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Shielding Effectiveness

We all know that microwave ovens heat liquids and cook food in record time, but very few of us question why the microwave oven itself does not get hot. This article will use example calculations to demonstrate why a microwave oven does not get hot. The standard microwave oven is a closed cavity, and the waves within it are not plane waves. Nonetheless, if we take a sheet of metal that is of the same thickness and area of the microwave and expose it to a time-averaged power density (Pav), say 1200 watts, the same as our food would be subject to, we can approximate the conditions within this cavity. Let us assume that the total walls of the microwave oven cavity is 1m2, a medium-sized oven, operating frequency at 2450 MHz with walls made of 1mm steel with a conductivity of 0.11E7 and a relative permeability of 200. The timed-averaged power density that pings on the steel sheets perpendicularly is 1200 W/m2. We expect little RF penetration because steel is a good conductor, but if we calculate the power dissipated into the steel sheet by first calculating the incident Electric field (Ei1) and the incident Magnetic field (Hi1) intensities at the steel interface, we can show why our microwave oven does not get hot. The incident Electric field (Ei1) and the incident Magnetic field (Hi1) can be calculated from time-averaged power density (Pav),and ?0, the intrinsic impedance of air, using Equations 1 and 2 below:

We discussed incident angles using Snell’s Law in our recent blog article “Shielding Effectiveness Testing & Incident Angles”, and examined electromagnetic waves encountering a boundary that is composed of different homogeneous materials, with intrinsic impedances, ?1, ?2, and ?3. We also calculated percentage of reflected power losses when an electromagnetic wave strikes the aforementioned mediums at a particular operating frequency, and plotted the percentage of Reflected Power vs. Frequency for various incidence angles. In this article, we will calculate a specific incident angle, Brewster’s Angle,?b, in which the reflected wave is canceled when the electric field component of a parallel-polarized (Transverse Magnetic) uniform plane wave strikes the plane of incidence as in the figure below.

In our last blog article about “Boundary Surfaces of Mediums in Shielding Effectiveness Testing”, we examined electromagnetic waves at three frequencies, 900 MHz, 1850 MHz and 2800 MHz, encountering a boundary that is composed of different homogeneous materials ?1, ?2, and ?3. In this article, we will examine the effects of varying the incident angle of electromagnetic waves on homogenous materials. We will use the example in our last article to calculate percentage of reflected power losses when the electromagnetic wave strikes air?fiberglass panel?air mediums at 1850 MHz, with the fiberglass panel thickness, d(1850 MHz) = 5.1280 cm. With their optical properties, electromagnetic waves are subject to refraction, reflection, diffraction and interference, just like light waves. When they bend around objects, it is referred to as refraction; when they bounce, it is a reflection; how they scatter is diffraction, and when they collide, it is interference. Using Maxwell’s equations, the above phenomenon can be eloquently explained. Snell’s law defines refraction through mediums; this includes the angle of incidence ?1, the angle of reflection ?2, and the angle of refraction ?3, including the refractive indexes n1 and n2 of the medium in the figure below:

We are sometimes asked to perform material shielding effectiveness tests such as transparency (a material’s inability to impede electromagnetic waves), attenuation (a material’s ability to impede electromagnetic waves) and reflection (the level of the incident electromagnetic wave reflected by the material). Other materials include conductive cloth used in Sensitive Compartmented Information Facility (SCIF) rooms (pronounced “skiff rooms”), enclosure composite domes for RADAR antennas, as well as materials used in the wireless industry for concealment of PCS cellular towers. Tower materials are typically made of fiberglass, polycarbonate plastic, and Styrofoam polystyrene panels. Electromagnetic waves encountering a boundary that is composed of different homogeneous materials will split into reflected waves propagating back to the first medium and into transmitted or refracted waves proceeding into the second medium. This is exactly the case when PCS wireless signals communicate with cell towers that are designed with the aforementioned panels in order to make them aesthetically-pleasing. Boundary surfaces exist between regions of different permittivity, conductivity, or permeability, e.g. air and fiberglass, air and gold, and air and metal respectively. Other applications involve boundary surfaces in which metal surfaces have been coated or dielectric materials used to either reduce reflection and/or to improve coupling of electromagnetic waves.

 

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