Broadband Planar Aperture-Coupled Antenna Array
for WLAN and ITS Beam-Steering Applications
R. Khajeh Mohammad Lou
, M. Naser-Moghadasi
, and R. A. Sadeghzadeh
Faculty of Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran,
Faculty of Electrical
Engineering, K. N. Toosi University of Technology, Tehran, Iran
A broadband multilayer beam-steering antenna array with a modiﬁed mushroom-like
radiating element is introduced. The two-layer aperture-coupled array consists of four equal sets of
mushroom-like elements, each of which is made of 16 unit cells in a 4 × 4 conﬁguration. The feeding
network is a modiﬁed quasi-Butler matrix, which beneﬁts from improved double-box branch line couplers.
The network has an isolation bandwidth between 4 and 7.9 GHz (65%) and a transmission bandwidth from
4.8 to 7.1 GHz (39%), which operates acceptably in most of the C-band, especially at wireless local area
network and Intelligent Transport Systems (ITS). A broadband Butler matrix uses broadband 3 dB couplers
and a middle network. The geometry of the element and feeding network is utilized in array form to
enhance the impedance bandwidth and radiation efﬁciency of the antenna. The results show that the
antenna impedance bandwidth covers 3.82 to 8.2 GHz (port 1) and 3.8 to 8.28 GHz (port 2) for
≤ À10 dB, which covers the C-band fully (4–8 GHz). Moreover, it has a 3 dB gain-bandwidth of
2.25 GHz that extends from 4.65 to 6.9 GHz with a peak value of 13.65 dBi. The antenna array has a 3 dB
axial ratio bandwidth of 1.65 GHz that extends from 4.9 to 6.55 GHz and involves wireless local area
network (5.15–5.825 GHz) and ITS (5.795–6.400 GHz). The radiation efﬁciency of the array for both port
excitations is in excess of 75% when circular polarization is achieved. Two ﬁring beams at elevation angles
of 20° and À35° are obtained for ports 1 and 2 upon excitation, respectively.
Switched-beam antenna arrays have been tested for wireless high data-rate mobile and satellite
communication systems because of their low-cost of fabrication (Aribi et al., 2014; Liu et al., 2011). Beam
steering can be introduced to antenna arrays through feeding networks such as the Butler and Blass matrices
and space-feed networks (Mailloux, 2005). The Butler matrix is popular beam-forming network for switched-
beam antennas that is simple and features low power loss. Several studies have investigated the Butler matrix
(Cheng et al., 2013; Liu et al., 2013; Nie et al., 2015); however, the conventional Butler matrix is large in size and
has a narrow bandwidth that is limited by the traditional branch line coupler (BLC). Because the BLC is a major
component of the Butler matrix, operation of the feeding network depends upon its performance.
Conventional BLC has a narrow intrinsic amplitude and phase bandwidth, and the use of branches of a
quarter wavelength increases the size at operating frequency. Several studies have increased the bandwidth
and miniaturized the BLC (Chiu & Xue, 2010; Karamzadeh et al., 2015; Tsai et al., 2011). Investigations indicate
that a modiﬁed BLC with equivalent Π and T networks having stubs instead of quarter-wavelength branches
can resolve these issues (Cheng & Yeung, 2012; Tseng & Chang, 2012).
Multipath fading, interference and polarization mismatch are drawbacks to wireless communication;
however, the use of suitable antennas with adjustable beams can overcome these disadvantages.
Polarization can have a major effect on the performance of a communication system, but polarization
inconformity and the effect of multipass reﬂections are negligible if circular polarization is used
(Shokri et al., 2014). Furthermore, minimizing polarization mismatch eliminates the requirement for alignment
between transceivers (Weily & Guo, 2009). Mutual coupling has also been generated by electromagnetic
interaction among elements, affect the radiation pattern, and input impedance of radiators. This last
phenomenon causes nonzero voltage at the input of other radiating elements (Hoi-Shun et al., 2009).
Several techniques have been reported to reduce the surface wave in antenna arrays (Exposito-
Dominguez et al., 2012; Farahani et al., 2010; Zhang & Pedersen, 2016). Most recently, metamaterial
structures, also known as electromagnetic band gaps (EBGs), have shown the ability to control wave
KHAJEH MOHAMMAD LOU ET AL. 200
• A two-layer array antenna is proposed
using a mushroom-like element and a
Butler matrix with enhanced
• The magnitude and phase bandwidth
of the feeding network are enhanced
using the proposed branch line
• The proposed array antenna provides
good impedance-matching and
radiation characteristics at the WLAN
and ITS bands, which make it suitable
for wireless systems
Khajeh Mohammad Lou, R.,
Naser-Moghadasi, M., &
Sadeghzadeh, R. A. (2018). Broadband
planar aperture-coupled antenna array
for WLAN and ITS beam-steering
applications. Radio Science, 53, 200–209.
Received 2 SEP 2016
Accepted 23 JAN 2018
Accepted article online 27 JAN 2018
Published online 21 FEB 2018
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