of moments code (MoM) without any problems. All simulation results are obtained using an MoM code. The goal is to produce the best possible horizontally polarized omnidirectional pattern. The reader should note that all gain values cited in this section do not include mismatch losses. Absolute gain, which includes matching networks, will be presented at the conference.

Introduction

Antenna design is mostly concerned with optimizing two properties: radiation pattern and input impedance. Radiation pattern optimization typically involves two tasks: designing the antenna geometry and materials to support a set of natural modes; and determining the feed point locations such that the desired modes are excited. Input impedance optimization involves designing the feed locations and excitations such that the current at the feed points does not change much over frequency and is well-matched to the feed transmission line. Optimization of both properties can be greatly assisted by the judicious application of characteristic mode theory [1], [2].