Accurate formulas for computer-aided design of shielded microstrip circuits

Design equations are derived for the calculation of capacitance, characteristic impedances, and effective dielectric constants of single and coupled shielded microstrip. The equations are in closed form and hence they are suitable for direct use in computer-aided procedures for the design of microwave circuits containing these elements. Results obtained using these equations agree well with those obtained by numerical techniques. The present method could either be used to compensate for the effect of the shield on the performance of the circuit, or to design circuits whose characteristics could be controlled by varying the shield height. A computer-aided circuit optimisation procedure has been developed and examples are given of circuits designed using these equations. Practical measurements show good agreement with theoretical calculations and the effect of varying the shield heights on the performance of the circuit is demonstrated.