Xu et al. / J Zhejiang Univ-Sci C (Comput & Electron) 2011 12(9):766-773
766
Miniaturized fractal-shaped branch-line coupler for
dual-band applications based on composite
right/left handed transmission lines
*
He-xiu XU
†1
, Guang-ming WANG
1
, Pei-lin CHEN
2
, Tian-peng LI
1
(
1
Electromagnetic Field and Microwave Technique, Missile Institute, Air Force Engineering University, Sanyuan 713800, China)
(
2
School of Electronics and Information Engineering, Northwestern Polytechnical University, Xi’an 710129, China)
†
E-mail: hxxu20008@yahoo.cn
Received Sept. 30, 2010; Revision accepted Jan. 13, 2011; Crosschecked July 29, 2011
Abstract: In this paper, novel dual-band (DB) branch-line couplers (BLCs) employing a composite right/left handed transmis-
sion line (CRLH TL) and fractal geometry are presented for the first time. The CRLH TL, with specified characteristic impedance
and phase shift, consists of lumped elements for the left handed (LH) part and fractal-shaped microstrip lines (MLs) for the right
handed (RH) part, which can be designed separately. Two designed BLCs are involved in size reduction, one using a 3/2 fractal
curve of first iteration, the other constructed based on a hybrid shape of fractal and meandered lines. A miniaturized principle for
CRLH TL realization is derived and an exact design method for fractal implementation is developed. For verification, an example
coupler was fabricated and measured. Consistent numerical and experimental results confirmed the design concept, showing that
the BLCs obtain DB behavior centered at 0.9 GHz and 1.8 GHz respectively with good in-band performance, except for slightly
larger coupled insertion loss for the hybrid-shaped BLC case. In addition, the proposed fractal- and hybrid-shaped BLCs obtained
a 49.7% and 64.1% size reduction respectively relative to their conventional counterparts working in the lower band. The most
important contributions of this article are the demonstration of compatibility between the fractal and CRLH TL techniques and the
provision of an alternative approach and a new concept for designing devices.
Key words: Fractal, Composite right/left handed transmission line (CRLH TL), Dual-band (DB), Miniaturization, Branch-line
coupler (BLC)
doi:10.1631/jzus.C1000343 Document code: A CLC number: O441
1 Introduction
Hybrids, such as rat-race, branch-line, and
coupled-line configurations, are a set of the most
important and widely used passive circuits for micro-
wave and millimeter-wave applications. When con-
centrating on a 90° hybrid, the branch-line coupler
(BLC) is preferred. Demand for increased portability,
reliability, and integrity of multi-standard communi-
cations has encouraged extensive research on minia-
turized and dual-band (DB) BLCs. There are nu-
merous publications on miniaturized BLCs for vari-
ous applications. Ghali and Moselhy (2004) using
fractal geometry achieved a significant 75.3% size
reduction and Chen and Wang (2008) a 64.6% size
reduction, based on a dual (Tang et al., 2008) and
discontinuous (Sun et al., 2005) transmission line,
slow-wave structure (Wang et al., 2007), and a
quasi-lumped elements approach (Liao and Peng,
2006). BLCs reported by Sun et al. (2005), Liao and
Peng (2006), Wang et al. (2007), and Tang et al.
(2008) obtained size reduction rates of 60%, 71%,
72%, and 63.9%, respectively. Although these BLCs
Journal of Zhejiang University-SCIENCE C (Computers & Electronics)
ISSN 1869-1951 (Print); ISSN 1869-196X (Online)
www.zju.edu.cn/jzus; www.springerlink.com
E-mail: jzus@zju.edu.cn
*
Project supported by the National Natural Science Foundation of
China (Nos. 60871027 and 60971118) and the National Basic Re-
search Program (973) of China (No. 2009CB613306)
© Zhejiang University and Springer-Verlag Berlin Heidelberg 2011