Permeability vs. permittivity. What it means in the real world.

Electronic devices become continuously smaller and more powerful, leading to more complex EMI environments. Shielding, grounding and EMI absorbing are popular EMI management techniques. But engineers need to select the right techniques and material sets to achieve specific targets for EMI noise reduction. There are two major material sets used to control EMI – conductive materials (foils, gaskets, shields) and absorbing materials (magnetic sheets, dielectric foams, etc.). Conductive materials provide low resistivity, allowing RF currents to travel away from protected components. Absorbing materials have specially formulated magnetic and/or dielectric properties for suppressing EMI.

For controlling EMI in near-field applications, it’s common practice to select materials with high magnetic permeability. As we move into higher frequencies with increased complexity, effective EMI control also requires knowledge and understanding of both permeability and permittivity.

Permeability is the response of a material to an external magnetic field. It is quantified by relative permeability (mr). Relative permeability consists of real and imaginary parts. The real part defines the total magnetic field in the material. The imaginary part defines the energy loss, or energy that strays from its intended path.
 

• Materials with high real permeability are effective at redirecting magnetic energy. Therefore, they protect components from noise induced by magnetic fields.
• Materials with high imaginary permeability are good absorbers of magnetic energy.

Permittivity is the response of a material to an external electric field. It is quantified by relative permittivity (er,) Relative permittivity consists of real and imaginary parts. The real part defines the total electric field in the material. The imaginary part defines the energy loss, or energy that converts to heat instead of signal or power processing.
 

• Materials with high real permittivity are effective at redirecting dielectric fields. Therefore, they protect components from noise induced by electric fields.
• Materials with high imaginary permittivity are good absorbers of electric energy.

In some cases the system’s design (such as materials used, geometry and placement) determines the electromagnetic modes of propagation, reflection and loss, and therefore  the absorber’s overall effectiveness. In these systems, the permeability and permittivity of the material play a significant role in EMI suppression – especially at high frequencies, where shorter wavelengths can cause variances more easily. Real-world applications with complex magnetic environments might require materials that affect both permeability and permittivity.  

One example is a trace line carrying conductive noise over from one component to another. Here the performance of the EMI absorber can benefit from high permeability and high permittivity. Another example is an IC circuit where two small conductive noise sources must be decoupled to retain a reliable signal. In this case, a material with high magnetic loss and low permeability is preferred. For cavity or enclosure noise reduction, permeability and permittivity should be chosen for proper impedance and wavelength matching. For shielding very small (compared to wavelength) magnetic sources such as high frequency inductors or loops in a circuit, a material with high permeability is required (permittivity here plays no significant role).

3M offers a broad range of EMI absorbers and magnetic shielding materials, with features and options that meet today’s challenges and allow for new applications. They include materials with high permeability and high permittivity for excellent EMI noise suppression.  

3M™ EMI Absorbers can be effectively used to control near-field EMI noise from 200 MHz to 10 GHz. 3M EMI absorbers in combination with 3M™ Magnetic Shielding Materials and 3M™ Electrically Conductive Tapes provide a complete solution for EMI noise control in complex electromagnetic (EM) environments.

3M completes comprehensive material testing to help provide materials with repeatable, reliable performance in the lab and in the field. Contact your 3M representative or reach out to one of our experts to learn more. We’ll help you choose the right EMI control solutions for your exact specifications.