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RF Absorbing Material

Foam absorbers in microwave and RF attenuation applications: here’s what you need to know

Are you in the process of defining specifications and determining what absorbing materials to incorporate in your antenna test range or EMC testing facility? No doubt you’re reading up about the various components and configurations with which you’re planning to optimize the effectiveness of your facility. One such component is the broadband absorber for which a foam absorber is the most common choice. This article provides more information on the advantages and disadvantages of RF absorbing foam.

Foam absorbers in microwave and RF attenuation applications: here’s what you need to know

Foam absorber

In the interest of transparency: DMAS does not currently offer foam absorbers. However, we felt it would be useful for potential clients to have a detailed comparison between foam and polystyrene (EPS) absorbers. After all, traditional foam absorbers are still the most common choice for EMC and microwave testing facilities even though they might not be the best ‘tool’ for the job in some cases (though they can definitely be in others). What this article aims to do is inform clients of the advantages and disadvantages of different types of RF absorbers in certain applications. Of course, considering the source, you might expect a bias in favor of EPS absorbers, but that is not the case. Like polystyrene, foam absorbers have their own advantages and we will certainly point those out where we find them. With that out of the way, we hope you find the following information useful. If you have any questions about the subjects covered in this article, get in touch with us. We’re happy to tell you more.

The basics

If you do any kind of research that involves the measuring of radiation patterns of antennas or electromagnetic interference, you either need an antenna test range or EMC test chamber to perform this research in. It goes without saying, but to properly assess whether a piece of equipment or a new device complies with EMC emissions standards, the first order of business should be to attenuate background or ambient radio waves. This makes it much easier to distinguish what’s coming from your product versus what’s coming from a myriad of other nearby (or even faraway) sources that might muddy your findings. That’s where the semi anechoic chamber (or SAC for short) comes in.

The SAC is a metal, RF shielded enclosure that is lined with radiation-absorbent material also known as RAM. For measurements below 1GHz the anechoic chamber is typically lined in the form of ferrite absorber tiles in combination with pyramid absorbers. The RF shielding structure itself acts as a faraday cage that blocks outside interference. Meanwhile, the ferrites and RF absorbers on the inside of the chamber perform an equally important task: they stop reflections of electromagnetic radiation from as many incident directions as possible. Without them, EM waves would be bouncing around in the EMC chamber causing an echo, making accurate measurements impossible.

Research goals such as immunity/emissions, EMC testing or antenna (pattern) measurements define the criteria for chamber design. This is where the type of the absorbers plays an important part. At DMAS you can opt for either a hybrid solution (ferrite base with pyramids on top) or the microwave absorber for antenna measurements.

Microwaves

The foam absorber

Foam absorber

Of course, these RF absorbers have to be made from a certain material and for quite a long time the material of choice has been polyurethane foam. There are a number of reasons for this: polyurethane is a fairly inexpensive material, it’s easy to handle (i.e. cut to shape) and its resistance to heat is generally very good. To increase its capacity for absorbing RF and microwaves, the open cell structure is loaded with carbon. Manufacturers each have their own methods to achieve this and the results are often great in terms of performance. That’s why, wherever you look, foam absorbers are the standard when it comes to the absorbing ‘agent’ used in over-the-air testing, wireless test chambers, compact ranges and radar cross sections.

So why change anything?

A wise man once said: ‘if it ain’t broke, don’t fix it.’ Which, generally, sounds like a good idea, but if everyone heeded these words, endeavoring individuals would never improve on anything either. So what are some of the downsides of foam absorbers? What are characteristics of the RF absorbing foam that can be improved? There’s a couple of things that spring to mind: rigidity, leakage, toxicity, modularity and durability.

  • Rigidity

While it is easy to create foam absorbers with very sharp tips, the tensile strength of polyurethane does have the tendency to degrade over time due to weight, susceptibility to humidity and age. This causes the tips to droop which negatively affects the attenuation capabilities and therefore the performance of the RF absorbing foam.

  • Leakage

Due to the open-cell structure of polyurethane foam, the carbon that is essential for the absorption process, can ‘leak’ from the absorber. This sounds inconsequential, but it actually leads to contamination of the anechoic room and adjoining areas of the facility as the small particles are easily carried everywhere, unlike RF absorbing foam.

  • Toxicity

Untreated, polyurethane is quite flammable and can ignite easily. This is obviously an undesirable quality. To counter this, polyurethane or foam absorbers are treated with fire retardant chemicals. Combined with their tendency to leak particles, that means foam absorbers are more toxic than some of the alternatives. In addition, foam absorbers are sometimes described as having a ‘peculiar odor.’

  • Modularity

To effectively attenuate electromagnetic waves in an anechoic chamber, you need to line the chamber with a significant number of absorbers. The shape of a foam absorbers is cut from a single piece of foam. With multiple pyramids ‘stuck’ to a single base, this means that if one tip of the RF absorbing foam is damaged, you need to change out an entire section of absorbers instead or expect performance degradation.

  • Durability

If you take all of the above into account, foam absorbers have definitely room for improvement in terms of their durability or sustainability. The performance, while excellent, degrades with age because the foam starts to sag. Over time, their effectiveness is also diminished by the leakage of carbon. And, if you accidentally damage one of the absorbers, you’re forced to replace a full section.

EPS absorber

A new challenger appears: EPS

EPS absorber

Which brings us to the polystyrene absorber that was mentioned in the introduction. A word of warning: DMAS does in fact offer absorbers that are fashioned out of this material (expanded polystyrene). In other words, this is where we recommend that you keep an eye out for potential bias or even excessive singing of praises. All kidding aside, the RF absorber made from EPS is a DMAS specialty and we have focused our efforts on perfecting this type of absorber specifically to address some of the shortcomings of the foam absorber. The result of these efforts is an innovative product with the following advantages:

  • Superior design and rigidity

Due to the characteristics of polystyrene, an EPS absorber is considered ‘lightweight’ when compared to a foam equivalent while also having greater rigidity. Furthermore, EPS is non-hygroscopic (resistant to changes in humidity) where foam acts as a sponge. This means that an EPS absorber tip does not droop and that its overall shape remains the same over time, resulting in very stable performance and a visually appealing chamber.

  • Closed cell structure (no leakage)

Both in polystyrene and foam absorbers, carbon plays a key role in attenuating RF and microwaves. A key difference is the material cell structure. As mentioned before, polyurethane (foam) has an open cell structure while polystyrene has closed cell characteristics. For DMAS absorbers, the carbon is coated onto each individual EPS bead resulting in a uniform carbon loading. What’s great about this is that the carbon particles do not leak, which leads to stable performance and repeatable lab measurements for the entirety of its long product life. Another benefit, DMAS absorbers are suitable (certified even) for use in clean room environments.

  • Modularity

Unlike foam absorbers that are cut from larger sections of polyurethane, polystyrene absorbers are crafted individually and then mounted to a baseplate grid. Inadvertently damage a single pyramid? Simply remove the pyramid from the grid and replace it with a spare. It’s cost effective, less wasteful, and therefore better for the environment.

  • Low toxicity

Even untreated, expanded polystyrene is less flammable than polyurethane foam, this means that there is a much lower need for (poisonous) fire retardant chemicals to achieve the level that is required for compliance with the relevant standards. Due to the absence of hazardous substances, DMAS absorbers are RoHS and REACH compliant.

  • Long product life (durability)

An added benefit of using a robust material like EPS is a product life that is way above average. In fact, our confidence in the product is such that the warranty that comes with a DMAS EPS absorber is for a full 25 years. The expected product life is even longer. There is no reason why a polystyrene absorber, not accounting for physical damage, shouldn’t last for 40 years (with no noticeable drop in performance).

Is there a catch?

The simple answer is no, though there used to be. This had to do with the tips of the absorber. It used to be much more difficult to get consistently sharp tips when using EPS which resulted in performance that was noticeably inferior to their foam counterparts. Thanks to recent product innovations and advancements during manufacturing, these issues have been ironed out and the performance of DMAS microwave absorbers is now almost on par with that of foam absorbers. Still, there are cases where foam absorbers are the preferred solution. Specifically in RCS test ranges with highly accurate testing requirements, in which each dB performance degradation is undesirable. 

In addition, for extreme high-power applications, there are better alternatives to both polystyrene and polyurethane absorbers. These are made of a honeycomb material, specifically designed for extensive testing in which particularly high field strengths are generated and therefore need to be able to dissipate heat effectively. However, the majority of testing does not have these requirements, making
EPS the better choice given the advantages in the areas of stability, sustainability and longevity.

EPS absorber

Professional advice

In conclusion, there’s something to be said for the foam absorber as well as polystyrene absorbers (no surprises there). You just have to be mindful of the application and the specific requirements of the research. If you’re still unsure what the best solution would be for the application you have in mind, we recommend that you get in touch with us for more information about this topic. As we’ve said before: we’re happy to share our expertise. Send us an email, download our brochures or give us a call to get started.

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