Wednesday, June 09, 2004

Small is beautiful

This kind of development is good news for those who want to put digital radio on the air....

PORTLAND, Ore. — A four-year skunk works effort at the University of Rhode Island in Kingston has cut the size of an antenna by as much as one-third for any frequency from the KHz to the GHz range.

Using conventional components the four-part antenna design cancels out normal inductive loading, thereby linearizing the energy radiation along its mast and enabling the smaller size.

"The DLM [distributed load monopole] antenna is based on a lot of things that currently exist," said the researcher who invented the smaller antenna, Robert Vincent of the university's physics department, "but I've been able to put a combination of them together to create a revolutionary way of building antennas. It uses basically a helix plus a load coil."

The patent-pending design could transform every antenna-from the GHz models for cell phones to the giant, KHz AM antennas that stud the high ground of metropolitan areas-Vincent said.

For cellphones, for example, Vincent said he has a completely planar design that is less than a third the size of today's cellphone antennas. And those 300-foot tall antennas for the 900-KHz AM band that dominate skylines would have to be only 80 feet high, with no compromise in performance, using Vincent's design, he said.

"With my technique, I reduce the inductive loading that is normally required to resonate the antenna by as much as 75 percent . . . by utilizing the distributed capacitance around the antenna," he explained.

"I looked at all the different approaches used to make antennas smaller, and there seemed to be good and bad aspects" to each, Vincent said. "A helix antenna is normally known to be a core radiator, because the current profile drops off rapidly; they are just an inductor, and inductance does not like to see changes in current, so it's going to buck that.

"What I found was that for any smaller antenna, if you place a load coil in the middle you can normalize and make the current through the helix unity; that is, you can maximize it and linearize it," he added.

Vincent has verified designs from 1.8 MHz to 200 MHz by measuring and characterizing the behavior of his DLM antenna compared with a normal quarter-wave antenna of the same frequency. He found that many of the disadvantages of traditional antennas were not problems for the much lighter inductive loading in a DLM.

To check his theory, Vincent analyzed and compared the current profiles, output power and a score of other standard tests for measuring antenna performance. All measurements were in reference to comparative measurements made on a quarter-wave vertical antenna for the same frequency, on the same ground system and same power input.

"I was able to increase the current profile of the antenna over a quarter-wave by as much as two to 2.5 times," said Vincent.

"The technology is completely scalable: Take the component values and divide them by two, and you get twice the frequency; take all the component values and multiply them by two, and you are at half the frequency," said Vincent.

Vincent said he is moving up into the GHz bands for use with cellphones and radio-frequency ID equipment. A problem in the past has been that as components are downsized, they become too small to utilize standard antenna materials. At 1 GHz, for example, the helix is only eight-thousandths of an inch in diameter and requires more than 100 turns of wire.

"So I came up with a new way of developing a helix for high frequencies that is a fully planar design; it's a two-dimensional helix," said Vincent.

With the new helix design, Vincent has built a prototype 7-GHz antenna that he claims is indistinguishable from a quarter-wave antenna in all but its size. "Because the new design is completely planar, we could crank these out using thin-film technologies," Vincent said.

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