Theory and Applications of RF Absorbers

In the RF / Microwave range, Absorbers are objects that weaken the energy of an electromagnetic wave. Absorbers are used in a wide range of applications to remove wandering or unwanted radiation that interfere with the function of a system. The Absorbers can be used externally to reduce the reflection of particular objects or to reduce their sending, as well as to reduce the fluctuations caused by the resonant cavity internally. They can also recreate an open space by removing the reflection in anechoic chamber.

Absorbers can have a variety of physical shapes and forms, such as flexible elastomer, foam, hard epoxy or plastic. They can be made to withstand weather and temperature extremes. Absorbers are known in some systems to reduce the interference between different components of the circuit as a very important element.

Absorber Theory

Absorbers generally consist a filler material inside a material matrix. The filler consists of one or more constituents that do most of the absorbing. The matrix material is chosen for its physical properties (temperature resistance, weatherability, etc.).Absorbers are characterized by their electric permittivity and magnetic permeability. The permittivity is a measure of the material’s effect on the electric field in the electromagnetic wave and the permeability is a measure of the material’s effect on the magnetic component of the wave. The permittivity is complex and is generally written as

ε*=ε′-jε″

The permittivity arises from the dielectric polarization of the material. The quantity ε’ is sometimes called the dielectric constant which is something of a misnomer when applied to absorbers as ε’ can vary significantly with frequency. The quantity ε” is a measure of the attenuation of the electric field cause by the material.

Absorbers Application

Near field absorbers: Near field absorbers are a class of absorbers that are placed near or directly upon a radiating element. Since the energy in the near field is predominantly magnetic, near field absorbers have high magnetic permeability and high magnetic loss. Also, since they are often in direct contact with circuit elements, they must have very low conductivity. Even the best designed circuit will contain elements that will resonate and radiate at particular frequencies. These radiators could be inductors or capacitors or connecting wires that will behave differently at certain frequencies. In the near field most of the radiated energy is magnetic so radiators are modeled as a loop antenna. Magnetic energy dies off very quickly with distance but can still interfere with nearby circuit components.

Loads: Microwave terminations are waveguide or coax sections that present low reflections to the incoming wave. Terminations are used in many microwave systems such as circulators or couplers to eliminate unwanted signals. A termination must be able to absorb the incident energy, hence the use of absorber load material. A load must absorb over the entire waveguide band and is therefore shaped to present an impedance taper to the incoming wave. Load materials are generally magnetic absorbers which are easily machined or molded.

Millimeter wave absorbers: Different modes of analysis and absorber types are needed in the RF/microwave band depending upon whether the absorber is used in free space or inside an enclosed cavity. Absorbers for free space reflectivity or insertion loss use a different design philosophy than for cavity resonance reduction. Most applications in the RF/microwave realm are clearly one or the other. The physics will change somewhat as we move into millimeter waves. Even a physically small cavity or enclosure could encompass several wavelengths at millimeter wave frequencies.

Reflection Reduction: Any system that transmits energy can experience interference from reflections back to the transmitter. Also, unwanted reflections can interfere with other systems. Often the reflection source can not be moved as with a building or a ship’s mast. Absorbers can then be used to reduce the reflection level. Typical reflectivity reduction for weather resistant outdoor absorber material is –20dB which will eliminate 99% of the reflection. Care must be taken that the chosen absorber is designed to absorb at the transmit frequency.

Radar Cross Section reduction (RCSR): Absorbers can also be used to reduce the radar cross section of a target object. By reducing the reflection level the object will present a smaller cross section. However, due to the narrowbanded performance of thin radar absorbent material (RAM) and the thickness and weight of broadband RAM, it is difficult to achieve effective radar cross section reduction using absorber alone.

Anechoic Chambers: Anechoic chambers are used to create a free space condition in an enclosed room. Very high performance absorber material is used on the walls, ceiling and floor to eliminate reflections. Reflectivity of absorbers used in anechoic chambers can be –50dB or better.