Page 47 - Antennas for Base Stations in Wireless Communications

20 Chapter One

Bandwidth can be improved using these techniques:
■ Using thick air substrate
■ Using parasitic resonators with resonant lengths close to the resonant
frequency 21
■ Using stacked elements 22
■ Varying the size of the ground plane 23
The size of the PIFA can be reduced using these techniques:

■ Using an additional shorting pin 24
■ Loading a dielectric material with high permittivity 25
■ Capacitive loading of the antenna structure 26
■ Using slots on the patch to increase the antenna’s electrical length 27

1.2.4 Planer Dipoles/Monopoles
Dipoles and monopoles are the most widely used antennas. The mono-
pole is a straight wire vertically installed above a ground plane; it is
vertically polarized and has an omnidirectional radian in the horizontal
plane. To increase the impedance bandwidth of the monopole antenna,
planer elements can be used to replace the wire elements. 28
Planer designs with different radiator shapes have been widely used
in which the bandwidth reaches 70%. These shapes include 17

■ Circular (BW from 2.25–17.25 GHz)
■ Triangular
■ Elliptical (BW from 1.17–12 GHz)
■ Rectangular (BW of 53%)
■ Ring
■ Trapezoidal (80% BW)
■ Roll monopoles (more than 70% BW)
The square planar monopole with trident-shaped feeding strip (shown
29
in Figure 1.11) was introduced with a bandwidth of about 10 GHz
(about 1.4–11.4 GHz). This bandwidth is three times the bandwidth
obtained using a simple feeding strip.
A compact wideband cross-plate monopole antenna (shown in
30
Figure 1.12) has been proposed. This antenna has a cross-sectional
area only 25% that of a corresponding planar cross-plate monopole
antenna and can generate omnidirectional or near omnidirectional