Why Is It So Hard to Choose the Right Absorber Material? A Look Inside My PhD Research
Inside a car, aircraft, or piece of electronic equipment, you will often find metal enclosures protecting sensitive electronics from electromagnetic interference (EMI). These enclosures help keep unwanted electromagnetic energy out and prevent electronics from interfering with other systems. But shielding an enclosure is not simply a matter of making it from a highly conductive metal. As part of the PARASOL doctoral network, my PhD research looks at how absorber materials can be used to improve the shielding performance of enclosures, while avoiding unnecessary material, weight, and cost. This fits directly into the Safe and Sustainable by Design (SSbD) approach: achieving the required performance while using resources as efficiently as possible.
Shielding is not just about blocking
It is easy to think of a metal enclosure as a kind of mirror for electromagnetic waves: the waves hit the metal and are reflected away. Metals such as aluminium and steel are indeed very effective at blocking electromagnetic energy. Real enclosures, however, are more complicated. They have seams, doors, ventilation openings, cables, and other features that can allow electromagnetic energy to enter or escape. More importantly, once electromagnetic energy gets inside, it can bounce repeatedly between the metal walls. A useful way to picture this is to think about sound in a room. In an empty room with hard walls, sound can echo and build up. Similarly, electromagnetic waves can bounce around inside a metal enclosure and create strong electromagnetic fields at certain frequencies, known as resonances. This is where absorber materials can help. Instead of simply reflecting electromagnetic energy back into the enclosure, an absorber converts part of that energy into heat, reducing the energy that continues to bounce around.
So, how do you know if an absorber is good?
This turns out to be less straightforward than it sounds. When looking at absorber materials, engineers encounter many different quantities: permittivity, permeability, reflectivity, reflection loss, transmission, attenuation, absorption, and absorption cross section (ACS). The challenge is that these quantities do not all describe the same thing. Some measurements tell us about the electromagnetic properties of the material itself. Others tell us how much energy the material reflects or absorbs under a particular measurement condition. And some, such as ACS, describe how an object interacts with an electromagnetic environment. In other words, there is no single number that tells the whole story. The right measurement depends on the question we are trying to answer.
This was the motivation behind our paper, “An Overview of Microwave Two-Port Measurement Methods for Absorber Materials,” recently published in IEEE Letters on Electromagnetic Compatibility Practice and Applications. In the paper, we review several commonly used approaches, including coaxial lines, rectangular waveguides, free-space measurements, and reverberation chambers, and explain what each method actually tells us. For example, coaxial and waveguide measurements can be used to determine intrinsic electromagnetic properties of a material. Free-space measurements can show how the material reflects electromagnetic energy at different angles. A reverberation chamber, on the other hand, can characterize how much energy an absorber takes from a complex field arriving from many directions. These methods are therefore not simply different ways of measuring the same thing. They answer different questions.
How much absorber do we actually need?
Once we understand how to characterize an absorber, another practical question remains: How much absorber is enough?
Adding more absorber may reduce electromagnetic fields inside an enclosure, but it also adds material, weight, volume, and cost. Ideally, we want to know how much is needed before building and testing multiple physical prototypes.
This is where my ongoing research on absorption cross section (ACS) comes in. ACS is a way of describing how much electromagnetic energy an object absorbs compared with the electromagnetic power incident on it. In a reverberation chamber, the electromagnetic field arrives from many directions and with different polarizations, making it particularly relevant for understanding what happens inside reflective environments such as electronic enclosures.
At EMC+SIPI 2026, I presented our experimental study on the linearity and sensitivity of ACS measurements in a reverberation chamber. We investigated how the measured ACS changes when the amount of absorber is increased. The key question is whether the effect of adding absorber behaves in a sufficiently predictable way. If it does, this could provide engineers with a useful way of estimating absorber requirements and reducing the need for repeated trial-and-error testing.
From measurement to better design
These two parts of my research are closely connected.
The first question is: How should we measure an absorber, and what does that measurement actually tell us?
The second is: How can we use that information to decide how much absorber is needed in a real enclosure?
Together, they are part of a broader goal: helping engineers make better decisions earlier in the design process.
This is also where the connection to Safe and Sustainable by Design becomes important. The goal is not simply to add as much absorber as possible and achieve the best shielding performance. More material is not necessarily a better solution. It can mean additional weight, volume, cost, and environmental impact. Instead, the aim is to achieve the required electromagnetic performance with the right material, in the right amount, for the right application. For me, that is an important part of what SSbD means in practice: not only asking whether a design works, but also asking whether we can achieve the same performance more efficiently and with less unnecessary material.
My PhD research is therefore not just about measuring absorber materials. It is about building a better link between material characterization, real enclosure performance, and more informed design decisions, helping move electromagnetic compatibility from trial and error towards a more systematic and sustainable approach.
