Page 44 - Antennas for Base Stations in Wireless Communications

Fundamentals of Antennas 17

Bandwidth can be increased using the following techniques:
■ Using thick and low permittivity substrates
■ Introducing closely spaced parasitic patches on the same layer of the
fed patch (15% BW)
■ Using a stacked parasitic patch (multilayer, BW reaches 20%)
■ Introducing a U-shaped slot in the patch (to achieve 30% BW) 13
■ Aperture coupling (10% BW, high backlobe radiation) 10–11
■ Aperture-coupled stacked patches (40–50% BW achievable) 14
■ L-probe coupling 15
The size of the patch antenna can be reduced by using the following
techniques:
■ Using materials with high dielectric constants
■ Using shorting walls
■ Using shorting pins 16

To obtain a small size wide-bandwidth antenna, these techniques can
be combined.

1.2.2 Suspended Plate Antennas
A suspended plate antenna (SPAs) is defined as a thin metallic conductor
bonded to a thin grounded dielectric substrate, as shown in Figure 1.8.
to 0.12 l
Suspended plate antennas have thicknesses ranging from 0.03 l 1 1
(l is the wavelength corresponding to the minimum frequency of the
l
well-matched impedance bandwidth) and a low relative dielectric con-
stant of about 1. SPAs have a broad impedance bandwidth and unique
radiation performance. 17
The use of thick dielectric substrates is a simple and effective method to
enhance the impedance bandwidth of a microstrip patch antenna by reduc-
ing its unloaded Q-factor. As the impedance bandwidth increases, however,
surface wave losses also increase, which reduces radiation efficiency. To sup-
press the surface waves a low permittivity of the substrate is required.

Advantages:
■ Easy to fabricate
■ Not expensive
■ Large bandwidth
■ No surface waves