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Microwave
Absorber: Finally, Some Respect?
By Dick Herman
“Let’s face it,” says Dan
Hirschnitz, president of Northern California-based SRC Cables,
“microwave absorber is without question the ‘Rodney
Dangerfield’ product in our industry today. Historically,
it was the last thing into the box, after everything else
failed to work. Not many engineers I know are willing to
talk about their applications, despite how critical this
material has become for newer, higher frequency designs.”
Whether it gets any “respect”
or not, microwave absorber has since become de rigueur for
today’s high performance products. A relentless march
toward higher-frequency, greater bandwidth, smaller form-factor,
densely-packed devices and more proliferation of wireless
devices seems destined to propel these materials even further.
“We think the worldwide market today
for absorber materials is approaching $300 million dollars,”
says Chip Madden, president of ARC Technologies, Amesbury,
MA. “I want to emphasize that’s an estimate,”
he adds, “but we’ve seen tremendous growth,
especially during the last two or three years.”
According to those who were interviewed for this article,
commercial applications are growing much faster than the
largest segment of the market--military and space. What
seems strange is the fact that so little is written about
a product that’s been around since the World War II.
Perhaps that’s because today’s products bear
so little resemblance. What began as “stealth”
technology has quietly evolved into hundreds of supplier
offerings finding their way into everyday consumer products.
$300 million. That’s a lot of polyiron,
magram and elastomer. Just where does it all go? Just about
anywhere you can imagine--cell phones, digital cameras,
servers, optical switching, antennas, automobile radars,
medical imaging equipment and more.
Unloved or Just Misunderstood?
What exactly is microwave absorber, and how does it differ
from more conventional materials? Although there are many
different types, all absorbers work through a process of
converting unwanted electrical and magnetic emissions into
small amounts of heat--a strategy of attenuation vs. isolation.
That strategy contrasts with “shielding,” which
is the process of essentially reflecting unwanted emissions
back towards their source.
Engineers and designers are very familiar with shielding,
fingerstock, gaskets and board level products, but they
are not always well versed in the capabilities or the appropriate
applications of absorber. Some still think of absorber as
a quick fix, or a band-aid for a rushed design, but that
sentiment is much less prevalent than before. While there
is no substitute for a solid design and good engineering
practices, designers today are faced with some pretty difficult
challenges.
ARC’s Madden elaborates: “At 1.9 GHz the wavelength
is less than 16cm. Normally, that wavelength wouldn’t
radiate through small openings. However, in a crowded system
enclosure, harmonics create a resonant cavity and traveling
waves develop, resulting in system unreliability. When the
enclosure is say, a handset, you can see there isn’t
going to be much room for redesign. The right absorber placed
near the offending circuitry is the solution, so people
are beginning to look at absorber as an integral part of
their product.”
“Someone designing a military radar, cell tower antenna
or a waveguide looks hard at absorbers much earlier in their
design process and they really look at things more from
an entire systems view, since impedance matching is so critical
at the frequencies they typically work with,” says
Hirschnitz. “I think the commercial world is rapidly
catching on to this, especially in areas like optical switching,
3G and 4G handsets, wireless backhaul, high-end imaging
and the test equipment manufacturers.”
Properties of Microwave Absorber
There are hundreds of products, as new demands for absorber
have evolved along with the materials themselves. It’s
not unusual to see a supplier’s product catalog with
thirty or forty different absorber part numbers--each of
them optimized in some way for a specific product application.
Supplier provided figures of merit will vary, and material
parameters don’t always seem to follow a common reporting
format, though “dB of Attenuation” is a common
metric. Everyone admits it can get a bit confusing.
“Just saying you have 20dB of attenuation isn’t
precise,” points out Michael Katz, president, Millimeter
Wave Technology, Passaic, N.J. “Are we talking about
insertion loss, dB down at normal incidence or specular?
Depending upon the application, this can be very critical,
and it can determine the right choice among numerous absorber
characteristics. For instance, the choice of whether to
use absorber optimized for specular reflection—such
as a free space foam—or whether you need to address
a near-field, cavity resonance problem with an ultra-thin,
magnetically loaded, non-conductive elastomer.”
Making sense of supplier offerings begins with understanding
just three major categories of absorber, and the characteristics
of the applica tions they best suit. Keep in mind, suppliers
have optimized their designs, so your ultimate choice may
depend upon tradeoffs about the thickness, amount of space
you have, environmental characteristics, cost and attenuation
that you need. Table 1 is a rough guideline,
but it’s best to consult with the supplier’s
catalog for more detailed specifications.
The Crystal Ball
Looking ahead, it seems reasonable to assume that the pace
of adoption for these highly-engineered products will continue,
especially within hot commercial markets like 3G /4G, high
performance computing, telecommunications backhaul and of
course, military and space applications.
It’s hard to predict just how many applications for
microwave absorber exist today, let alone how many there
will be in the near future, other than to say that there
are more and more every day. Talking to the industry leaders,
one gets the sense that each company has a fairly unique
market niche-- whether it is military, space, automotive,
medical or communications. These companies have invested
the time and the intellect to get to know their customers’
design challenges well enough to anticipate some of their
needs, and to keep their products evolving to meet those
needs.
What else does this future promise? Even if users of microwave
absorber remain so secretive—and it is likely they
will—the race to develop newer, higher performance
and more cost effective absorbers is well under way. Though
even the suppliers are tight-lipped about what’s in
R&D, there are already indications of several major
trends.
“Even after forty years, we’re only at the
tip of the iceberg of what we can do with better materials,
more options to integrate the product into our customers’
designs, and our ability to improve the unit cost so our
products will find their way into higher-volume consumer
applications, as well as the high-dollar test and high-frequency
equipment,” says Dan Hirschnitz. “One of the
areas we’re working on is how our industry comes to
grip with making the testing of these materials more universal,
and helping our customers’ engineers integrate their
thinking about ‘absorber’ into the rest of the
impedance issues they have, the cables, the connectors,
those sorts of things.”
“The pace of adoption follows trends we see in the
marketplace,” says Mike Katz. “Where once the
ICU of a hospital was considered off-limits to any kind
of emission whatsoever, we’re seeing that attitude
begin to abate as better diagnostic and medical imaging
applications demand more wireless connectivity. That in
turn drives the demand for our products.”
Is there some kind of “tipping point” developing
in microwave absorber? Is it likely that commercial applications
will one day overtake military and space--long the source
of most innovation and adoption of these materials?
“We’re hiring more R&D staff than any
other area at the current moment,” says Chip Madden.
“Like most suppliers, I suppose, we’ve seen
the future and it’s not just more of the same, but
also an opportunity for us to make our products perform
better, bring the unit costs down as adoption of microwave
absorber expands into higher volume production, and especially,
to find ways to integrate these materials directly into
our customers’ product components--to make them part
of the product, so they are easier to assemble and they
can get their products to market quicker.”
Abraham Lincoln once said, “The best way to predict
your future is to create it.”
Say goodbye to Rodney Dangerfield.
About the Author
Dick Herman is a Northern California business strategist
and writer. He writes about the adoption of new technologies,
especially for electronics, medical and high tech manufacturing.
All photos are courtesy of SRC Cables. For more information
please visit
www.rfmicrowaveabsorber.com
Dick Herman
www.rfmicrowaveabsorber.com
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