A new method for determining the propagation constant and the effective width of Substrate Integrated Waveguides (SIWs) is presented in this paper. SIW cells can be viewed as two consecutive rectangular waveguide with same height but different width. By considering higher mode effects, using a reactive element, transmission ABCD matrix of a unit cell of SIW structure is obtained. By applying Floquet theorem, overall transfer matrix of the unit cell is achieved and propagation constant of the SIW structure is calculated. Also a closed form formula for effective width is derived. A review and comparisons with published and simulation results are also presented to show the accuracy of the proposed method.

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