Miniature Metamaterial Backward Wave Oscillator With a Coaxial Coupler

A coaxial coupler is proposed to develop a miniature metamaterial (MTM) backward wave oscillator (BWO). With respect to those using waveguide couplers as output structures, the MTM BWO adopting the coaxial coupler exhibits obvious miniaturized characteristics. For the MTM BWO, its high-frequency structure including the MTM slow wave structure (SWS) and the coaxial coupler has longitudinal length of
$\sim 1.25\lambda $
. (
$\lambda $
is the free-space wavelength at the operating frequencies.) Importantly, the transverse size of
$\sim \lambda $
/5 is only ~1/3 of those based on waveguide couplers. Furthermore, the transmission experiment is carried out. The experimental results show that the coaxial coupler with low loss property can effectively transform the TM-dominant mode in the MTM SWS to the TEM mode. The particle-in-cell simulations predict that the proposed MTM BWO yields MW-level peak output powers at frequencies from 2.85 to 2.903 GHz when the beam current is 30 A, and the beam voltage ranges from 65 to 110 kV. The simulation results confirm the feasibility of miniaturization as well as high output powers of the MTM BWO.