RF shielding is the process of reducing the electromagnetic field in an area. This reduction in RF occurs by the blocking or absorption of waves with materials that are either conductive or magnetic. The blocking materials are called shielding. Shielding can be present on cables, power cords or the entire device’s enclosure.
The ability of a material to block or absorb electromagnetic radiation is considered its shielding effectiveness. Shielding effectiveness testing is the process of determining what signals a material blocks.
What materials block or absorb RF?
Metal is the best material to block electromagnetic energy. The metal could be sheet metal, metal screens or metal foam. However, metal is rigid, heavy and expensive. Because of this, many other materials have been created to provide the RF blocking properties of metal without the drawbacks. These materials are lighter, flexible and sometimes less expensive.
Resins and fabrics have been designed with RF blocking material woven in them. There are some paints, inks and coatings that have RF blocking properties. These materials often consist of a carrier metal loaded with very small particulates of another metal such as nickel or copper. Copper is used for RF shielding because it absorbs electromagnetic and RF waves.
What items should be RF shielding tested?
As a shielding effectiveness test lab, we test a lot of different products. Starting on the small side, we test a lot of material samples. With technology advancements, manufacturers must push the envelope and find more flexible and cost-effective materials that can block RF. Some of the materials we have tested include, fabrics, resins, glass, paints and inks.
The majority of our RF shielded testing involves cables and entire electronic devices. In testing an electronic device, we are essentially testing the enclosure. There are two focuses when testing an enclosure. First, how well does the enclosure keep RF noise generated by the device inside the device. It is important that the device’s operations do not interfere with other devices. The second consideration is how well the device keeps outside electromagnetic noise outside of the device. It is equally important that a device’s operations are not interrupted by other devices or signals.
In testing an enclosure, the most common points of concern are cables, fans and inputs and outputs. Essentially, what are the holes in the enclosure and are the gaskets, filters, insulation and other components sufficiently blocking RF.
The last test item we test are facilities. The most common facilities are military, medical, industrial and utilities. Electromagnetic Pulse (EMP) attacks are a growing concern and have prompted new standards and hardening requirements.
How is electromagnetic shielding testing completed?
The process of shielding effectiveness testing depends on what is being tested. The testing or materials for RF shielding is completed by having two connected EMC test chambers with a shared wall. This shared wall has an opening in which the material sample is placed. The RF shielding test lab will place a receiving antenna in one chamber and a transmitting antenna in the second chamber. The transmitting antenna will transmit a set of RF waves. The receiving antenna will record all of the electromagnetic signals received.
To determine how well the shielding material performed, an analysis of what was transmitted and what was received is conducted. Any RF noise transmitted but not received was effectively blocked.
To test electronic devices, an EMC test chamber should be used. An EMC test engineer will operate the device to be tested. For part of the testing, a receiving antenna receives and records all of the EMI generated by the device. Upon completion of that, a transmitting antenna will transmit signals at the device. The objective is to determine if there are unacceptable impacts on its operation.
What is determined to be unacceptable will depend on the device and its purpose. For instance, a blip on the screen of a video game might be acceptable whereas a blip on an aircraft navigation system might not. The test standard often details what is acceptable and what is not.
Testing the shielding effectiveness of a facility is similar to the testing of a material sample. Antennas are placed on both sides of the structure. The structure could be a vent, a wall, a window, a door or any other physical barrier. One antenna receives and the other transmits. The shielding effectiveness of facilities is very important for certain industries such as military, aerospace, health care and industrial facilities.
Keystone Compliance has developed the reputation of being a leader in electromagnetic shielding testing. Our engineers not only provide testing, but can also assist in the design and implementation of shielding effectiveness test improvements. Please contact us for more information on how Keystone Compliance can assist you with determining and improving the RF shielding of your product.
Common RF Shielding Test Standards:
- ASTM D 4935-10: Standard Test Method for Measuring the Electromagnetic Shielding Effectiveness of Planar Materials
- ASTM D 4935-99: Standard Test Method for Measuring the Electromagnetic Shielding Effectiveness of Planar Materials
- IEEE 299-2006: Measuring The Effectiveness Of Electromagnetic Shielding Enclosures
- MIL-DTL-83528C: General Specification For Gasketing Material, Conducting Shielding Gasket, Electronic, Elastomer, EMI / RFI
- MIL-G-83528B: General Specification for Gasketing Material, Conducting Shielding Gasket Electronic, Elastomer, EMI / RFI
- MIL-STD 1377: Measurement And Effectiveness Of Cable, Connector, And Weapon Enclosure Shielding And Filters In Precluding Hazards Of Electromagnetic Radiation To Ordnance
- MIL-STD-188-125-1: High-Altitude Electromagnetic Pulse (Hemp) Protection For Ground-Based C4I Facilities Performing Critical, Time-Urgent Missions (Transportable Systems)
- MIL-STD-188-125-2: High-Altitude Electromagnetic Pulse (Hemp) Protection For Ground-Based C4I Facilities Performing Critical, Time-Urgent Missions (Transportable Systems)
- MIL-STD-285: Military Standard For Attenuation Measurements For Enclosures
- MIL-STD-907B: Engineering, Design, And Shielding Effectiveness Criteria For Shelters (Expandable & Non-Expandable)