[opendtv] Anetnna design breakthrough?
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 18 Jun 2004 15:14:02 -0400
This could be really interesting. Here's my take. Traditional
antennas have current and voltage peaks and nulls along the length
of each element. If an antenna is designed for constant current
throughout each element, then its efficiency can be greatly
increased. So the element -- he used a monopole design -- is
provided with distributed L and C tuned to the carrier frequency.
The results are amazing. Could be a big improvement to indoor
antenna design, for example.
The article addresses transmitter antennas, but the same should
work for receiving antennas. More effeciency for the same size,
or smaller sizes than traditional indoor antennas for the same
signal strength. The 21 MHz example went from a 12 to 24 foot
monopole to a 1.5 foot monopole.
These are vertical monopoles. I'd like to see how this
translates to horizontal dipoles, and the effect it might
have on antenna height. At very least, you should see VHF
antennas that are about the size of UHF antennas.
This seems to be an elaboration of the traditional load coil
used, for example, to match an FM monopole to CB frequencies.
Bert
------------------------------------
Antenna design boosts efficiency per given size
By R. Colin Johnson , EE Times
June 09, 2004 (2:28 PM EDT)
URL: http://www.eet.com/article/showArticle.jhtml?articleId=3D21600147
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.=20
"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 cell phones, for example, Vincent said he has a completely
planar design that is less than a third the size of today's cell
phone 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.
"When looking at these antennas, you pretty much have to forget
everything you ever knew about antennas and keep an open mind,
because some of the things I have done are very radical," said
Vincent. "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."
NIMBY factor
Vincent, an amateur radio operator, embarked on his project after
he moved to a new neighborhood and his neighbors objected to the
140-foot tall antenna he planned to erect for a quarter-wave 1.8-MHz
transmitter. So he surveyed the literature, took the best of the
best designs and combined them into a 21-MHz test antenna that was
18 inches high, as opposed to the 12- to 24-foot height of the
antennas normally used for that band. Building on that work, he
eventually devised a 46-foot-tall 1.8-MHz antenna his neighbors
could accept.
"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. "But 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."
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.
"For instance, in a normal quarter-wave antenna the current
continually drops off in a sinusoidal shape, but these antennas
don't do that," said Vincent. "The current at the top of the
antenna is 80 percent of the current at the base."
The reason more current can be pumped into a DLM design than in a
conventional equivalent at the same size, Vincent theorized, is that
the DLM distributes energy more evenly along the antenna's length.
Using a DLM antenna one-third to one-ninth the size of standard
quarter-wave antenna, he measured nearly 80 percent efficiency,
when conventional wisdom would dictate that an antenna the size of
a DLM should be only 8 to 15 percent efficient.
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. "That is,
the magnitude of the current in these antennas is two to 2.5 times
larger than for a normal quarter-wave antenna.
"However, if you measure the current profiles for both antennas and
integrate the area under the curves, you come out with the same
volume, indicating that the much smaller antenna is filling the
airwaves with the same amount of radio energy."
Vincent plans to publish the results in a scientific journal soon,
but with a patent decision imminent, he couldn't hold off a
preliminary announcement that his theories regarding DLM antennas
were being supported by the experimental results. According to the
researcher, the DLM antenna profiles look just like the
theoretically ideal antenna profile-operating on a single frequency
with very high efficiency, while not producing any interfering
frequencies or wasting thermal energy.
"The phase and amplitude of this antenna are a perfect mimic of the
universal resonance curve," said Vincent. "This makes the antenna
completely predictable well beyond its bandwidth. Another unique
feature is that these antennas have no harmonic response whatsoever;
as a matter of fact, to a certain extent I used filter synthesis to
design the antennas."
Nondescript
To the naked eye, the DLM antenna looks unremarkable, said Vincent,
who jokes that you could put a flag on his antennas and they would
look like flagpoles. But under the skin are four main sections to
the antenna (from bottom to top): an inductive helix, a capacitive
midsection, an inductive load coil and a capacitive top section.
The different lengths of the mid- and top sections give them
different resonant frequencies, which, together with the exact
values of inductance and capacitance, define the antennas design
specifications for any desired frequency.
"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. "There are two poles in the antenna,
and where I place the poles in relation to one another-how much I
bring the two resonant frequencies together or spread them apart-
enables me to emulate different antennas, from a quarter-wave to
a five-eighths wave."
Vincent said no existing modeling software could adequately model
his antenna design. So he rolled his own simulation with Mathcad,
making use of some of Mathcad's filter design algorithms for the
inductive/capacitive-canceling effect.
"Eight years ago, antenna design was 90 percent black magic and 10
percent theory," said Vincent. "But now, with my design, they are
10 percent black magic and 90 percent theory."
The antennas are also well-behaved, with wide bandwidth and easy
to connect to standard equipment, according to Vincent. For
instance, they can directly connect to standard 50-ohm antenna
inputs without any adapters.
"All I have to do is tap the helix at its base, and you get a
perfect 50-ohm match with out any lossy networks [as are required
for other advanced antenna designs]," said Vincent.
For the future, Vincent is moving up into the GHz bands for use
with cell phones 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.
Vincent received the 2004 Outstanding Intellectual Property Award
from the University of Rhode Island's Research Office, joint
applicant for the patent.
Copyright =A9 2003 CMP Media
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