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Borrowing
from Nature to Miniaturize Antennas:
Fractal Technology at Work
by David Brown and Marlys
Amundson
Antennas for the next generation of cell phones and other wireless
communications devices may bear a striking resemblance to the Santa
Monica mountain range, the California coastline, or even the tree
in your backyard.
A team of UCLA researchers led by electrical engineering chair Yahya
Rahmat-Samii is using fractals -- mathematical models of mountains,
trees, and coastlines -- to develop antennas that meet the challenging
requirements of the more sophisticated technology in new cell phones,
implanted devices, automobiles, and mobile communications devices.
These antennas must be miniature and they must be able to operate
at different frequency bands simultaneously.
“Manufacturers of wireless equipment, and particularly those in
the automotive industry, are interested in developing a single,
compact antenna that can perform all the functions necessary to
operate AM and FM radios, cellular communications and navigation
systems,” notes Rahmat-Samii. “Users demand light, compact and aesthetically
beautiful terminals to keep our world connected anywhere at anytime
and with anyone.”
Fractals, or fractional dimensions, are mathematical models originally
used to measure jagged contours such as coastlines. Like a mountain
range whose profile appears equally craggy when observed from both
far and near, fractals are used to define curves and surfaces, independent
of their scale. Any portion of the curve, when enlarged, appears
identical to the whole curve -- a property known as self-symmetry.
“The theory of fractal geometry describes how a simple formation
can be evolved into a complex formation by many subsequent iterations,
each of which creates a reduced replica of the original form,” says
Rahmat-Samii.
In general, fractal shapes are greatly detailed and complex as the
number of iterations grows. However, Rahmat-Samii and his research
team have shown that only several iterations of the fractal formation
are required to obtain the necessary properties for antenna applications,
enabling easier and practical construction of fractal antennas.
“Based on Maxwell’s equations and the radiating properties of antennas,
it is natural to invoke the self-symmetry and space filling features
of fractal geometry for antenna designs,” notes Rahmat-Samii.
Using fractal designs with these unique features, his group has
developed antennas that meet two critical challenges presented by
the new generation of wireless devices: they require less space
and can operate simultaneously at several different bands. Rahmat-Samii's
team has constructed novel, tightly packed fractal antenna arrays
for potential use in communication devices by overcoming the multi-path
effects.
"The unique packaging properties of compact fractal elements allow
for the beams of array antennas to be scanned at a wider angle without
severely suffering from the creation of unwanted gating lobes in
the antenna radiation pattern," notes Rahmat-Samii. "A fractal antenna
may not cover the entire band uniformly, but will perform well at
several designated frequencies."
Much of the early research on internal antennas was
conducted in Rahmat-Samii's UCLA antenna laboratory in the
early 1990s, and the university is one of the leading research institutions
exploring fractals for antenna design. Additionally, Rahmat-Samii's
team made pioneering contributions in characterizing the interaction
between handset antennas and humans. More recently, his research
group is addressing the challenges of developing and characterizing
implanted antenna devices for medical applications, an area in which
fractal antennas could play an important role.
UCLA is also among the first to use fractals to create frequency
selective surfaces (FSS), which, acting as filters allow only selected
frequencies to go through while deflecting others. Among the potential
applications for these filters are military situations where deflected
frequencies can be used to hide the presence of an object, in advanced
reflector antenna systems utilized as ground terminals, and on spacecraft
for space missions and satellite communications.
Rahmat-Samii and his colleagues have pioneered the concept of integrating
MEMS technology into FSS to enhance the performance of these devices.
With actuators embedded into the periodic elements of an FSS, it
becomes possible to tilt the elements to enhance its performance.
They are also considering potential uses of fractals in developing
artificial surfaces that respond to electromagnetic signals as a
way of facilitating additional antenna designs.
His group also has developed powerful electromagnetic computation
tools that enable them to precisely predict antenna performance
for different fractal arrangements, allowing them to employ specific
fractal patterns for different antennas.
Rahmat-Samii explains, "The intricacy of fractal geometry demands that advanced
computation tools are used at the outset of a design in order to
properly assess the advantageous or disadvantageous properties of
a particular design before building the antennas."
Their modeling tools incorporate electromagnetic theory, antenna
theory, and numerical and computation simulation to demonstrate
how the current moves on a fractal design, and to predict, in depth,
the performance of a given design. The
simulation tools also allow them to compare fractal topologies
to match a structure to a desired performance, and to catalogue
the
results for future use. These tools allow Rahmat-Samii's team
to "close the loop" on the design and development of fractal
antennas through comparison of the predicted performance simulations
to results from model antennas tested in the lab.
Rahmat-Samii recently added a third anechoic chamber to his lab,
one bigger than the other chambers, with larger cone-shaped absorbing
material that allows his team to measure antennas at lower frequencies.
"The chamber's spherical near field measurement equipment
enables us to accurately characterize small antennas with broad
radiation patterns," notes Rahmat-Samii. "This is critical
to assessing the performance of antennas used in personal communications
applications."
For additional information on Dr. Rahmat-Samii's antenna research,
please see http://www.ee.ucla.edu/faculty/bios/yrs.htm.
Photo: Todd
Cheney, UCLA Photography |
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