Microwave Absorbers for the Layperson

Without microwave absorbers, we simply wouldn’t have as many electronic components passing FCC regulations, nor would they have the required performance needed for their intended use. These materials are used in a wide variety of markets, from military and aerospace to consumer electronics. The applications for microwave absorbers vary just as widely, from reducing the radar cross section of a stealth aircraft to the lining of antenna shrouds in order to direct the signals properly. Following, is a brief explanation of absorber types and uses for the novice.

The EM Spectrum and Microwaves

First, what is a microwave? It is a portion of the electromagnetic spectrum. The spectrum is a way of classifying various types of energies, such as heat, visible light, radio waves, Xrays, and microwaves. These are measured by how short or long the waves are. Microwaves, as suggested by their name, have very short waves ranging from 300 MHz to 300GHz. A very wide array of technologies use the microwave spectrum for uses such as communications, radar, radio astronomy, GPS, heating, and more. However, in order to effectively use microwaves they have to be channeled correctly.

Why are absorbers needed?

Traditional shielding materials are highly conductive materials designed to block, contain or redirect stray radiation. As operational frequencies increase, the wavelength decreases and the gaps can cause shielding failures. Absorbers attenuate (absorb) the stray energy. At higher frequencies, absorbers can solve a shielding problem by absorbing the stray radiation. In general, when you think of electronics, both consumer and non-consumer goods, there is a need to provide more speed and capabilities. Speed and functionality require more components to get the job done, in an increasingly smaller package. When more components are crammed into smaller spaces, more design challenges arise. Design challenges include, component incompatibility, need for more power, EMI issues caused by higher power and higher frequency interference, and the need for lower cost/higher performing products. Cavity resonance problems will result from this. Microwave absorber materials can be used as a solution to cavity resonance problems.

Microwave absorber types

There are two types of Absorbers; Dielectric types, in which the absorbing filler acts on the electric field. Commonly used materials are carbon and silicon carbide, which can be impregnated into polyurethane foams or rubber materials. The second absorber type is magnetic, in which absorbing fillers act on the magnetic field. Carbonyl iron and ferrites are common materials that are used. There are many varieties of rf absorbers and which one you use depends on the application. The absorber figures of merit include insertion loss, reflectivity, and attenuation.

Rubber sheet microwave absorbers: These absorbers can be used inside of cavities to prevent microwaves from bouncing around inside. This maintains a clear signal and prevents other devices from reacting to the device while in use (crosstalk). They can also be placed directly on top of circuits, since rubber doesn’t conduct electricity. These absorbers can be either broadband or specifically tuned to a desired frequency.

Foam absorbers: These are used primarily around high powered antennas and wireless equipment. They are often used to cover metal structures in the path of RF energy to eliminate unwanted reflections.

Rigid and Castable absorbers: Typically filled with iron, these materials are used to produce waveguide loads and absorptive housings, and they can be cast directly into RF device packages to attenuate unwanted noise and resonances.

Honeycomb absorbers: These microwave absorbers are used in high energy applications. Their design also allows them to be light and strong, making them perfect for aerospace. The honeycomb design permits forced air cooling for these high power applications.

Anechoic absorbers: Anechoic absorbers are pyramidal in shape and are typically used in a chamber environment to isolate a test set up from environmental factors.

For more information about microwave absorbers, please contact Cuming Microwave Corporation at 508 521-6700 or at cmcsales@ppg.com