Antenna Design Choices
The type of antenna employed in a wireless sensor network
node depends greatly on its application. Usually an internal antenna is
preferred, due to its inherent portability and protection from mechanical and
environmental damage. Several types of internal antennas are available.
The first option is often the use of a circuit board trace as an
antenna. This option has the advantage of low cost because no components must be
purchased, handled, or placed on the board; however, because circuit board area
is not free, its cost is not zero. It is also very thin, an advantage for
low-profile network node designs. The most significant disadvantage to this type
of antenna is its poor performance. Due to the thinness of the circuit board
trace, its series resistance can be relatively high despite the low resistivity
of copper; the low quality of the circuit board material often adds dielectric
loss. Being in the circuit board itself also enables the circuit board trace
antenna to more readily couple to lossy components and other circuit board
traces, and to sources of noise. Finally, the tuning of circuit board antenna
can be subject to significant variation caused by etching variations during the
circuit board manufacturing process. This variation is often difficult to
control without significant expense because the required etching accuracy is
typically greater than that needed for simple connectivity of the circuit
board.
A second alternative is the use of simple wire or metal strip
antennas placed as components on the circuit board. These antennas can have
remarkably improved performance over that of the circuit board trace antenna,
due to their significantly lower loss and the fact that they are above the
circuit board. (Being above the circuit board, however, they are susceptible to
better coupling to some noise sources, such as the inductors in switching power
supplies.) Wire antennas can be either dipoles or loops; nodes designed for
on-body use, such as health monitoring devices, often employ loops as so-called
H-field antennas (i.e., magnetic field probes) in a plane normal to the body. Wire antennas often
require dielectric (e.g., plastic) supports to maintain their mechanical shape
and, therefore, their frequency of resonance, to the required tolerance. They
are often difficult to robotically place in an automated assembly line, and must
be inserted and soldered by hand. Nevertheless, wire antennas are often a good
compromise between the low cost and efficiency of the circuit board antenna and
the relatively high cost and high efficiency of the external antenna.
A third choice is the use of specialized ceramic antenna
components. These components are simple to place by automatic equipment, are
physically smaller than wire antennas, and do not require tuning. They are
usually more expensive than wire antennas, however, and are often available only
for the most widely used frequency bands (e.g., the 915- and 2450-MHz
Industrial, Scientific, and Medical [ISM] bands).
Because they usually are free of the size limitation placed on
internal antennas and are removed from the noise sources present in the network
node itself, external antennas can have very high performance. In applications
where the utmost in range is desired and a directive antenna must be used, an
external antenna is almost certainly required. Designers of nodes for use in
white goods (refrigerators and other large household appliances) may prefer
external antennas to avoid the shielding of their metallic housings. In
applications in which several different antennas must be used, for example, to
meet differing range requirements of multiple applications, external antennas
are the obvious choice. Although external antennas offer the highest performance
and design flexibility, they usually are the most expensive; not only must the
antenna be purchased, but usually one or more high-quality RF connectors, as
well. Losses associated with any feedline (usually miniature coaxial cable)
between the network node and the antenna itself must be included in the network
node design. A hidden cost not always considered is associated with the
frequency selectivity of the antenna. Because it is under the control of the
node designer, the selectivity of an internal antenna may be used as part of the
transceiver system design (e.g., Zolfaghari and Razavi[9]). Because users may replace
external antennas with those of unknown selectivity, when using an external
antenna, the node designer may often be required to include additional RF
selectivity in the node design, at additional product cost.
External antennas need not always be expensive, however; for
example, Ross et al.[10]
describes a covert wireless sensor network node (used in security applications)
that uses the doorknob housing the network node as its antenna.
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