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This article takes an in-depth look at RF shielding.
Read further and learn more about topics such as:
What is RF shielding
Nature of radio frequency interference
RF shielding materials
Common forms of RF shielding
And much more…
Chapter 1: What is RF Shielding?
Radiofrequency (RF) shielding is the practice of blocking radiofrequency electromagnetic signals that cause radio frequency interference (RFI). RFI can disrupt the electrical circuits of a device from working normally. RF shielding is accomplished by installing barriers made of conductive and magnetic materials around the circuitry of electronic devices, cable lines, and potential sources and victims of an electromagnetic field to completely isolate them from the environment. The effectiveness of RF shielding in reducing the amount of interference depends on the properties of the shielding material, design, thickness of the shield, electromagnetic frequency, and the size of discontinuities present on the shield.
Radiofrequency interference is a problem that can decrease the performance of electronic and communication devices. Every type of device has distinct responses on RFI. It can result in systems and information losses, data and security breaches, and even failure for some devices. However, in the real world, RFI is widespread in electronics and cannot be eliminated. Electrical circuits can emit radiofrequency electromagnetic signals and become susceptible to those signals at the same time. The use of RF shielding is a measure to safeguard our devices and equipment from the harmful effects brought by RF interference.
Chapter 2: Nature of Radio Frequency Interference
Electromagnetic waves carry energy. They consist of an electric and a magnetic wave oscillating at right angles with each other. Electromagnetic waves are characterized by their wavelength and frequency. The continuum of these waves is visualized in the electromagnetic spectrum.
Electromagnetic Interference and Radio Frequency Interference
Electromagnetic interference (EMI) occurs when unwanted electromagnetic waves or signals disturb the proper functioning of electrical devices. It is often referred to as “electromagnetic noise” or simply “noise.” But how does EMI differ from RFI?
Electromagnetic radiation at any frequency can cause interference. Radiofrequency interference (RFI) is a form of EMI when the electromagnetic waves involved are in the radio frequency band of the electromagnetic spectrum. The frequencies of radio waves widely range from 3 kilohertz to 300 gigahertz. The terms EMI and RFI are often used interchangeably, but the former is commonly used. RFI is a form of EMI in the radio frequency of the spectrum.
Types of Radio Frequency Interference
Radiofrequency interference can be grouped according to its source, duration, and bandwidth.
Naturally occurring RFI is generated due to astronomical phenomena such as lightning strikes, solar flares, static electricity, cosmic noise, dust storms, and snowstorms.
Electronic and electrical devices can emit electromagnetic radiation, affecting other devices and equipment within their vicinity. The man-made sources of RFI are further categorized as either unintentional or intentional sources:
Unintentional Sources Equipment such as electric motors and generators, lightings, rectifiers, inverters, satellites, and transmission lines disrupt other nearby devices as a result of switching large electrical currents. However, the emission of electromagnetic radiation is purely incidental.
Electronic devices operating using wireless signals such as cellphones, laptops, Bluetooth mice and speakers, wireless routers, and remote controls are abundant sources of RFI. As these devices become faster, the frequency increases and more electromagnetic radiation is emitted in the surroundings. The electromagnetic radiation that leaks out from these devices would cause interference.
Intentional Sources Intentional RFI sources are devices designed to emit electromagnetic energy. These sources include radars, jamming devices, and radio transmitters.
Continuous radiofrequency interference refers to the RFIs continuously emitted by a source through conduction or radiation. On the other hand, impulse radiofrequency interference occurs intermittently or within a very short duration. Switches and lighting commonly cause impulse RFI to disrupt the voltage and current equilibrium of connected nearby devices. Natural or man-made sources can make both types.
The bandwidth refers to the range of frequency with which the RFI is experienced.
When the RFI emitted consists of a single frequency or a narrow band of frequencies, it is considered a narrowband RFI. It can be generated by a form of an oscillator or as a result of spurious signals due to different kinds of distortion in a transmitter. Narrowband RFI has a minor effect on electronic devices but must be kept within acceptable levels. This RFI can be emitted by devices such as mobile phones and Wi-Fi routers.
Broadband RFI occurs in a wide range of frequencies which can comprise a large portion of the electromagnetic spectrum. The interference does not occur in a single or discrete signal. Broadband RFI can occur in different forms and be caused by a natural or man-made source. A naturally occurring source of this interference is the sun, which masks desirable satellite signals from communication systems. This interference can also be caused by arc welding, defective power lines, faulty brushes in motors and generators, and others.
The coupling mechanism describes how electromagnetic waves or signals from a source reach the receiver or the affected device, resulting in RFI. The coupling mechanisms are as follows:
Radiation coupling is the most commonly encountered RFI coupling mechanism. In this mechanism, electromagnetic waves are propagated from the source to the receiver through the air. The source and receiver do not involve physical contact and can be separated by a large distance.
Conduction coupling occurs when RFI travels across conductors such as wires and cables connecting the source and the receiver. It is more common in power supply lines and is heavily dependent on the magnetic component of the electromagnetic wave. This type of RFI coupling is mitigated by installing shields over electrical wirings.
Capacitive coupling occurs when an electrical charge from a source is passed to a receiving circuit because of charge differentials. It happens between two circuits in a system with very close proximity to each other, typically less than a wavelength apart.
Magnetic or induction coupling occurs when a varying magnetic field exists between the conductor loops of the source and the receiver. This consequently transfers RFI to the receiver as a result of electromagnetic induction. It often happens between conductors that are close to each other.
The shielding effectiveness depends on the electrical conductivity and magnetic permeability of the material and the frequency of the electromagnetic wave and the geometry of the shield. High conductivity enables the material to block or reflect the electric component of electromagnetic waves. A high magnetic permeability, on the other hand, allows the material to offer a low reluctance path for the magnetic flux which is beneficial in absorbing and drawing magnetic fluxes around the shielding area. Materials selection is based on the relative strengths of the electric and magnetic components of the electromagnetic field.
The common RF shielding materials are the following:
Copper is the most reliable RF shielding material because it is highly effective in absorbing and attenuating the electric and magnetic components of electromagnetic waves. It has high electrical conductivity.
Copper is easy to manufacture and formed into a variety of preferred shapes; copper-based shields can be installed as RF shields to electronic devices with relative ease. Aside from these, copper is naturally corrosive resistant and can resist oxidation brought by the environment.
Copper alloys such as phosphorus bronze, beryllium copper, brass, and bronze are used as RF shielding material. The elasticity of phosphorus bronze and beryllium copper makes them useful in contact applications for batteries and springs. However, despite its excellent RF shielding properties, copper costs more compared to other materials.
Nickel Silver (Copper Alloy 770)
Nickel silver, or commonly referred to as copper alloy 770, contains varying amounts of nickel, copper, and zinc and is widely used as an RF shielding material in highly corrosive environments. It is effective in attenuating RFI from mid-kHz up to the GHz frequency range. It has a permeability of 1, which makes them ideal in constructing RF shielding for MRI machines, where magnetic waves are not permitted.
Nickel silver does not require post plating to become corrosion-resistant and solderable. It is a highly aesthetic material with a bright silver appearance, although it does not contain silver.
Aluminum is a non-ferrous metal that has high electrical conductivity and a strength-to-weight ratio. It is a versatile material. Thin aluminum sheets block low-frequency radio waves. Aluminum can be used in constructing the enclosures of electronic devices to offer built-in protection against radiofrequency. Aluminum has 50-60% of the conductivity of copper; the thickness of aluminum RF shielding must be greater to achieve the same shielding effectiveness as copper. However, aluminum is prone to galvanic corrosion and oxidation. Its exposure to the environment will cause oxides to form on its surface. It also has poor solderability on its own.
Steels are manufactured in different ways and have assorted alloy content. Steels and other ferromagnetic materials provide low-frequency magnetic-field shielding that is lacking in copper alloys and aluminum. They possess a wide range of RF shielding and mechanical properties, depending on the type of steel.
Low carbon steels have higher permeability and saturation point than high carbon steels. These properties are critical in drawing the magnetic component of electromagnetic waves in the RF shielding. Saturation point refers to the magnetic flux density that a material can contain at a specific thickness.
Annealed steels have enhanced magnetic properties. Annealing enlarges the grain structure and relieves the internal stress of steel. The grain orientation of steels must be in the same direction as the magnetic flux to provide a low reluctance path. Lastly, cold-rolled steels possess better magnetic shielding properties, but hot-rolled steels have better mechanical properties.
Mu-metal is a soft ferromagnetic alloy consisting of approximately 80-82% nickel, 5% molybdenum, alloying elements such as copper and silicon, and iron as balance. It has superior magnetic permeability. It has good ductility and malleability, allowing it to be easily formed into a variety of shapes. Mu-metal RF shielding is used in electric power transformers, MRI equipment, hard disks, sensors, and other sensitive electronic devices.
Pre-Tin Plated Steel
Pre-tin plated steel is a low-cost RF shielding material that captures electromagnetic waves from the kHz to the lower GHz range of the spectrum. Its tin plating provides corrosion resistance and enables it to become solderable during board assembly.
Coating techniques such as plating, arc or flame spraying, metallization, and sputtering are performed on certain metals to increase corrosion resistance and solderability or compatibility between different surfaces. Common coating materials include tin, tin-lead, zinc, gold, and chromates.
Due to its flexibility, elastomeric materials can be easily processed into different forms of RF shielding, such as gaskets, O-rings, and linings. They are made electrically conductive to block electromagnetic waves by coating and loading them with metal fillers such as nickel graphite, silver copper, and silver aluminum. These materials provide a conductive path along shielding seams and other openings in electronic closures to block electromagnetic fields from the environment completely. Elastomer-based RF shielding with adhesive backing is available for easy installation.
The common elastomeric materials used in RF shielding are silicone rubber, fluorosilicone rubber, EPDM, and neoprene. These materials are highly flexible synthetic elastomers. They have good resistance to UV rays, ozone, oxidation, weathering, and many chemical substances. They possess high thermal and dimensional stability and are ideal for outdoor and harsh environments.
Conductive fabrics are lightweight textile materials coated or blended with metals such as nickel, copper, silver, gold, and carbon. These materials exhibit good shielding effectiveness and are used in mitigating RFI in enclosed rooms and spaces. Fibers used in the synthesis of conductive fabrics include polyester, cotton, silk, and nylon.
Chapter 4: Common Forms of RF Shielding
The following are the common forms and types of RF shielding, as well as their operating principles and design considerations:
Wire Mesh and Screens as a Faraday Cage
A Faraday cage is a continuous and conductive enclosure made of wire mesh or screens that blocks static and non-static electromagnetic fields. It works by distributing the electromagnetic waves around the exterior of the cage. Without an electric field, the electric charges within the conductive cage are evenly distributed around the material. When an external electric field is applied to the cage, it will cause the charges to immediately redistribute and cause electron flow around the cage. Thus, a secondary electric field in the opposite direction is created. Both electric and incoming fields cancel each other; hence, the net electric field is zero.
Faraday cage works differently when absorbing and attenuating magnetic fields. The magnetic permeability of a material redirects the flux lines of the incoming magnetic field. In addition to that, the movement of the incoming magnetic field induces eddy current within the conductor. These eddy currents will generate a secondary magnetic field that opposes the incoming waves. Therefore, the interior of the Faraday cage will have a lower net magnetic field. The concept of how Faraday cage is the principle behind the operation of some other types of RF shielding.
Faraday cages heavily attenuate low-frequency electromagnetic waves well. However, it is not advisable when dealing with high-frequency waves (e.g., HF RFID) as they can penetrate the shielding. The size of the cage holes must be smaller than 1/10 of the wavelength of the electromagnetic wave to be blocked.
Solid enclosures are rigid cases that block electromagnetic fields from penetrating and leaving the system. They are made of continuous metal and have relatively fewer openings; hence, they have lesser discontinuities and can block a wider range of electromagnetic waves compared to wire mesh and screens. However, solid enclosures provide little air ventilation to the shielded component. Solid enclosures are grounded to divert stray currents on their surface, eliminating electric shock.
Gaskets and O-rings
Openings such as doors and lids are placed on Faraday cages and solid enclosures to access the shielded electronic components. The seams around these openings break the neutralizing effect of the shielding, thus decreasing the shielding effectiveness drastically. Therefore, it is recommended to install RF gaskets or O-rings to seal the cage completely.
Gaskets and O-rings can be made from elastomeric materials reinforced and loaded with metal fillers. Elastomers are preferred due to their resiliency. Metal gaskets are rigid and strong, but they tend to deform under the pressures required for sealing. Gaskets and O-rings must be compatible with their mating surfaces.
A cable shielding is wrapped around and runs coaxially with the insulating layer of the power-carrying conductor. It is used in instrumental wiring to prevent external electromagnetic waves from interfering with the signals to be delivered to the instrument. It is also used in power cables to block electromagnetic waves from escaping and interfering with nearby electrical or electronic devices. Cable shielding is usually grounded. There are three types of cable shielding:
Foil shields block frequencies greater than 15 kHz. They can be made from materials such as aluminum, copper, conductive polymers, or metal-coated textiles. Since foils are thin materials, they might be damaged when excessively flexed or stretched and cause discontinuities.
Spiral shields consist of strands of copper wires spiraling around the insulator of the conductor. It is easy to install in cables and inexpensive. However, the winding can loosen and cause discontinuities if the cable is stretched, bent, or twisted.
Braided shields are made of woven wires made from bare or tinned copper, nickel, or silver. They have high strength and flexibility. They can be easily terminated when crimping or soldering to a connector. However, they do not guarantee full shielding coverage, and small discontinuities may be present. Shielding coverage depends on the tightness of the woven wires. Braided shields are recommended for blocking low-frequency waves.
Shielded vents, or sometimes referred to as shielded honeycomb vent panels, facilitate airflow and dissipate heat to cool the electronic components housed in solid enclosures. One cannot simply create openings for ventilation. They are designed to meet the air cooling requirements of the device without compromising the shielding integrity of the enclosure.
A board-level shielding blocks electromagnetic signals from penetrating or leaving the board-level components. It is available in one-piece or two-piece construction and is composed of either a cage or a solid enclosure and several pieces of RF seals and fittings.
RF shielding can be incorporated in the construction of IT, healthcare, military, banking, business, government, research, and testing facilities to avoid penetrating external RF signals. An RF enclosure secures sensitive and confidential information and blocks attempts of espionage, cyberattacks, and data and security breach. It protects the accuracy and reliability of the results and procedures performed by the electronic equipment confined in the facility.
The MRI room is a popular example of an RF-shielded facility. Magnetic Resonance Imaging (MRI) machines use strong magnetic fields and radio waves to examine body parts through imaging. Interference with external RF signals and magnetic fields distorts the images. MRI machines also emit electromagnetic radiation that can disrupt other medical equipment.
RF-shielded facilities use conductive sheets, usually made from copper, aluminum, and steel, to cover walls, ceilings, doors, windows, floors, and partitions. Conductive fittings line the seams. It is critical to cover the six sides of the room, as discontinuities defeat the purpose of the RF shielding.
Radiofrequency (RF) shielding is the practice of blocking radiofrequency electromagnetic signals that cause radio frequency interference (RFI).
RFI decreases the performance of electronic devices and can induce devastating effects.
The use of RF shielding is a measure to safeguard our devices and equipment from the harmful effects brought by RFI. RF shielding is accomplished by installing barriers around potential sources and victims of electromagnetic fields.
RF shielding materials must have high conductivity and magnetic permeability.
The common metal RF shielding materials are copper, aluminum, nickel silver, pre-tin plated steel, and Mu-metal.
Elastomers and textile fibers can also be used as an RF shielding material by making them conductive first by adding metal fillers and coating. The common elastomers are silicone rubber, fluorosilicone rubber, EPDM, and neoprene, while the common textile fibers are polyester, nylon, silk, and cotton.
The common forms of RF shielding are wire mesh and screens, solid enclosures, gaskets and O-rings, cable shielding, shielded vents, board-level shielding, and RF-shielded facilities.
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