Design of low-pass microstrip filters with equal-ripple passband and finite attenuation poles

This paper describes the development of a practical approach for realizing low-pass elliptic filters using cascaded microstrip rectangular elements. The low-pass prototype is based on the numerical approximation method. With an appropriate transformed variable, the numerical accuracy of the synthesis procedure is enhanced and the values of the elements are attained. The realization in microstrip configuration consists in making the equivalence between the geometry of a planar rectangular structure and the basic cell of the lumped element prototype. The physical design and electrical performance of a microstrip filter of sixth order with cutoff frequency in the C-band is given. There is good agreement between the experimental and theoretical results.