Abstract: The electromagnetic waves propagation attenuation in the emi-circular?arched can not be calculated accurately by the classical waveguide?theory.?Using?the lowest?attenuation mode of the rectangular tunnel to be equivalent to that of the semi-circular arched tunnel, it can be approximately described. The equivalent results show that for the?larger size of the?semi-circular?arched tunnel, using the rectangular tunnel of same sectional area and same shape factor achieves the best equivalent results, and the error rate between the equivalent attenuation and the calculate attenuation of the frequency band 900MHz to 1000MHz is small, and the greater the frequency, the effect of more obvious; for the small tunnel, using the rectangular tunnel of same sectional area and same shape factor, or of same sectional area and fixed shape factor 1.7 is better. Studies have shown that the equivalent method is feasible.

Key words: semi-circular arched tunnel, electromagnetic wave, the lowest attenuation mode, equivalent analysis method, shape factor