UHF Bandpass Filter Using a Combination of Semi-Lumped and Lumped Components

This paper presents the design of compact planar bandpass filters for sub-gigahertz applications using a combination of microstrip semi-lumped components and the commercial lumped elements resulting in a low cost implementation. Design procedure starts by synthesizing the lumped circuit of the filter for a specific filtering characteristic and center frequency. Then, the circuit elements are mapped to the geometrical dimensions of the filter. The proposed design concept is verified by fabrication and measurement of a filter prototype with 140% fractional bandwidth at a center frequency of 0.38 GHz. The filter has a compact size of $0.0021 \lambda_{g}^{2}$ with $\lambda_{g}$ being the guided wavelength and shows less than 1. 1 dB passband insertion loss.