Setup and Characterization of a Backward Wave Oscillator Delay Line Scaled Down to Centimeter- and Millimeter-Wave Ranges

The output power of backward wave oscillators and carcinotrons decrease considerably at high frequencies. This drop in power can be due to the increase in losses and manufacturing defects in their delay lines. In this study, we present measurement methods and setups developed for the characterization of delay-line performance. The developed methods were validated on two scaled-down models of a TH4229 carcinotron operating in frequency bandwidths around 10 and 100 GHz. Measurements of losses and dispersion diagrams were made using two different wave couplings with propagating waveguides and radiating horn antennas. Electromagnetic simulations were performed using finite-difference time-domain (FDTD) code. For both delay lines, the results obtained from measurements with different couplings presented a very good level of consistency with simulations. The measured losses, normalized per delay line elementary cell, were 0.04 and 0.12 dB/cell at 9.5 and 95 GHz, respectively. Accordingly, the normalized ohmic losses were extrapolated at THz frequencies and at 0.95 THz, 0.38 dB/cell was obtained. As a first approximation, only the skin-effect ohmic losses were considered. The ability to characterize delay lines using radiating coupling was demonstrated.