Q:

For a concrete to fully block an electromagnetic wave or electromagnetic radiation, what should be its dimension (height, width and thickness) with respect to wavelength? Does its distance from the EM radiation source must be considered?

- Gin (age 20)

Philippines

- Gin (age 20)

Philippines

A:

Hello Gin,

Electromagnetic waves passing through a conducting medium will be attenuated. Usually the amplitude will fall off as exp(--z/d) where d is the so-called "penetration depth" or "skin depth" and is equal to SquareRoot(2ρ/ωμ) where ρ is the resistivity of the material, ω is the frequency of the wave in radians per second and μ is the magnetic permeability of the material. [This works if you're careful to use the right unit system.] In the case of concrete the resistivity can vary by factors of 10 due to differences in water content but a typical value would be around 10 Ohm-Meters. The magnetic permeability of concrete is not much different than that of free space,

1.25x10^{-6 }m kg s^{-2}A^{-2}. Plugging in all the numbers you get the result d ~ 1535m /sqrt(f/Hz). As an example for f = 10^{6 }Hertz, d ~ 1.5 meters.

This result should be taken as very approximate. The resistivity of concrete can vary enormously depending on age, water content, aggregate, and other variables.

LeeH

*(published on 06/07/2014)*