Page 273 - Antennas for Base Stations in Wireless Communications

246 Chapter Seven

7.2.1 Materials, Fabrication Process, Time
to Market, Deployment, and Installation

7.2.1.1 Material
A. Dielectric Substrate Low-cost dielectric substrates, such as Flame
Retardant-4 (FR-4) with an e r ≈ 4.4 and a tand ≈ 0.02, are widely employed
for printed circuit board (PCB) antenna designs usually up to 6 GHz.
At higher frequencies, the dielectric loss incurred becomes greater. The
thickness of the FR-4 material ranges from 5–60 mils, and it normally
comes with copper cladding with a thickness ranging from 0.5–2.0 oz.
The dielectric substrates that can be used for high frequencies, such
as Rogers 4003 (e r ≈ 3.38, tand ≈ 0.002) and RT/Duroid 5880 (e r ≈ 2.2,
tand ≈ 0.0004), are generally not selected for mass production as they
are relatively more expensive. Instead, the air substrate is commonly
adopted for low-cost and broadband solutions.

B. Conductor Conductors with good conductivity, such as copper
7
(5.8 × 10 S/m) and brass, are commonly employed as radiators and
feeding networks in antenna design. Aluminum and galvanized steel
with appropriate surface treatments are normally used for the antenna
ground plane and casing. Aluminum varies from the 1xxx–7xxx series.
Its oxide and magnesium contents make aluminum difficult to process
by using conventional low-temperature (225–490ºC) soft-soldering.
Mounting an RF connector on an aluminum ground or casing is carried
out by riveting instead of soldering.

C. Radome A wide variety of antenna radome materials are avail-
able in the market. They include fiber reinforced plastics (FRPs), glass
reinforced plastics (GRPs), polypropylene (PP), acrylonitrile butadiene
styrene (ABS), and a variety of polycarbonates. The radome should be
electromagnetically transparent with respect to the operating frequency
of the antenna. In addition, it should be thin, lightweight, homogeneous,
and uniform in thickness. The radome used for outdoor applications
has to be weatherproof and resistant to vandalism. Polypropylene is
commonly used in injection molding, but is susceptible to marring or
chalking after prolonged UV irradiation.

D. Supporting Structures In order to lower the Q-value of the antenna,
and provide ample support, Styrofoam TM material (e ≈ 1.1), nylon
r
≈ 3.5) stubs, rods, and screws are commonly used as supporting mate-
(e r
rials. Metallic screws are also used as shorting pins or as support at the
noncritical portions of the antenna, e.g., the center of a patch antenna.