A Novel ${\rm TM}_{0{ n}}$-TEM Mixed-Mode Converter

Overmoded relativistic backward wave oscillators cannot preserve output mode purity, generating a number of TM
_0n
modes in general. Until now, terminations for high power microwave (HPM) devices are only matched to a single operating mode. It is quite difficult to properly radiate a combination of TM
_0n
modes using HPM antennas. In this paper, a novel efficient TM
_0n
-TEM-mode converter is proposed. With the method of power dividing and field matching described, a mixture of TM
₀₁
, TM
₀₂
, and TM
₀₃
modes generated by an HPM device can be successfully converted into a pure coaxial circular waveguide TEM mode utilizing the outer and inner coaxial waveguide structure with an efficiency of over 99% at the center frequency. Such a mode transition will greatly improve the radiation efficiency of HPM antennas.