Ben and His Kite – Don’t Try This At Home!
Ben and His Kite – Don’t Try This At Home!

A Faraday cage (sometimes called a Faraday shield, RF cage or EMF cage) is a shielded enclosure formed by conducting material or by a mesh of such material.  The enclosure is like a sealed tin can; it keeps the electromagnetic fields inside, and blocks the entry of external electric fields and radiofrequency waves. Faraday cages are named after English scientist Michael Faraday, who is credited with their invention in 1836.

Quick history:   Although the development of the faraday cage shielding effect has been attributed to Michael Faraday, it was actually Benjamin Franklin in 1755 who observed the effect by lowering an uncharged cork ball suspended on a silk thread through an opening into a metal container.  He noted that the cork was not attracted to the inside of the enclosure as it would have been to the outside and although it touched the bottom, when pulled out it was not found to be “charged” by the touch. Thus, Franklin actually discovered the behavior of what we now refer to as a Faraday cage or shield.

Faraday’s famous ice pail experiments duplicated Franklin’s cork and can trial. He conclusively demonstrated the electrostatic shielding effect when he built the first formal Faraday cage in 1836. With it, he observed and documented that the charge on a charged conductor remained only on its exterior with no influence on the interior.  His Faraday enclosure was a room coated with metal foil. Faraday made high voltage discharges from an electrostatic generator to strike the outside of the room.  Using an electroscope, he demonstrated that the discharges caused no deflection when located inside the room. Since it did so outside, no response meant absence of an electric charge on the inside of the walls.

Quick physics: The operation of a Faraday shield is best understood considering a hollow conductor. Externally applied electric fields produce forces on the charge carriers (usually electrons) within the conductor, generating a current that rearranges the charges. The rearranged charges cancel the applied field within and the current stops. There are additional factors to consider when applying an alternating current, particularly one in the radio frequency (RF) domain. The walls of a Faraday box or enclosure only shield the interior from external electromagnetic (RF) radiation if the walls are thick enough to reduce skin effect penetration and any holes are much smaller than the radiation’s wavelength.

Applications: The usefulness of the RF Faraday cage is its ability to protect personnel, operating systems, sensitive test equipment, volatile materials, and other things from radio frequency waves.  For instance, in the hospital environment sensitive operating room instruments are protected by RF shields from the bombardment of environmental RF energy.  In research and test labs and industrial shops similar protection is often afforded by faraday enclosures similar to the LBA EMFaraCage® (http://devlbagroup.com/technology/faraday-cages.php).

Technicians Applying LBA Room Shielding Systems
Technicians Applying LBA Room Shielding Systems

The security of wireless communications is often protected using the Faraday principle by shielding building areas and forensic test facilities with architectural materials such as LBA’s SM-10 shielding fabric or CPC-54  conductive paint (http://devlbagroup.com/technology/emi-rf-shielding-materials-fabric-paint.php). Coaxial cable such as used for cable television, actually includes a continuous Faraday shield to protect the internal conductors from electrical noise and to limit external radiation of the enclosed RF. Finally, the military uses this Faraday cage technology for protection of defense equipment for both RF protection, and protection from electromagnetic pulse attacks.

Ironically, the technology can also be used for illicit purposes.  A shopping bag lined with aluminum foil acts as a Faraday cage assisting shoplifters to steal RFID tagged merchandise. Hackers can set up Faraday cage test cells for “black” development of wireless LAN intrusion tools, for instance. On the other hand, university researchers use faraday cages to study anti-hacking in a secure RF environment. An excellent example is the program at East Carolina University that utilizes a custom LBA EMFaraCage® Faraday box as discussed in Technical Note 127: Keeping Black Hat In The Box.

University “Black Hat” Test in an LBA FC-10 EMFaraCage® Faraday Cage
University “Black Hat” Test in an LBA FC-10 EMFaraCage® Faraday Cage

LBA Faraday cage solutions: The EMFaraCage® shown above is one of a line of innovative portable desktop Faraday cages developed by LBA for production testing, laboratories, and other applications. Many other RF shielding requirements may be accomplished through LBA designed Faraday cages of architectural shielding materials. We design and furnish RF cages integrated into room or building construction using the most effective RF fabrics and RF conductive paints. We can also furnish architectural shielding solutions against EMF effects of nearby power lines, such as found in utility telecommunications sites.

For assistance on shielding system design, or to purchase EMFaraCages® or shielding materials, contact Byron Johnson at byron.johnson@lbagroup.com or at 252-757-0279.

 

About The Author

LBA Group, Inc. has 50 years of experience in providing RF asset solutions and risk management for industrial and telecommunications infrastructure assets. The company is comprised of LBA Technology, a leading manufacturer and integrator of radio frequency systems, lightning protection, and EMC equipment for broadcast, industrial, and government users worldwide; the professional engineering consultancy Lawrence Behr Associates, and LBA University, providing on-site and online professional training. The companies are based in Greenville, N.C., USA.

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