Page 25 - Antennas for Base Stations in Wireless Communications

xxiv Introduction

reradiation system in a shadow area are presented. Fifth, we will show
that the vertical radiation pattern due to a number of vertical array ele-
ments can be controlled by changing the phases of each array element.

Chapter 6
Chapter 6 introduces several wideband unidirectional antenna designs
based on microstrip antenna technology. All designs employ electrically
thick substrates with a low dielectric constant for achieving wide imped-
ance bandwidth performance. Moreover, these antennas using the twin
L-probe feed, meandering probe feed, or differential-plate feed not only
achieve wide impedance bandwidths, but also possess excellent electrical
characteristics such as low cross polarization, high gain, and symmetrical
E-plane radiation. After that, the chapter proceeds to illustrate a new type
of wideband unidirectional antenna element—a complementary antenna.
This novel wideband unidirectional antenna is composed of a planar elec-
tric dipole and a shorted patch antenna that is equivalent to a magnetic
dipole. A new Γ-shaped feeding strip, comprising an air microstrip line
and an L-shaped coupled strip, is selected for exciting the dipole and
the shorted patch. This configuration of antenna structure accomplishes
excellent electrical characteristics, such as wide impedance bandwidth,
low cross polarization, low backlobe radiation, nearly identical E- and
H-plane patterns, a stable radiation pattern, and steady antenna gain
across the entire operating frequency band. In addition, two alternative
feeding structures, T-strip and square-plate coupled lines, demonstrate
the flexibility of antenna feed design. All antennas presented in this chap-
ter find practical applications in many recent wireless communication
systems like 2G, 3G, WiFi, ZigBee, and so on.

Chapter 7
Chapter 7 provides a general description of the standards and deploy-
ment scenarios of WLAN (WiFi). Designs are considered from a system
perspective, including materials, fabrication process, time-to-market, as
well as deployment and installation. The application of MIMO technol-
ogy in WLAN systems in order to provide reliability and high-speed wire-
less links is also discussed. In MIMO systems, antenna performance will
greatly impact capacity through the cross-correlation of the signals in
transmission and reception. The mutual coupling between the antennas
will, therefore, play a critical role in antenna design, which includes ele-
ment selection and array configuration. The optimized antenna designs
with low mutual coupling will enhance the diversity performance of the
MIMO systems. Furthermore, the MIMO systems are also advantageous
for various types of diversity techniques, for instance, space, pattern,
and polarization diversity when applied simultaneously.