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On the multimodal analysis and design of guided filters in circular waveguide technology

Circular waveguides are widely used in the fabrication of many passive and active components (such as waveguide filters [1], traveling-wave tubes [2], orthomode transducers [3] and turnstile junctions [4]), and they can be considered as key elements of current space communications devices in both microwave and millimeter-wave range [5-8]. In this work, a multimodal technique for the efficient analysis and design of waveguide filters implemented in circular waveguide technology, and including off-centered circular irises, is proposed. The developed CAD tool is based on an integral equation technique, which provides a full-wave representation of the elementary blocks of the waveguide filters in terms of an equivalent multimode network. The computational efficiency of the developed tool has been drastically improved by expressing the radial variation of the modal solutions of the circular waveguides in terms of sinusoidal functions, thus avoiding the use of the more cumbersome Bessel's functions employed in the classical formulation. Besides, instead of using surface integrals to compute the coupling coefficients related to the waveguide junctions present in the considered filters, the developed CAD tool resorts to transform such surface integrals into line integrals. Consequently, the CPU effort (both in time and memory) is drastically reduced. The obtained results show that the relative position of the considered circular irises can be considered as a new design parameter with an important influence on the electrical response of the filter. The accuracy of the proposed method has been successfully validated by comparing the obtained results with data extracted from both the technical literature and a commercial software based on the finite-element method.