Abstract: In one embodiment, an underground communication system is described. The system includes a low to medium frequency transmitter and a plurality of receivers. The transmitter comprises an output coupler and a signal selection unit. The transmitter is operable to generate a low to medium frequency signal. Each of the plurality of receivers includes an antenna and at least one sensory output unit. The plurality of receivers is configured to detect a low to medium frequency signal.

Abstract: Systems and methods for communication in enclosed areas. Implementations may include a high-frequency (HF) conversion side including an HF modulator and HF demodulator which is coupled with an HF antenna. A medium-frequency (MF) conversion side including an MF modulator and an MF demodulator which is coupled with an MF antenna may also be included. The HF conversion side may be coupled to the MF conversion side at the HF demodulator and the HF modulator. The HF conversion side may be physically separate from the MF conversion side.

Abstract: Systems and methods for communication in enclosed areas. Implementations of a method may include sending a first high frequency (HF) signal using a first HF radio, receiving the first HF signal with a first medium frequency (MF) repeater, converting the first HF signal to an MF signal using the first MF repeater, and sending the MF signal to one or more second MF repeaters located in an enclosed space. The method may include receiving the MF signal with the one or more second MF repeaters, converting the MF signal to a second HF signal using the one or more second MF repeaters, and sending the second HF signal to a second HF radio. Implementations of radio communication systems may utilize MF repeaters with MF antennas which may be omnidirectional.