Page 43 - Antennas for Base Stations in Wireless Communications

16 Chapter One

Advantages:
■ Planer (and can be made conformal to shaped surface)
■ Low profile
■ Ease of integration with microstrip technology
■ Can be integrated with circuit elements
■ Ability to have polarization diversity (can easily be designed to have
vertical, horizontal, right-hand circular (RHCP), or left-hand circular
(LHCP) polarizations)
■ Lightweight and inexpensive

Disadvantages:
■ Narrow bandwidth (typically less than 5%), requiring bandwidth-
widening techniques
■ Can handle low RF power
■ Large ohmic loss

The most common microstrip antenna is a rectangular patch. The
rectangular patch antenna is approximately a one-half wavelength long
section of rectangular microstrip transmission line. When air is the
antenna substrate, the length of the rectangular microstrip antenna
is approximately one-half of a free-space wavelength. If the antenna is
loaded with a dielectric as its substrate, the length of the antenna
decreases as the relative dielectric constant of the substrate increases.
The resonant length of the antenna is slightly shorter because of the
extended electric fringing fields, which increase the antenna’s electrical
length slightly. The dielectric loading of a microstrip antenna affects
both its radiation pattern and impedance bandwidth. As the dielectric
constant of the substrate increases, the antenna bandwidth decreases.
This increases the antenna’s Q factor and, therefore, decreases the
impedance bandwidth.

Feeding Methods:

■ Coaxial probe feeding
■ Microstrip transmission line
■ Recessed microstrip line
■ Aperture coupling feed 10–11
■ Proximity-coupled microstrip line feed (no direct contact between the
12
feed and the patch )