In a terahertz radiation detection system, an array of harmonic transmitters/receivers can be scanned across a field of view. By using an unbalanced pair of Schottky diodes in the mixer of a heterodyne receiver, the receiver can be converted to a transmitter, generating terahertz radiation at even harmonics of the local oscillator. By using local oscillators of different frequencies in the transmitter and a receiver, the receiver can detect the generated radiation. In alternative arrangements, the local oscillator of the transmitter might have the same frequency as that of the receiver but be modulated, either directly or simply as a result of the transmitter being scanned across the field of view. If modulated directly, it might be amplitude or frequency modulation, these having different advantages. A receiver can be switched between acting as a transmitter and as a receiver by turning on and off an electrical bias to the Schottky diode pair. Embodiments of the invention have the further advantages of high signal to noise ratio in the output and the use of narrow bandwidth processing of the IF output.