Attenuation Measurements in the 1 to 1000 MHz Frequency Range in Wet Granite Tunnels

Attenuation measurements were made in the 1 to 1000 MHz frequency range in two wet-granite tunnels leading to an underground facility. The purpose of the measurements was to determine the vulnerability of the facility to interfering electromagnetic radiation. Continuous-wave signals were propagated within the tunnels and the signal levels at various distances from the transmitter were measured with a spectrum analyzer. Measurements were made using antennas to launch and receive the signals and also using baluns to conduct the signals directly to longitudinal metallic structures. These structures consisted of ventilation ducts and of the sheaths on power cables and coaxial cables. At low frequencies, the path attenuation was on the order of 2 dB/100 ft. (0.07 dB/m). The results show that this low attenuation probably resulted from the presence of the longitudinal metallic structures in the tunnels. The maximum path attenuation occurred at 50 MHz in one tunnel and at 200 MHz in the other (smaller tunnel). Then, at higher frequencies, it is thought that propagation occured in a waveguide-like manner. At the higher frequencies, it was found that bends in the tunnels significantly increased the attenuation. It is concluded that if interfering signals in the MHz range are anticipated, tunnels free of longitudial metallic structures should be used. To prevent interference by higher frequencies, where waveguide-like propagation occurs, tunnels should contain bends.