# Microwave interference inside a leaking Faraday Cage

Let's consider a perfect cubic Faraday-cage with solid metal walls of several centimeters' thickness. Radio waves from the microwave region are fully blocked by this cage, and the fieldstrength inside this cage is zero.

What happens when six narrow slits (with a length of multiple wavelengths, and a width multiple times lower than a wavelength) are made in all the six walls of the cube? Is there anything left of the initial shielding inside? And more important: do these slits become sources of radiation themselves, as the two url's below suggest? And will the emitted wave patterns interfere with each other, forming nodes and anti-nodes inside the cage? Moreover: can this result in area's inside the cage with an even higher intensity than that of the incoming field? And finally: is positioning of the slits possible in such a way that their emitted waves cancel each other out?

These two url's suggest that slits or holes in a Faraday-cage will form new sources of radio-waves that will radiate inside the Faraday-cage:

• here is a good rule of thumb, if on a metal surface whose thickness is at least 3 skin depths and has slit whose width is less than $\lambda/20$ then a linearly polarized wave whose polarization is parallel with the slit is essentially reflected from the slit and will not penetrate. Nov 28, 2022 at 22:59
• Is this rule you mention especially for slits? Or do you mean the broadly used rule of thumb for holes in the cage walls? For what I know, one must consider the longest dimension of a hole in the wall. For a slit that is the diagonal of the slit. In my question, these diagonals are way longer than the wavelength of incoming waves. Nov 28, 2022 at 23:19

## 1 Answer

Even narrow slits are very leaky to radiation polarized perpendicular to them unless they are short compared to a wavelength. They function as slot antennas, coupling the inside of your enclosure to the outside.

Generally, inside a box, the field will have peaks and nulls. At frequencies where the box resonates, the peaks may be considerably higher in amplitude than the incoming radiation. Computing the resonant frequencies is not simple, as they are affected by the slits.

• So a leaky Faraday-cage can do more harm than good in terms of shielding? Nov 29, 2022 at 19:12
• @Birdy Yes, if a resonance is at a troublesome frequency. Nov 29, 2022 at 19:16