Design of improved microwave low-pass filters using strip-line techniques

Low-pass filters find application as components in narrow band transmitting, receiving and test equipment to control spurious responses, harmonic output and interference. Strip-line low-pass filters are easily designed and give very satisfactory performance at a fraction of the cost of other types in the frequency range from 500 to 7000m.c. This report gives a design procedure for strip-line low-pass filters including two improvements over conventional designs. It is shown that by cutting away a small portion of the dielectric material surrounding the fine conductor in the high impedance or inductive sections of the filter, the stop band width can be effectively extended by as much as 100%. A formula for the characteristic impedance of the resulting two dielectric, high impedance transmission line structure is derived. A method not involving stubs for realizing m-derived sections is given. The resulting m-derived sections have the advantage of smaller size and do not become antiresonant at approximately twice the cut off frequency as do the quarter wavelength open circuited stubs commonly used. An approximate method for taking fringing capacitance into account is given. The problem of spurious responses caused by waveguide mode propagation is discussed and two solutions are offered.