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Monday, March 04, 2013
Microstrip Antennas
INTRODUCTION
HISTORY
FEED METHODS
ADVANTAGES AND DISADVANTAGES
RADIATION EFICIENCY AND BANDWIDTH
B9702122
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Microstrip Antennas
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INTRODUCTION
Microstrip antennas (often called patch antennas) are widely
used in the microwave
frequency region because of their simplicity and compatibility
with printed-circuit
technology, making them easy to manufacture either as
stand-alone elements or as
elements of arrays. In its simplest form a microstrip antenna
consists of a patch of
metal, usually rectangular or circular (though other shapes are
sometimes used) on top
of a grounded substrate, as shown in Figure 1.
History
The origin of microstrip antennas apparently dates back to 1953,
when G.A.
Deschamps proposed the use of microstrip feed lines to feed an
array of printedantenna elements. The printed antenna elements
introduced there were not microstrip
patches, but flared planar horns. The microstrip patch antenna
was first introduced by
Robert E. Munson in a symposium paper in 1972, which was
followed by a journal
paper in 1974. These papers discussed both thewraparound
microstrip antenna and
the rectangular patch. Shortly after Munsons symposium paper, J.
Howell also
discussed rectangular patch antennas in another symposium paper
in which he credits
Munson with the basic idea by referencing a private
communication.
In a later journal paper, Howell introduced the circular patch
as well as the circularlypolarized patch antenna. Soon after the
introduction of the microstrip antenna, papers
appeared describing methods of analysis for these antennas,
including the
transmission-line model, the cavity model, and the
spectral-domain method.
FIGURE 1 (a) Rectangular microstrip patch antenna and (b)
circular microstrip patch antenna
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Microstrip Antennas
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Feed Methods
Various methods may be used to feed the microstrip antenna, as
shown in Figure 2 for
the rectangular patch. The coaxial probe feed shown in Figure 2a
is one of the most
common feeds for a stand-alone element. The inset feed in Figure
2b is common for
array applications. The proximity-coupled feed in Figure 2c
requires multilayer
fabrication, but reduces spurious radiation from the feed line.
The aperture-coupled
feed shown in Figure 2d has the advantage of eliminating
feed-line radiation (at the
expense of some back radiation from the aperture) and also
allows for relatively thick
substrates, since probe reactance is not an issue.
FIGURE 7-2 Feeding methods for a microstrip antenna: (a) coaxial
feed, (b) inset feed, (c)
proximity-coupled feed, and (d) aperture-coupled feed
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Microstrip Antennas
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Advantages and Disadvantages
Microstrip antennas usually have the important advantage of
being low profile, and if
the substrate is thin enough, they may also be conformable,
meaning that the substrate
can be bent to fit a curved surface, making the antenna very
unobtrusive. Because the
lateral size of a microstrip antenna on a substrate board is
typically on the order of a
half wavelength in the dielectric, size considerations usually
dictate that these
antennas are used in the UHF frequency band or higher, up
through millimeter-wave
frequencies, with microwave frequency applications being the
most common. The
main disadvantages of microstrip antennas include potentially
lower radiation
efficiency compared with other antennas (although this depends
significantly on the
substrate permittivity and thickness) and small bandwidth.
Radiation Efficiency and Bandwidth
Radiation efficiency depends largely on the substrate
permittivity and thickness. A
substrate that has a higher permittivity or that is thicker will
suffer from increased
surface-wave excitation, which will lower the efficiency. On the
other hand, if the
substrate is too thin, the efficiency will be low due to
conductive and dielectric losses.
The bandwidth increases with the substrate thickness and
inversely with the substrate
permittivity, so bandwidth is made larger by using thicker
low-permittivity substrates
at the expense of increased lateral size and vertical
thickness.
