A Microwave Insider’s overview on EMI, RF, & MF Shielding

In an effort to provide information on Conductive and Absorptive Materials for EMI RF Shielding, below please find a Microwave overview from a Microwave Geek (an old college roommate).

Basic applications:

Applications for Conductive materials:

Electrically conductive materials are used to contain electronic signals within an enclosure by forming a uniform conductive barrier to signal propagation. Conductive coatings may also be applied to the exterior of objects to reduce radar cross section. Conductivity inconsistencies  in an objects surface can create reflective hot spots that increase radar cross section.  These materials may be formed (extruded or molded) in custom shapes, sheets, applied or overmolded to plastic or metal parts.

Nickel and Silver loaded epoxy paints are common shielding coatings.

Iron or nickel loaded silicone rubber sheets are often cut into shapes and sandwiched between conductive panels.

Strip gasket material consisting of either a beryllium – copper spiral spring encapsulated in a pliable extruded material or a pliable extruded material loaded with conductive material is often used in a dual role as a moisture and EMI gasket.

Shielding Effectiveness:

Shielding effectiveness is normally expressed in – dBc as a measurement of signal level decrease relative to the contained signal when measured at 1cm.

Shielding Effectiveness also changes with the frequency (wave length) of the RF Signal.  Generally, the higher the frequency the more effective a conductive material is as an RF shield. However, at extremely high frequencies the signal wavelength becomes smaller so even tiny voids in the conductive surface can allow signal propagation through the shielding material.

A good example of this phenomenon is the Faraday shield used on your microwave oven to contain the radiation. The microwave oven operates at 2.4GHz with a signal wavelength of ~ 100mm. As long as the holes in the Faraday screen are smaller than 1/4 of the signal wavelength ~ 25mm, the screen is an effective EMI shield even though you can see through it.

Conversely, if the same Faraday shield was used to contain 60GHz radiation it would be ineffective because the 1/4 wave length at 60 GHz is ~ 1.25mm would propagate through the small holes in the screen. The same rules would apply to conductive materials  encapsulated in compounds.

Absorptive Materials:

Materials can also be loaded with resistive materials most often carbon to absorb, capture and attenuate RF radiation.  Where conductive material might actually cause interaction from reflected radiation, absorptive materials reduce parasitic interaction by absorbing and trapping stray radiation.

These materials are extremely frequency-sensitive since the amount of loading (concentration of resistive material) is directly proportional to the wave length of the signal to be absorbed.  Lower frequency signals with longer wave lengths are very difficult to absorb because the thickness of the absorber material must approach a 1/4 wave length to be effective. Higher frequency radiation with smaller signal wave lengths can be absorbed is very small, relatively lightly loaded, physical structures. Heavily loaded materials that may be absorptive at lower frequencies can be reflective to higher frequencies.  Typically these materials are compounded for a specific frequency of operation.

Absorptive materials generally come in 2 basic forms; resistively loaded foam materials, load rubber sheets or custom shape material and coatings loaded with resistive material. The foam materials can be rigid or flexible open or closed cell. Sheet rubber or custom shape thickness and loading concentration will vary for frequency of use and coatings vary in thickness and loading concentration.

Common applications for absorptive materials include the placement of load rubber gasket or part within electronic modules to absorb and attenuate tray signals and coating objects with absorptive materials to reduce radar reflection.  Absorptive foam material is used for anechoic testing facilities or applied to antenna reflectors to control radiated side-lobe energy.

Absorptive material can be like a band-aid to correct previously unknown, parasitic signal propagation issues. Often this material is sold in prototyping kits with different frequency materials and shapes to provide flexible solutions while systems are in development.

Coal has also been employed as an absorptive material.   Since coal is predominantly carbon and readily available, it has become a popular option. Coal can also be pulverized mixed with a binder material and molded into rigid shapes.  The molded pieces or glass balls can then be selectively metalized to yield the end result of controlled conductivity within an absorptive cavity.

We hope this information has been helpful. Please visit this site: Microwave101 to get more answers for your Microwave questions.

Sincerely,

Casco Bay Molding

Liz Thornton