Electromagnetic and absorption properties of some microwave absorbers

Abstract

Electromagnetic properties of a thermoplastic natural rubber (TPNR), a lithium–nickel–zinc (Li–Ni–Zn) ferrite and a TPNR–ferrite composite subjected to transverse electromagnetic (TEM) wave propagation were investigated. The incorporation of the ferrite into the matrix of the TPNR was found to reduce the dielectric loss but the magnetic loss increased. The absorption characteristics of all the samples subjected to a normal incidence of TEM wave were investigated based on a model of a single-layered plane wave absorber backed by a perfect conductor. It is evident from a computer simulation that the ferrite is a narrowband absorber, whereas the polymeric samples show broadband absorption characteristics. Minimal reflection of the microwave power or matching condition occurs when the thickness of the absorbers approximates an odd number multiple of a quarter of the propagating wavelength. This is discussed as due to cancellation of the incident and reflected waves at the surface of the absorbers. The Li–Ni–Zn ferrite exhibits another matching condition at low frequency when the magnitude of the complex relative dielectric permittivity ${\mathrm{(\epsilon }}_r^{*})$ equals that of the complex relative magnetic permeability ${\mathrm{(\mu }}_r^{*}\mathrm{).}$ The specular absorber method provides a simple theoretical graphic aid for determining the absorption characteristics and the location of the matching conditions in the frequency domain. The result for the ferrite sample was tested and confirmed directly from terminated one-port measurements.