A compound or an offset inclined slot fed by a rectangular waveguide has been analysed using the image method for the evaluation of internal admittance in the Method of Moments framework. The internal admittance has been evaluated from the 2D infinite planar array equivalent image representation of a slot in a waveguide. The advantages offered by this method include the freedom to work in slot coordinates rather than the waveguide coordinates, the reuse of external admittance for air filled waveguides, and the flexibility in the choice of the mutual admittance evaluation technique. Unlike the conventional mode method, the proposed technique does not run into difficulties while evaluating the fields from longitudinal magnetic current for points in the same transverse plane. The θ-algorithm has been used for the convergence acceleration of the series of mutual admittances in the internal admittance evaluation and has been shown to yield better results than other convergence acceleration algorithms investigated. The results obtained from this method have been shown to agree within 0.5% average with those from other theoretical techniques and within 1% average with measurements. The proposed method is useful in the design of compound slot arrays and can be extended to other configurations where the image representation is possible and for various slot aperture distributions.
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