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This article introduces a radial staggered double vane (RSDV) backward wave oscillator (BWO) using a radial sheet electron beam (RSEB) to overcome the limitations of traditional Ka-band BWOs. Traditional BWOs face limitations due to a decrease in interaction space as frequency increases, leading to lower efficiency and considerable challenges in mitigating thermal stresses. In addition, these devices typically require an external magnetic field for beam focusing to counteract space charge effects, adding complexity to their fabrication. The RSDV slow wave structure (SWS) is designed, and its high-frequency characteristics are numerically analyzed. Particle-in-cell (PIC) simulations are conducted for the RSDV BWO using a converging RSEB without an external magnetic field, at a beam voltage of 20 kV and a current density of 22.43 A/cm2. The device demonstrates enhanced efficiency over traditional Ka-band BWOs, achieving an average power of 117.61 kW and an efficiency of 10.69% at 31.02 GHz. Furthermore, the performance using a diverging RSEB setup is evaluated and compared with that of a converging RSEB. Coupled particle-thermal simulation confirms the device’s capability to effectively manage thermal stresses. The device, consisting of 18 periods, is fabricated and tested; cold-test results indicate an ${S} _{{11}}$ parameter around −10 dB at desired frequencies, aligning with simulated results. Moreover, a comparison between analytical, simulated, and measured dispersion curves based on ${S} _{{11}}$ phase is provided.

The inductive output tube (IOT) remains a popular microwave amplifier for sub-gigahertz (GHz) applications due to its cost, efficiency, and size. This article presents a study on a gridless IOT with an L-shaped, high-Q cavity, with an experimental cold test of the cavity. The L-shaped cavity is designed to maintain a compact IOT diameter. The cavity is coupled to a coaxial line to obtain output power in the TEM mode, at an operating frequency of 431 MHz. The tapered configuration of the coaxial line allows for power extraction in the TEM mode, without the use of conventional TEM coupling loops. Time-domain solver in CST Studio is used to obtain the impedance locus for Q-factor calculation and optimize the geometry of the output line. Particle-in-cell (PIC) results are presented to validate IOT design, show the gridless operation of the annular beam, and analyze the beam-cavity power transfer. The IOT PIC simulation is performed with a 70-kV beam voltage, controlled by applying a modulation signal to nonintercepting modulating disks, and an axial magnetic field of 0.1 T. IOT simulations show 2.24-MW average output power, a maximum gain of 16.5 dB, and an average efficiency of 53.9%. Cavity simulations show an unloaded Q-factor ${Q}_{{0}} = {2025}$ and ${R}/{Q} = {39.4} {\Omega }$ . Experimental cold test shows ${Q}_{{0}} = {1812}$ , ${R}/{Q} = {32.7} {\Omega }$ , and an acceptable 0.9% deviation in frequency from simulation.