Electromagnetic Shielding With Concrete
Most recent answer: 06/07/2014
- Gin (age 20)
Philippines
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-2A-2. Plugging in all the numbers you get the result d ~ 1535m /sqrt(f/Hz). As an example for f = 106 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)