Rectangular Coaxial Line Low-Pass Filters

A new type of coaxial line low-pass filter in which the high impedance lines have rectangular enclosures is analyzed, built and tested. The modes (TEM, TE and TM) in the coaxial line with rectangular enclosure are obtained using full wave mode matching method. The generalized scattering matrices of the discontinuities between rectangular and circular coaxial lines with different cross-sections are obtained. A new equivalent circuit of the discontinuity is presented for the first time. The complete frequency response of the rectangular coaxial line low-pass filter is computed. The theoretical results show excellent agreement with the measurement of an experimental filter.