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1.INTRODUCTION
Vehicular land transportation has become an indispensable and
necessary element of contemporary society activities. However, the vehicle
transportation environment has been facing severe situations with increasing
number of accidents and congestions in many metropolitan areas and
highways worldwide. This necessitates the adaptation of intelligent
transportation system (ITS) that could improve vehicle driving environment
with respect to safety, efficiency, and information availability. This demand
leads to wireless access for vehicular environments (WAVE) [1], which is also
regulated by the IEEE 802.11p standard. WAVE systems operate in 5.850–
5.925 GHz frequency band assigned by the Federal Communications
Commission (FCC). The advanced orthogonal frequency-division
multiplexing (OFDM) modulation is adopted to achieve high-speed data rates
up to 6–27 Mbs/s.
In an 802.11p system, access points (APs) are installed along
the road and, hence, are either in front or behind a moving vehicle. An
antenna that is specifically tailored for this kind of application should provide
two switchable beams, one toward the front and one toward the back of the
vehicle. A car can use its forward beam to connect an AP in front of it and
switches to the backward beam after it passes the AP to maintain the good
connection.
An end fire beam switchable antenna array used in vehicular environment
Fig. 1 shows what an ideal pattern should look like in 802.11p
systems. Because the roadside AP is far away most of the time, the radiation
pattern needs to have a narrow beamwidth in the elevation plane. In the
azimuth plane, the road system is not always perfectly straight, and vehicles
also need to change lanes from time to time, so the radiation pattern needs to
have certain beamwidth in this plane to guarantee a stable connection to the
roadside AP. In all antenna array configurations, the endfire array is the best
candidate to generate the ideal pattern shown in Fig. 1. Unlike broadside
array, an endfire array could be designed to have only one peak in the
azimuth plane, thus eliminate the requirement of a reflector. By selecting the
total number of array elements, the beamwidth in the azimuth plane can be
easily adjusted. Endfire arrays have been investigated for many years and
An end fire beam switchable antenna array used in vehicular environment
may include Yagi and log-periodic antennas [2]. Wideband antenna elements
have been used in the designs of endfire arrays. Diamond-shape dipoles have
been used to achieve 100% bandwidth with a stable endfire pattern [3]. Fanshaped
bow-tie antennas can also be used to achieve an endfire radiation [4].
In both [3] and [4], grounds are used as the reflector to improve the front-toback
ratio of the antenna. Therefore, they are only suitable for fixed beam
application. Drossos et al. [5] proposed a bidirectional endfire array, which
radiates at both endfire directions. Its gain is around 3 dB lower than a
unidirectional array. Kamarudin et al. [6] proposed a switch beam array that
can cover all azimuth angles; to achieve that capability, a complex feeding
network and multiple switches must be used. The endfire array proposed in
this letter is an improved version of endfire beam switchable array proposed
in [7], which is a linear array with four L-shaped elements separated by a
quarter of a wavelength. The feed network of the array has two isolated
inputs and four outputs. Each input can generate equal amplitude signals
with 90 incremental or descending phase distribution on fouroutput ports,
thus forming two orthogonal endfire radiation patterns.