A mode-matching approach to determine the shielding effectiveness of a doubly-periodic array of apertures in a thick conducting screen

The transmission of electromagnetic waves through apertures in a conducting screen is a problem that has been examined many times before. Several techniques have been used when the apertures are periodically arranged, and computational approaches have allowed for the analysis of complex aperture shapes. However, past literature is typically concerned with screens whose thickness is comparable to or smaller than the aperture dimensions (i.e. thin screens). Further, the usual focus is on transmission within a narrow band of frequencies.; The shielding properties of planar, periodic structures have been considered in prior efforts. For a thick conducting screen of apertures, one approach for estimating the shielding effectiveness is to treat the screen as an array of cylindrical waveguides. This is referred to as the waveguide below cut-off principle. The result is dependent on the attenuation constant of the aperture and the aperture length. This technique is limited by the fact that it was developed to describe the attenuation of waves propagating in an opening whose length is at least five times its width. In addition, this approach is only relevant when the frequencies of interest are below the cut-off frequency of the dominant waveguide mode.; This dissertation uses mode-matching to determine the shielding effectiveness of a doubly-periodic conducting screen of apertures whose thickness can be several times the aperture size. This is accomplished by modeling the screen as an array of cylindrical waveguides. This study considers rectangular and circular apertures, and the fields within them are represented using waveguide modal fields. The reflected wave above the screen and the transmitted wave below the screen are found by applying Floquet's Theorem, thereby exploiting the doubly-periodic nature of the screen of apertures. After enforcing boundary conditions and building a system of linear equations, the system is then truncated to produce a matrix equation which is solved using standard techniques. The shielding effectiveness of the screen is determined by comparing the transmitted power to the incident power carried by a plane wave. It is clear that as the thickness of the screen increases, the transmitted power is greatly reduced at frequencies below the cut-off frequency of the dominant waveguide mode. However, increasing the thickness also increases the attenuation of the higher-order waveguide modes, leading to non-convergent solutions to the matrix equation. By selectively eliminating higher-order modes from consideration, meaningful solutions are found. Results also show the effect of increasing the number of Floquet modes, varying the incidence angle, and changing the incident plane wave polarization.; The mode-matching results for rectangular apertures are very similar to data obtained by applying the waveguide below cut-off principle. However, the mode-matching approach can be used in cases where the frequencies of interest are above the cut-off frequency of the dominant waveguide mode, when higher-order modes will begin to propagate. Comparisons are also made to previously published data using the mode-matching approach. The data curves are in strong agreement in each comparison. However, it should be noted that the previously published data considers the principal Floquet mode as the only propagating mode. That approach is inconsistent with the definition of the propagation constant for Floquet waves. Experimental data using commercial-grade aluminum honeycomb is also presented as another comparison for the mode-matching results. In each case, the curves are in good agreement in describing the transition from strong shield to weak shield.