From DC to Daylight — How Innovations in Microwave Absorbers Shield the Warfighter
The evolution of military electronics is marked by an endless series of measures and countermeasures. The enemy meets an enhancement in U.S. fighter jet technology with an improvement in its missile-guidance capabilities, forcing additional innovations from the U.S. The never-ending effort to enhance the effectiveness of military technology and protect the warfighter on the battlefield propels forward innovation in the industry.
Today, there's a battle underway that is dramatically expanding the frequency ranges used by the military for communications — soldier-to-soldier, soldier-to-satellite, aircraft-to-ground, missile-to-target, and more. Whereas military electronics used to operate in a narrow band of frequencies, today, military design engineers must protect equipment from damaging signal interference and enemy detection at a wider range of frequencies than ever before including radar at frequencies as low as 100 MHz and as high as 95 GHz.
This challenge is a complex one and addressing it requires a mix of materials science ingenuity, particle science innovation, and measurement and modeling sophistication. The solution? Changes in the makeup of a passive material called the microwave absorber.
The Crucial Role of Absorbers in Military Electronics
When effective, microwave absorbers:
Eliminate stray or unwanted radiation that could interfere with a system's operation.
Reduce the reflection of electromagnetic energy from or transmission to an object.
Minimize oscillations caused by cavity resonance.
Absorbers can take many different physical forms including flexible elastomers, foam, or rigid epoxy plastics. In the military, absorbers play a key role in enhancing:
Survivability - Absorbers can eliminate the “return energy” from an object, which keeps it hidden from radar detection.
Antenna patterns - Absorbers reduce side lobes and backscatter and improve the isolation of an antenna, thereby keeping communications clear.
Absorbers consist of a filler material inside a material matrix. The external coating protects the absorber from its environment. The use of resilient fluorosilicone, instead of traditional silicone, can protect an absorber from harsh substances, such as jet fuel and deicing fluid, improving its reliability and longevity.
An absorber's internal filler material has magnetic and dielectric properties that enable signal attenuation. To understand the effect of any material on electromagnetic energy, an engineer must first understand the material's conductivity, the electric permittivity, and the magnetic permeability.
Conductivity is a measure of the current that flows when a voltage is applied. For absorber material, the conductivity must be very low to enable absorption of electromagnetic energy. If a material has high conductivity, the electromagnetic energy will reflect at the surface and the material won't be able to absorb it.
The permittivity is a measure of a material's response to an electric field. A material's permittivity arises from polarization within the material. When subject to an electric field, charge carriers will align themselves in a direction that opposes the field. The greater this polarization field, the greater the permittivity.
Analogous to the electric permittivity is the magnetic permeability. The permeability is a measure of the material's response to a magnetic field. Materials with high permeability will concentrate magnetic field lines and increase magnetic energy density.
Knowing these parameters helps engineers develop absorbers that minimize reflection of electromagnetic energy and maximize its absorption.
Expansion in Frequencies Drives Innovation in Absorbers
Historically, military design engineers needed absorbers that were effective within a narrow range of 2-18 GHz. Magnetic absorbers for this frequency range are typically filled with standard magnetic powder. Today, however, this range has greatly expanded. As enemy radar capabilities improve, absorbers must prevent return energy at low frequencies. On the other end of the spectrum, as data transfer requirements grow, absorbers must enable clear and effective communications at high frequencies including 5G communications in the millimeter wave range.
Expanding the frequency range for absorbers involves using or designing filler materials with high absorption properties in the required frequency ranges. Maximizing the loss component of either the permittivity or permeability (or both) is very important. Also, increasing the permittivity and permeability will increase the attenuation per unit distance (or volume) due to the reduction of the wavelength inside the material.
Material absorption is significantly lower at lower frequencies due to the longer wavelengths. In these frequency ranges, engineers must increase permittivity and permeability to maximize wavelength compression and attenuation. With spherical particles, there are limits on permittivity and permeability; however, using shaped particles aligned in thin layers can greatly increase particle interaction and permittivity and permeability. So, in low frequency ranges, engineers must use metallurgical work to change a particle's size and shape (or morphology).
The picture changes as we move into the millimeter wave range, where wavelength compression is less important. Magnetically active fillers become less effective as we move into the higher frequency realm and dielectric materials alone can often effectively attenuate electromagnetic energy in these frequency ranges. But the challenge in these higher frequencies is to get the energy into the absorbing material. As a result, the shape of the absorber takes on greater importance than it does in low frequency ranges. The creative use of multilayers and pyramidal shapes, for example, can enhance the effectiveness of absorbers in these high frequency ranges. Further, nanostructure materials can boost performance while allowing engineers to control the weight and viscosity of the absorber, which provides them flexibility as they shape and design the absorber.
A Key Enabler of Innovation: Modeling and Measurement
None of this innovation in absorbers is possible without sophisticated measurement and modeling capabilities. Design engineers must understand the electromagnetic properties of the various materials that comprise their absorbers. Then, they must be able to see how the absorber — made up of these materials — performs under various conditions.
The key parameters for absorbers are the complex electric permittivity and magnetic permeability. These values will vary with frequency. The most common method for measuring and characterizing absorber parameters is to measure complex reflection and transmission values from a sample of a known thickness in a closed waveguiding system. Within a material property database, a design engineer can perform multilayer performance prediction in stacks and run iterations. This database allows the military and aerospace industries to accurately and efficiently identify the proper mix of absorber materials for a specific application.
From there, a design engineer can deploy sophisticated electromagnetic modeling to essentially create a “map” of the electromagnetic behavior of an absorber. Engineers can use modeling software to calculate the form of electromagnetic fields within a volume, based on stimulus and configuration. They can use boundary conditions to complete the model and assign values. Electromagnetic modeling can help design engineers devise solutions in applications with multiple energy sources and constraints. At the same time, it helps them anticipate potential future problems and reduce the number of iterations required during the design phase of a project.
Winning on Today's Battlefield of Innovation
While the flashiest innovations in the military and aerospace industries — everything from new weapons capabilities to new fighter jets — attract the most attention, advancements in RF/microwave absorbers are crucial enablers of these larger innovations. As design engineers seek to stay ahead in the non-stop battle of measures and countermeasures with adversaries around the world, they can take comfort in knowing that the innovations taking place in the absorber space will help them improve survivability capabilities, strengthen communications and in the end, shield the warfighter.
This article was written by Paul Dixon, staff scientist, and Rick Johnson, aerospace and defense director, at Laird Performance Materials. For more information, visit here .
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