An accurate model is given for the widely used practical topology of a coaxial feed. The probe connecting the inner conductor of the coaxial topology to the planar structure may cross any number of layers. The core of the model is the comparison of the actual configuration with a parallel plate system. It is shown that most of the parallel plate modal fields can be transferred to the actual configuration without modification. This allows to build a highly accurate model. It is shown that in practical single and double layered cases, the model reduces to adding a capacitor in parallel.
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