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International Journal of Antennas and Propagation
Volume 2013, Article ID 960138, 5 pages
Research Article

Microstrip Tunable Bandpass Filter with the Colinear Resonators

1School of Automation and Electrical Engineering, University of Science and Technology Beijing, Beijing 100083, China
2Department of EECE, University of Birmingham, Birmingham B15 2TT, UK
3Faculty of Science and Technology, Anglia Ruskin University, East Road, Cambridge CB1 1PT, UK

Received 15 February 2013; Accepted 16 June 2013

Academic Editor: Mandeep Singh Jit Singh

Copyright © 2013 Z. P. Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


This paper presents the lumped element circuit and transmission line equivalent circuit for a varactor-tuned bandpass filter. The filter consists of transmission lines, fixed capacitors, and a varactor diode. The colinear resonant sections, in this filter, are not configured in parallel, as they are in a conventional combline filter. For this reason the overall area of the filter is reduced. The passband of the filter can be tuned from 0.69 GHz to 1.20 GHz by varying the capacitance of the varactor diode. The insertion loss of this filter changes from 1.2 dB to 2.1 dB across this bandwidth.