Design and Millimeter-Wave Measurement of a Wideband Power Coupling Structure for Sheet Electron Beam Devices

Two power coupling structures based on a Y-type branch waveguide (YTBW) coupler for sheet electron beam devices are presented in this paper. The YTBW coupler is a good choice for the power coupling due to its broad bandwidth property and compact configuration. However, according to the previous studies, it is difficult for a YTBW coupler to obtain good isolation, small port reflection and, meanwhile, high transmission in a broad frequency band. To address this issue, the following two approaches were proposed: 1) introducing multiple branch slots between branch waveguide 1 and branch waveguide 3 of the YTBW coupler to cancel out the electromagnetic (EM) wave propagating toward the isolation port and 2) enlarging branch waveguide 1 at the junction of the three branch waveguides to achieve a port impedance matching and guide the EM wave to propagate to the port connecting with the slow-wave structure. The measured results demonstrated that the bandwidth of port reflection lower than -12.5 dB, isolation coefficient smaller than -12.0 dB, and meanwhile, the transmission higher than -1.0 dB, was 13.5 GHz (50.0-63.5 GHz), corresponding to a fractional bandwidth of 23.8%.