What happens if I put the receiver antenna inside a metal enclosure?
A conductive enclosure behaves like a Faraday cage and short-circuits the antenna's near field; with the cover closed the received signal can drop by tens of dB and cut range down to a few metres. Since a 6 dB loss halves the range, an attenuation of 20 dB reduces the theoretical range to roughly one tenth. The antenna must always be brought outside the enclosure through a cable gland.
A metal housing does two separate kinds of damage. The first is shielding: a closed conductive box traps the radio wave inside and also suppresses the field arriving from outside. The second is impedance degradation: metal a few centimetres from the antenna body loads the antenna's near field, shifting its resonance and lowering radiation efficiency.
In practice you see the following:
- A system that works with the enclosure cover open and drops to a few metres of range when it is closed.
- A link that cuts out when the operator changes direction and comes back at the same spot.
- The braid of the antenna cable starting to radiate and range changing with where the operator's hand is.
The correct practice is to move the antenna outside the enclosure, preferably to the top surface and at least a quarter wavelength away from the metal body. In AXI's IP67 sets the set contents list includes antenna and cable glands as separate items; this is so that you can take the antenna outside the enclosure. When bringing the antenna outside, do not forget that the extension cable also brings loss: about 0.44 dB/m for RG58, 0.55 dB/m for RG174 (around 400 MHz). So 3 m of RG58 costs about 1.3 dB; that does not even come close to the price of leaving it inside the enclosure.
Full article: 433 MHz or 2.4 GHz?