The electromagnetic waves in a partially-filled plasma waveguide, placed in an external d.c. magnetic field are numerically and experimentally investigated. Compared with the homogeneous filled waveguide, two new effects are observed. The inhomogeneous structure improves the cyclotron mode resolution. When the plasma filling of the waveguide is decreased, the coupling of both the EH and HE high-frequency modes with the cyclotron modes is possible. It is shown experimentally that increasing both the magnetic field and the discharge current (i.e. the plasma density) improves the cyclotron mode resolution.