A new method is proposed for investigating the current carrying superthermal tail using the electron cyclotron O-mode at downshifted frequency. The Letter discusses emission and transmission of radiation for propagation normal to or nearly normal to the magnetic field, with the emitting and receiving horns both located on an external side port in the equatorial plane, which requires a reflector for the O-mode radiation. A perfectly reflecting mirror is provided by the O-mode cut-off layer. Several applications are considered, and it is shown that the system permits local measurements of the tail properties.