Radio-frequency interference (RFI) is a type of electromagnetic interference (EMI) that falls within the radio frequency spectrum (20 kHz to 300 GHz). Although these frequencies are essential for wireless communication, their transmission can have undesirable effects.
RFI can significantly degrade the performance of nearby electrical and electronic equipment or prevent it from functioning entirely. In severe cases, RFI can even permanently damage solid state and digital devices. These issues are of particular concern in the medical field where the slightest loss of system performance can often have disastrous consequences.
To address these effects, RFI absorbing materials block or reduce the transmission of radio frequency electromagnetic radiation. Primarily used to protect sensitive electrical or electronic circuits and prevent various health and safety hazards, RFI absorbing materials are employed in numerous medical and consumer electronics including medical imaging devices, laboratory instruments, cell phones, computers/laptops, and audio equipment.
RFI Material Testing Methods
The requirements for RFI absorbing material protection are typically application specific. For example, RFI absorbing materials used in consumer electronics may have vastly different specifications than those used in the medical industry.
With this in mind, materials must be tested to measure and characterize their various performance metrics. Testing also helps analysts predict how well a particular material will perform in a specific application, as well as determine the required size and thickness of the absorbing material.
Reflectivity and insertion loss testing are two of the most common tests used to evaluate RFI absorbing materials.
Reflectivity Testing
Reflectivity testing, as its name suggests, assesses the ability of a particular material to reflect incoming radio frequency signals. The NRL Arch Test, initially developed by the United States Naval Research Laboratory, is considered to be the standard for measuring the reflectivity of materials. This test consists of a transmitting antenna connected to a signal generator and a receiving antenna connected to a signal detector positioned at various points along an arch.
The test material, which rests on a metal plate, is placed within the arch below the transmitting and receiving antenna. The transmitting antenna directs a signal toward the test target while the receiving antenna measures the residual energy reflected off of it. The results are then analyzed to assess the material’s reflectivity performance.
Insertion Loss Testing
Insertion loss testing quantifies the amount of energy lost when a signal is transmitted through an RFI absorbent material in its path. The test consists of two antennas – a transmitter and a receiver – both connected to a signal generator and detector respectively. Unlike the NRL Arch Test, where the antennas are positioned in a coplanar manner on an arch, insertion loss testing requires that the antennas face each other on opposite sides of the test material.
The insertion loss test is relatively straightforward. A signal is directed toward the test material on one side, while the response is measured at the other side of the test material by the receiver. The energy transmitted through the RFI absorbing material is indicative of its level of performance when subjected to the signals of a specific wavelength.
RFI Absorbing Materials in the Medical Industry
RFI materials are crucial in the medical industry as the disruption of electronic medical devices can often have life-threatening consequences. Many relatively small or portable medical devices, such as blood pressure monitors, heart monitors, ultrasound devices, and infusion pumps, utilize thin-gauge shielding components, including gaskets and seals constructed from various RFI absorbing materials.
Larger devices, such as MRI and X-ray imaging equipment, are placed in special rooms layered with RFI absorbing materials. These enclosed areas, known as Faraday Cages, help to contain EMI/RFI radiation, thus preventing emitted signals from affecting other nearby medical devices and components.
Some of the most common RFI absorbing materials used in the medical industry include:
- Metal-filled elastomers
- Foil tapes/copper foil tapes
- Wire mesh
- Fabrics laminated with conductive adhesives
RFI Materials in the Electronics Industry
Electronic devices contain sensitive circuits that are vulnerable to the effects of RFI. Electromagnetic induction, electrostatic coupling, and conduction in unshielded electronic devices, such as personal computers, mobile phones, and LCD cables, can lead to several issues including loss of signal transmission, increased error rate, and decreased system performance.
Integrated circuit and semiconductor packaging, for example, consists of metal, plastic, glass, or ceramic casings that encapsulate sensitive components to shield them from RFI/EMI originating from internal or external sources.
RFI absorption materials used in the electronics industry are similar to those employed in the medical field. These include:
- Ferrite cores and accessories
- Tapes (foil tapes, copper foil tapes, shielding tapes, insulation tapes)
- Metal-filled elastomers
- Absorbing materials and pads
- LCD screen applicators
- Silicone
- Fluorosilicone
Testing the Effectiveness of RFI Absorbent Materials
The key to selecting the best RFI absorbing material for a specific application lies in adequate laboratory testing and proper assessment of the expected frequency ranges of the given application. Reflectivity and insertion loss testing of various materials can help analyzers determine the expected performance level and physical characteristics of the chosen material.
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