In addition to the above-mentioned cable-supported systems, which are suitable for underground rooms, there are also "earth current" systems where two communication devices are not directly connected to one another, but rather via extremely low-frequency electromagnetic waves that propagate in the ground.
If you have to work underground, (cell phone) signals can be routed inside using a repeater station. PMR radios or the like can also be used for communication within large caverns. Caves and especially tunnels, depending on their geometric dimensions, represent a so-called “high pass” for radio frequencies. This means that devices that transmit on higher frequencies can be better used for communication within them. Example: In a vehicle tunnel that is closed to traffic, two people face each other at a distance of 200 meters. They cannot establish radio contact via CB handheld radios on 27 MHz, although they see each other; in the same situation with PMR devices on 446 MHz, this works without any problems. The cutoff frequency for vehicle tunnels is roughly in the VHF radio range.
Some fire brigades have handheld radios in the 4m range (approx. 70 MHz) and consequently cannot communicate with them in narrow tunnels. To solve this problem, many railway and vehicle tunnels are equipped with so-called "slotted cable technology". A slotted cable is basically a coaxial antenna cable with slots in the shield. This is laid lengthways, parallel to the tunnel axis over its entire length. Via this special cable, the transmission signal can escape to a certain extent. Received signal. Together with a signal repeater that is connected to the slotted cable, radio coverage can be ensured.
Reinforced concrete jacketed rooms shield radio waves relatively strongly. In order to establish radio contact with the outside world, the signal must therefore be fed in and out using suitable technology. If the room has windows, you can receive this signal unless it is heat-insulating, metallized windows. Such shields are also very strong against electromagnetic waves.