Faraday Cage
A closed conductive surface largely prevents the electromagnetic field inside it from passing outside; this effect is called a Faraday cage. A metal enclosure behaves the same way and can cut the range of an antenna placed inside it by tens of dB. Because the effect appears the moment the door closes, it is often mistaken for a fault in the field.
Currents induced on the conductive surface produce a counter field that suppresses the incoming field. Shielding effectiveness depends on the ratio of the openings to the wavelength: holes small relative to the wavelength do not leak, while large openings pass the signal.
- Symptom: a remote control that works with the enclosure door open and whose range collapses when it is closed.
- Half measures are not enough: putting the antenna in a plastic compartment inside the enclosure does not remove the cage effect.
- Correct solution: bring the antenna outside the enclosure through a cable gland and mount it on the outer surface.
In AXI's IP67 sets, "antenna and cable glands" are included in the set contents; this is so the antenna can be brought outside without breaking the sealing. Passing the cable through a hole and leaving the gland untightened, on the other hand, lowers the protection rating. Bringing the antenna outside removes both the cage effect and near-field detuning at the same time, and it is usually the single highest-return intervention that can be made in the field.
Context where this term is used: Wireless remote control installation: pairing, relay, safety