Multisection bandpass filters using capacitively loaded transmission lines

In the frequency range from 50 to 500 MHz, lumped element inductors may have internal resonances, and the distributed nature of the physical structure is solved using transmission-line theory. Stub filters (shorted or opened stubs separated by sections of line) are quite reliable and easy to manufacture, but their physical size is a disadvantage at these frequencies. In this article, the size reduction process is illustrated by a series of design examples, starting with a bandpass filter at 200 MHz that uses half-wavelength tapped-stubs and has multiple resonant sections separated by quarter-wavelength sections of transmission line. By using this technique the filter area is reduced from 243.67 square inches to 5.99 square inches, an impressive factor of about 40×! Theoretical results are verified by measurements. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 1107–1112, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24252