Ij

Overcoming UWB Antenna Design Challenges

With FDTD Software

By Remcom, Inc.

Aether Wire & Location, Inc. has pioneered the development of ultra-wideband (UWB)

technology capable of identifying the position of many devices to centimeter accuracy

over kilometer distances. This technology is applicable to many military and commercial

applications ranging from tracking the position of soldiers on the battlefield to inventory

control in a factory. As a pioneer in UWB technology, Aether Wire faced the challenge

of developing antennas that preserve the original waveform over a wide frequency range

while avoiding coverage gaps. The company's engineers discovered that finite difference

time domain (FDTD) electromagnetic simulation software provides the ability to simulate

antenna performance to a high degree of accuracy while providing reasonable

computational times. "FDTD simulation helps us evaluate a wide range of antenna

designs, making it possible to optimize antenna performance which in turn was crucial to

the success of our UWB technology," said Bob Fleming, Chairman and Co-Founder of

Aether Wire, Sunnyvale, California. "Over time we have gained a solid understanding of

the factors that affect UWB antenna performance while saving a huge amount of time and

money that would have otherwise had to be spent building and testing prototypes."

Aether Wire use the XFDTD software package to carry out these calculations.

Over a decade ago, well before the name UWB had even been coined, Aether Wire set

out to develop small, low-power transceivers that can be used for position location and

low data-rate communications. Global positioning system (GPS) technology was in its

infancy then but it was already clear that GPS was not the solution to this problem

because it isn't accurate enough and because it cannot operate within buildings, urban

areas, forests, etc. For most applications, what is desired is location relative to other

people or objects. Aether's technology uses small, low-power transceivers that determine

position by cooperatively exchanging an electromagnetic signal. The accuracy of this

range determination is proportional to the relative bandwidth, the ratio of the signal

frequency to the carrier frequency. With conventional sinewave technology, relative

bandwidth is very small, at most a few percent using spread spectrum. The zero-carrier,

spread spectrum technology pioneered by Aether Wire, on the other hand, provides a

relative bandwidth approaching 100%.

As seen in the April 19, 2007 edition of the RF Globalnet (www.rfglobalnet.com) newsletter.

Feature Article

Advantages Of Spread-Spectrum Technology

Spread-spectrum technology makes it possible to achieve centimeter-level accuracy

without using expensive microwave technology, because GHz bandwidth is obtained

without a carrier. Aether Wire developed very small, low power, low weight, and low

cost transceivers by integrating the electronics in CMOS without any inductive

components. MEMS can be used to integrate the resonator for the timebase on chip as

well. The antennas can be equally small, and can be driven directly by CMOS, because

they are non-resonant, current-mode, and low voltage. UWB signals form a shadow

spectrum which do not interfere with the sinewave spectrum. The transmitted power is

spread over such a large bandwidth that the amount of power in any narrow frequency

band is very small. The good features of spread spectrum are shared, including multipath

immunity, tolerance of interference from other radio sources, and inherent privacy from

eavesdropping. UWB signals have very good penetrating capabilities so transceivers can

operate within buildings, urban areas, and forests.

Aether Wire engineers designed integrated circuits and found they could easily determine

position by measuring time of flight between two localizers. They used impulse doublets

consisting of positive Gaussian impulses that directly generate a very wide instantaneous

bandwidth signal according to the time-scaling properties of the Fourier transform

relationship between time and frequency. It soon became clear that the engineers faced

significant antenna design challenges. They used a current mode antenna design that

builds up resonance in a dipole until current rises to a level of approximately 8 amps. One

of the major problems seen in early designs was a tendency of the electromagnetic field

produced by the antenna to oscillate, crossing the zero axis after the point at which the

impulse should end.

Move From Build And Test To Simulation

"In the early phases of our work, we built and tested a considerable number of antennas

and saw this problem reappear over and over again," Fleming said. "It was taking us

about a week to build and test each antenna design. It became clear that we might need to

go through hundreds of designs to find one that worked the way we wanted. It was far too

slow and expensive to reach our goals using this method. We tried several open source

packages that had been developed at universities as well as a commercial package that at

that time was the standard tool for antenna design. None of them worked they way we

wanted them to. The best that we tried was an open source version of the FDTD method

but it wasn't stable enough for industrial use. Then we heard about a commercial

implementation of the FDTD method, XFDTD from Remcom Inc. We tried it and

discovered that it provided both the accuracy and the robustness that we needed. We have

used it every since as our primary antenna design tool."

A key advantage of XFDTD is that it not only predicts antenna performance but also

provides a wide range of diagnostic information not available from physical testing.

As seen in the April 19, 2007 edition of the RF Globalnet (www.rfglobalnet.com) newsletter.

Feature Article

Aether Wire engineers have used the software to formulate guidelines, some of which

that have since become basic to UWB antenna design. For example, the software helped

engineers determine that current mode antennas generate an electric field in the near zone

that is the opposite polarity of the current generated in the far zone. XFDTD also helped

understand how the transition in the polarity works. Aether Wire engineers produced

approximately 100 simulations one time step apart to demonstrate how the field is

generated and converted them to an animation to help customers understand the

phenomena.

Figures 1 , 2, and 3: Near field electric field magnitude snap shots in time.

Figure 1

Figure 2

As seen in the April 19, 2007 edition of the RF Globalnet (www.rfglobalnet.com) newsletter.

Feature Article

Figure 3

Iterating To An Optimized Design

Aether Wire engineers can create and evaluate approximately six new designs per day,

making it possible to iterate much more quickly to an optimized design. Furthermore,

simulation helps engineers gain an understanding of the sensitivity of various

performance characteristics to design parameters much faster than in the past. For

example, this method made it possible to determine relatively quickly that the key to

solving the undershooting problem described earlier is to use very low impedances.

"With a normal sine wave antenna you can use a driver to match the impedance to the

antenna," Fleming said. "On the other hand with UWB, every time you design a new

antenna you have to design a new driver. Problems with drivers are very difficult to

detect. XFDTD helps us go through enough parameter changes to quickly get a sense of

what affects driver performance. That's not to say that it provides exact results. But, it

consistently is very close. For example, our simulations may tell us that the antenna

optimizes at 0.1 ohm. That puts us in the ballpark and perhaps we will build a few

iterations and discover that optimized value is actually is 0.12 ohm."

Managing Tradeoffs

The size of the antenna is often critical because when the antenna is less than a ¼

wavelength long the delay needs to be increased which reduces efficiency. On the other

hand, a larger antenna tends to distort the impulse because the impulse is not able to

illuminate the entire lop. Aether Wire engineers find that simulation provides an essential

tool for managing this tradeoff in order to keep efficiency high while also maintaining the

As seen in the April 19, 2007 edition of the RF Globalnet (www.rfglobalnet.com) newsletter.

Feature Article

desired waveform. Using these methods, the company has been able to maintain its

position at the leading-edge of UWB technology despite the upsurge of interest in this

field and many competitors that have followed in its footsteps.

The combination of communication and accurate position location capability within very

inexpensive devices has opened up a host of applications. A sampling of military and

commercial applications includes:

Monitoring large numbers of sensors dispersed over an area for nuclear,

biological, or chemical threats.

Geospatial registration for warfighter visualization.

Synthesis of large aperture antennas for tight beam communication, using

scattered transceivers that know their precise relative location and

synchronization.

Survey and construction.

Keeping track of mines, armaments, equipment, vehicles, etc.

Keeping track of personal items, such as one's children, pets, car, purse, luggage,

etc.

Inventory control in stores, warehouses, shipyards, railroad yards, etc.

Finding fire fighters in a burning building, police officers in distress, or injured

skiers on a ski slope.

Arbitrating rules in a sports event, playback of motions for coaching, or viewing

the re-creation of an event.

Keyless locks and rooms that adjust the light, temperature, and music sound level

of an office or home.

Automatically adjusting motion picture camera focus and motion-tracking for

matching digital effects

Using XFDTD for electromagnetic simulation is helping Aether Wire quickly develop

products that meet the specialized needs of these applications.

As seen in the April 19, 2007 edition of the RF Globalnet (www.rfglobalnet.com) newsletter.