Printed Antennas Tuned by Transversely Magnetized Ferrite Operating at a Novel Resonant Mode

Abstract

The excitation of a novel single mode in the negative efi area is studied for rectangular patch antenna printed on magnetized ferrite substrates or tuned by a ferrite inclusion. In all cases the DC magnetic fleld is considered perpendicular to the substrate plane. The ferrite inclusion-post shape is considered either cylindrical or rectangular. Numerical simulation results reveal the existence of this novel resonating mode inside the negative efi area for both the rectangular microstrip patch printed on ferrite substrate or tuned by a rectangular ferrite post. Also, the input impedance characteristics of these patch antennas are studied. 1. INTRODUCTION A number of published works (e.g., Pozar (1)) show that in the case of patch antennas printed on transversely magnetized ferrite substrates, there is a cutofi frequency range where there is not any propagating mode in the substrate below the radiator. In this range the efiective permeabil- ity efi of the ferrite is negative, so the corresponding wavenumber becomes imaginary leading to an evanescent mode. The resonant frequency of circular and ring patch antennas printed on transversely magnetized ferrite substrate was analytically studied in our previous works (2,3). The analysis was performed using the perfect magnetic walls approximation and the ferrite losses were also taken into account. Closed form expressions were given for all the geometries studied. By including ferrite losses in our analysis and with the proper mathematical handling of the involved Bessel functions, the solution of the characteristic equation in the negative efi area became possible revealing a novel single propagating mode. There is only one similar work published in the litera- ture reporting on this mode as propagating in a grounded ferrite substrate (4). Baccarelli etal. (4) have also considered a lossy ferrite substrate, but biased with an HDC parallel to the ground plane. Assuming propagation along the substrate, but in a direction transverse to the HDC, they indeed found a single mode denoted as TE1+ in the region of negative efi . Our previous studies on axially symmetric patches proved that the novel mode exists in the negative efi range independent of the speciflc patch shape or the tuning ferrite inclusion. This observation motivated us to study the rectangular patch printed on transversely magnetized ferrite substrate and the same patch tuned by a circular or rectangular ferrite post. Thus, the aim of the present work is to identify the existence of any resonant mode in the negative efi range, for rectangular patch radiators.