Propagation Characteristic of Electromagnetic Waves in Empty Circular Tunnels

The formulas of EM wave attenuation rate and the mode equations in an empty circular tunnel are given, and they are solved through numerical analysis method, and the main conclusions we obtain are as follows: when the conductivity of tunnel is high, the attenuation rate obtained from conventional metal wave guide approximation is correct; if frequency is higher, the attenuation rate is smaller; it is beneficial for EM waves to propagate when tunnel radius is larger; for TE modes, the changes of dielectric constant and magnetic permeability of tunnel have little effect on that of the attenuation rate; for TM modes, the dielectric constant or magnetic permeability of tunnel is bigger, the attenuation rate is higher.