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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