The paper presents the results of a theoretical study of the dispersion characteristics of electromagnetic waves (EMW) existing in a transversely magnetized antiferromagnetic (AFM) semiconductor with loss. An AFM semiconductor is an infinite bi-gyrotropic medium, the effective material parameters of which are twice negative in several frequency ranges. It was found that these frequency regions are in the terahertz range and there are four backward EMEs in them, two of which are TE waves, and the other two are TM waves.