Novel materials absorb microwaves

Microwave absorbers are increasingly being used to enhance shielding performance at higher frequencies.

Microwave absorbers are increasingly being used to enhance shielding performance at higher frequencies.

Products including die-cut elastomers, foam, thermoplastics and other custom solutions can aid in a wide variety of problems such as internal cavity resonances, antenna pattern shaping and high-frequency interference.

There has been a growing and widespread interest in microwave-absorbing material technology.

As the name implies, microwave-absorbing materials are coatings whose electrical and/or magnetic properties have been altered to allow absorption of microwave energy at discrete or broadband frequencies.

There are several techniques to achieve these properties.

The goal of the absorber manufacturer is to balance electrical performance, thickness, weight, mechanical properties and cost.

Altering the dielectric and magnetic properties of existing materials will produce microwave absorbers.

For purposes of analysis, the dielectric properties of a material are categorised as its permittivity and the magnetic properties assists permeability.

Both are complex numbers with real and imaginary parts.

Common dielectric materials used for absorbers, such as foams, plastics and elastomers, have no magnetic properties, giving them permeability of 1.

Magnetic materials, such as ferrites, iron and cobalt-nickel alloys, are used to alter the permeability of the base materials.

High dielectric materials, such as carbon, graphite and metal flakes, are used to modify the dielectric properties.

The simplest type of resonant absorber is the Salisbury Screen.

It consists of a resistive sheet spaced one-quarter wavelength from a conductive ground plane.

The resistive sheet is as thin as possible with a resistance of 377ohm/square, matching that of free space.

A wave incident on the surface of the screen is partially reflected and partially transmitted.

The transmitted portion undergoes multiple internal reflections to give rise to a series of emergent waves.

At the design frequency, the sum of the emergent waves is equal in amplitude to, by 180 degrees out of phase with, the initial reflected portion.

The inherent problems of the Salisbury Screen are poor flexibility, poor environmental resistance and increased thickness, especially at lower frequencies.

Distributing dielectric and/or magnetic fillers into a flexible matrix, such as an elastomer, can produce a more practical absorber.

Increasing the permeability and permittivity of the layer increases the refractive index ue, thus reducing the required thickness by 1/ue.

The dramatic difference in thickness achievable can be illustrated by comparing two microwave absorbers.

RFSS-10 is a Salisbury Screen-type absorber tuned to 10GHz and is nominally 6.4mm thick.

RFSB-10 is an elastomer loaded with carbonyl iron filler and is 1.7mm thick.

The same electrical performance can be achieved in a material that is 25% as thick (although a weight penalty must be paid).

The RFSB absorber is also very flexible and adaptable to outdoor environments.

Resonant materials can also be produced to absorb at multiple frequencies.

By controlling the critical magnetic/dielectric loading and thickness of each layer, two discrete frequencies can be tuned.

These flexible dual-band absorbers are standard production products and have the added advantage of broadband absorption.

For example, a dual-band absorber with appropriate resonant points will have greater than 15dB absorption over an octave bandwidth The performance indicated for resonant absorbers is at normal angles of incidence.

The effectiveness of these materials drops off as the angle of incidence increases.

Materials have been developed for situations where performance is needed at angles of incidence of 65 degrees and greater.

These absorbers are generally thin and heavily loaded with magnetic fillers.

Such high-permeability absorbers have a greater than critical impedance at normal angles of incidence, thus resulting in performance that is poorer than the resonant type at normal angles but improves as angle of incidence increases.

They are generally tuned for a high angle of incidence and horizontal polarisation.

The other absorber category is the graded-dielectric absorber.

Its principle of operation is quite different from that of the resonant type.

Absorption is achieved by a gradual tapering of impedance from that of free space to a highly "lossy" state.

If this transition is done smoothly, little reflection from the front face will result.

Anechoic chamber materials accomplish this via the pyramidal shape of the absorber.

The absorbing medium is a conductive carbon in polyurethane foam.

Absorption levels of greater than 50dB can be obtained with pyramids many wavelengths thick.

These are impractical for electromagnetic interference (EMI) or radar cross-section (RCS) reduction.

Good levels of reflectivity reduction (greater than 20dB) can be achieved in materials less than one-third wavelength thick.

In this case, a very open-celled (10 pores per inch) foam is used.

A gradual transition is achieved via a conductive carbon coating.

This method of gradual impedance transition can be applied to other materials.

Foams, honeycombs and netting are three such matrices where practical absorbers are being produced.

A wide variety of absorber materials are available for use in EMI and RCS reduction.

There are tradeoffs involved in the use of each candidate material.

To optimise the use of absorbers in a design, there are three sets of parameters that should be critically analysed: electrical, physical and application.

Although the "DC to daylight" goal has not been achieved, considerable strides have been made to broaden frequency coverage across the microwave region.

In optimising absorber use, the requirement must be defined as completely as possible.

The following questions should be asked.

First, what frequency bands need coverage?.

Secondly, is coverage needed over the entire region or just at specific frequencies?.

For example, if coverage cannot be achieved over the entire 2 to 18GHz region, will absorption at specific frequencies provide enough protection?.

Thirdly, what is the order of importance in coverage? Perhaps at F0, 20dB absorption is needed.

However, at F1, only 12dB is needed; at F2, 7dB is acceptable.

By setting these priorities, a design can be more easily reached.

And fourthly, will the absorber be used to absorb specular energy, or is the application such that high angles of incidence radiation and surface waves must be attenuated?.

By answering these questions, the various tradeoffs in electrical performance can be examined and an optimum absorber solution derived.

The following are valid electrical performance guidelines.

The broader the frequency coverage, the thicker, heavier and more expensive the absorber.

The lower the minimum frequency coverage, the thicker and heavier the absorber.

Normal incidence performance is better than off-normal performance for most types of absorbers, although they can be designed for off-normal performance.

Millimetre-wave materials are now being developed and used.

Of equal importance to the material's electrical performance is its physical performance, which includes environmental characteristics, temperature characteristics and mechanical properties.

Again, a series of questions can help clarify the parameters of major importance.

First, what is the application environment? Will the absorber be enclosed or subjected to the outdoor environment?.

Secondly, what environmental forces will be degrading the absorber? Some examples are salt, water, ozone, oxygen, ultraviolet light, fuels, oils, chemicals, nuclear and stack gases.

Thirdly, over what temperature range will the material be subjected, and within what thermal range must the material perform?.

Fourthly, what mechanical stresses will be placed on the absorber? Examples are vibration, thermal shock, elongation or wind.

And fifthly, what is the expected lifetime of the absorber? For example, missile applications may not require the same degree of physical integrity as a shipboard application.

The following are valid physical performance guidelines.

Elastomeric-type (rubber) absorbers have better environmental resistance than the broadband foam types.

These types have been used successfully on surface ships for more than 40 years.

A variety of elastomers are available to aid in designing for a specific environment.

Hypalon is widely used in naval applications because of superior weather resistance and colour fastness.

Nitrile is used for fuel and oil resistance.

Fluoroelastomers and silicones have an excellent operating temperature range.

Broadband absorption is obtainable with the dual-layer elastomeric absorbers.

Broadband foam materials can be used for external environments, but steps must be taken to protect the absorber.

Open-cell foams can be filled with low-loss plastics to make rigid panels for use outdoors.

Broadband absorbers can be encapsulated in fibre-reinforced plastics to form flexible absorber panels that can be draped over reflectors.

The useful temperature range of most absorber material is -55 to +120C.

Certain materials are available with higher maximum temperatures.

When considering absorber types, thin, flexible elastomeric absorbers are best for outdoor use.

The method of application is adhesive bonding to a metal substrate.

Adhesives vary with the type of elastomer chosen and include: epoxies, urethanes, contact adhesives and pressure-sensitive adhesives (PSA).

In general, Hypalon and Nitrile are the easiest elastomers to bond and have a variety of compatible adhesive systems available.

Bond strengths in excess of 70kPa are typical.

In some cases, it is necessary to cover a tight radius or complex curvature.

An alternative to flat sheet material is conformally moulded parts.

Conformal moulds increase the ease of bonding and reduce the likelihood of applying any built-in stresses into the material.

For gasket applications, the elastomeric absorber may be extruded.

To improve weather resistance, the absorber is painted.

Typically, an epoxy or urethane-based paint is used.

To avoid gaps between sheets, absorptive gap fillers are used to minimise any impedance mismatches from sheet to sheet.

This technique also limits the formation of surface waves and reflections.

Newer noncorrosive fillers, such as iron silicide, are also available for corrosive environments.

Open-cell foam absorbers are normally used in a protected environment, such as radomes or nacelles.

Therefore, application becomes much less critical than for those on the exterior of a vehicle.

The typical method of application is adhesive bonding.

Again, a wide class of adhesives may be used, including contact cements, epoxies and acrylic PSA.

In general, cohesive failure of the material will result before adhesive failure.

The front surfaces may be painted or coated to further protect the absorber.

R and F uses two methods to produce broadband absorbers for external use.

The first method involves taking broadband foam or netting absorber and encapsulating it in a reinforced coated fabric.

The bagging material is completely enclosed around the absorber making it weather proof.

This radar-absorptive cover can then be used in external environments with no physical degradation to the absorbing medium.

A second method uses a closed-cell foam filling technique to produce rigid structural absorptive panels.

The absorber, RFRigid, is lightweight and may be moulded to a variety of shapes.

It has broadband absorptive characteristics similar to the flexible foam RFRET absorbers.

The rigid, closed-cell form may be painted and will be impervious to external environments.

A variety of high-strength, lightweight, flexible fillers for RFRigid are being developed.

RFRigid and absorptive honeycomb may be used as the inner core for structural panels.

The panel would consist of face sheets of glass fibre or Kevlar facing the radar and graphite or metal as the ground plane.

These panels are lightweight and high strength and can be used as structure in certain applications.

The two largest applications for radar-absorbing materials are for EMI and for RCS reduction in military and commercial electronics.

Back to top