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In this letter, a modified optimum bandstop filter (BSF) design is presented. Broad stopband and sharp rejection characteristics are achieved by realizing three transmission zeros near the passband edges. Stopband rejection depth and bandwidth can be controlled by the impedances of the configuration. A lossless transmission line model analysis is used to derive design equations. Further, a compact geometry is chosen by replacing the low impedance section with its equivalent dual-high impedance lines, which facilitates convenient folding. To validate the theoretical prediction, a single unit sharp-rejecting prototype BSF having a 20 dB rejection bandwidth of 126% at 1.5 GHz has been fabricated.Index Terms-Bandstop filter (BSF), sharp rejection, transmission zeros, ultra-wideband (UWB).

In this article, the design of a broadband branch-line coupler (BLC) with reduced size and suppressed harmonic passband response is presented. The proposed approach can be used to replace the low impedance k/4 lines of the conventional BLC by an equivalent structure almost k/12 in length. The main advantage of the proposed BLC is that, it has approximately the same bandwidth as that of a conventional BLC. A prototype broadband coupler having fractional bandwidth >50% at 1.1 GHz and of size less than one third of a conventional three-section wideband BLC topology is realized. In addition, at least 20 dB suppression of up to fourth harmonics is achieved.

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In this letter, design of compact size rat‐race coupler (RRC), based on impedance‐controlled simple transmission line structure is proposed for predetermined harmonic suppression. The present approach has the advantage that within the fabrication limit of high‐impedance lines, a large number of harmonics are suppressed. Design guidelines are provided. Theoretical predictions are verified by implementing a prototype microstrip RRC, with upto twelfth harmonic of the operating frequency suppressed along with 75% size reduction compared with the conventional RRC. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 30–34, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24832

This article presents the design of harmonic suppressed compact microstrip balun, using shunt open-stubs. Two approaches using P-shaped and T-shaped open-stub configurations are investigated. Because of the stopband effect of the open-stubs, the harmonic passband responses are suppressed effectively. A compact-size balun is realized because both kinds of structures have slow wave characteristics. To validate theoretical predictions, two prototype baluns operating at f 0 ¼ 1 GHz, showing size reduction up to 75% and no spurious passband up to 4f 0 compared with the conventional design are fabricated in microstrip form.

A compact rat-race coupler (RRC) using asymmetric tapped-stub units for wide-range spurious passband suppression is presented. The main advantage of the present approach is that, under similar conditions, asymmetric T-shaped unit exhibits slow-wave and stopband effects, superior to those of traditional symmetric T-shaped unit; this result in extra size reduction and extended harmonic suppression within the fabrication limit of high impedance line. Design equations along with graphs are presented. A fabricated prototype RRC, at f 0 ¼ 0.5 GHz, has circuit area only 28% of that of the conventional one and no harmonic passband up to 10 f 0 . The method also features good 20-dB input matching bandwidth and sharp-harmonic rejection (>30 dB at second harmonic) performances.

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