# How can I create hindrances to radio waves?

How can I create hindrances to radio waves?

• – j.c. Nov 19 '10 at 3:33
• In elevator/lift I have seen range in my cell phone dropping to zero. Maybe you can hide there :p – Pratik Deoghare Nov 19 '10 at 6:11
• In the presence of much thermal noise, I would think radio waves stand no chance of carrying whatever information they might contain. – user172 Nov 19 '10 at 11:44

## 4 Answers

Any conductive material will block any EM radiation, including radio waves. However, the thickness of material required to fully block the radiation depends on the wavelength of the radiation and the properties of the material. With a perfect conductor, the radiation is stopped completely by any thickness of the material, but in any real material (except for a superconductor) the radiation is attenuated as it travels through the material. The strength of the radiation will fall off with an exponential decay, and it will do so more quickly in a better conductor. The important number here is called "skin depth" which is given by:

$$\delta = \frac{c}{\sqrt{2\pi\sigma\mu\omega}}$$

Where $\sigma$ is the conductivity of the material, $\mu$ is the magnetic permeability of the material, and $\omega$ is the frequency of the radiation. The skin depth is the distance into the material at which the radiation will have fallen to a fraction of $1/e \approx 0.37$ relative to its original intensity.

As you can see, the skin depth will be smaller for a better conductor or higher frequency radiation. For example, for Aluminum, radiation at 60Hz will have a skin depth of over 8 millimeters, while optical radiation around 600 THz will have a skin depth around 3 nanometers! This is why metal blocks visible light so well, yet radio waves easily penetrate into your house.

Sources:

1. "Lectures on Electromagnetism" page 280. Ashok Das; Hindustan Book Agency, New Delhi, 2004.

2. http://en.wikipedia.org/wiki/Skin_effect

• Your answer also seems to answer this question – CMR May 9 '11 at 20:12

A large sheet or volume of matter that contains plenty of freely moveable charge carriers. Examples: metal, which contains electrons not assigned to particular atoms, but free to move; plasmas which consist of electrons and positive ions free to move in space; or a cloud of charged dust particles. If you just want to slow it down not stop the radio waves, any dielectric material will do, the the index of refraction can vary greatly with frequency.

Have enough of this material to block the line of sight between all points of the emitter and all points of the sensor/observer, and more beyond that to reduce radiation sneaking around due to diffraction. To say much more, the question needs to be more specific.

In short, just build a Faraday Cage out of conducting material with a mesh size much smaller than the wavelength of the electromagnetic wave you want to block.

Let's go for a somewhat more unconventional answer. Consider this: if you have a radio wave of certain frequency and wavelength, you can "cancel out" its effect by emitting another wave of same frequency, wavelength and amplitude, just offset by half of its period. Something like this:

(forgive the crude drawing)

The two waves would neutralize each other–so to an observer there would be effectively no radiation.