Modelling the Damping of Screened Room Resonances by Ferrite Tiles Using Frequency Dependent Boundaries in Tlm

A new technique which allows the simulation of thin ferrite absorbing tiles in a coarse TLM mesh is described. Results are presented which show a good fit to manufacturers' reflectivity curves. Experimental and simulation results for a screened room partially lined with ferrite are presented to demonstrate the accuracy of the method. INTRODUCTION The Transmission Line Matrix (TLM) method of numerical electromagnetic analysis with the symmetrical condensed node is well known [1]. The representation of lossy materials is described in references [2], [3], and [4]. To represent the operation of a lossy dielectric or magnetic material using these methods the material blocks must be about 6 mesh units deep. This is often feasible for lossy dielectric radio absorbent materials, such as those used to line anechoic chambers, as the material depth is relatively large compared to the mesh size. Ferrite absorbing tiles are only a few millimetres in depth and cannot easily be represented as material blocks when the mesh size required to model a typical anechoic chamber may be a significant fraction of a metre. In order to overcome this problem an efficient means of approximating the reflectivity of ferrite absorbers with frequency dependent boundaries is presented. It allows the simulation of ferrite tile absorber by the use of frequency dependent boundaries. Thus allowing the use of a much larger mesh size than would be possible if the absorber were represented as material blocks. This means that it is realistic to simulate the effect of fully, or partially ferrite-lined screened enclosures with the TLM method. The method requires the storage of only 4 values for each mesh-unit sized boundary patch. This is very efficient compared with the alternative possibility of using multi-grid or graded mesh TLM which would require a large ratio of mesh sizes, and a large number of mesh elements to adequately represent the tiles. FERRITE ABSORBING TILES Material Parameters The variation of permeability of ferrite materials can approximated by the function: rm = 1 + ( ) 'r0 1