A novel broadband multimode extended interaction klystron (EIK) circuit is proposed to overcome the bandwidth limitation of the conventional single-mode EIK. The uniform-gap and stagger-gap extended interaction circuits are comparatively studied; the multimode operation is demonstrated; stability and feasibility of this multimode circuit are analyzed. A −3-dB bandwidth of 860 MHz (2.51% relative bandwidth) in Ka -band is predicted by a particle-in-cell simulation. Meanwhile, comparative study on the uniform-gap and stagger-gap circuits also explains the working mechanism of the conventional single-mode ( ${\pi } $ mode) stagger-gap circuit.

A slow-wave structure named coupled double folded waveguide (CDFW) is proposed for traveling wave tube operating at terahertz region. Its electromagnetic characteristics and beam-wave interactions are presented in this letter. This structure, evolved from a single folded double ridge groove waveguide (DRGW) by intersecting the groove along its transverse direction, can operate at high-order TM ₂₀ resulting in increased horizontal dimension. Furthermore, this novel CDFW can interact with two pencil beams and have high beam-wave coupling impedance, which is beneficial to output high power at terahertz band. The proposed CDFW, when applying a dynamic velocity taper technique, can produce an output power of 210 W at 340 GHz with a 3-dB bandwidth of 13 GHz, and it is 80 W higher than a folded DRGW with a single beam at the same condition, when the beam voltage and beam current are 27.98 keV and 0.25 A, respectively.

This paper proposes a wideband differential fed CP antenna (48% overlapped bandwidth, which is the minimum value of AR bandwidth, gain bandwidth and impedance matching bandwidth) with multiple beams radiation across V- and W-band enabled by a parabolic reflector, a long-slot radiator and a wideband 3D printed polarizer. The long slot structure is an array consists with SIW based open slot elements (the vias besides a conventional SIW slot are moved, to form an open slot), which yields wideband characteristics when compared with traditional slot array. Cooperating with wideband parabolic reflector and modified 3D printed polarizer, it finally achieves 48% CP bandwidth. Finally, by setting multiple ports on the feeding network, multi-beam radiation can be achieved. For validation, a multi-beam CP antenna operating from 58 to 94.5 GHz with 20.5 dBic peak gain level and three radiation beams is fabricated and measured. Based on the authors’ extensive research, the proposed antenna yields a superior bandwidth when compared with recently reported mm-wave long slot structures and high-gain CP antennas.

The $\pi $ -mode operation in the extended interaction circuit is studied by the numerical simulation. Circuit features like dispersion characteristics, $S$ -parameters, external quality factors, characteristic impedance, coupling impedance, frequency interval, and electric field distribution are analyzed. It shows that the uniform-gap circuit with conventional coupling method could not support the $\pi $ -mode operation, instead, the stagger-gap circuit could. The analysis is then verified and applied in the comparative circuit design of a $W$ -band $\pi $ -mode-operating extended interaction oscillator (EIO) and a $W$ -band $\pi $ -mode-operating extended interaction amplifier (EIA). Moreover, the test results are well consistent with the simulation results. Not only the single-mode ( $\pi $ mode) operation but also the multimode operation in the extended interaction circuit is preliminary studied.

A novel methodology for the beam-wave division/combination in a sheet beam traveling wave amplifier (SB-TWT) is proposed. The design, microfabrication, and measurement of an H -band (220–325 GHz) input–output coupling structures, which originates from an E -plane Riblet coupler with one shorted port, are presented in this article. To experimentally validate the proposed methodology, the designed input–output coupling structure was manufactured by using nanocomputer numerical control milling and measured by using a vector network analyzer. A 3-dB transmission bandwidth of ∼94.0 GHz (231.0–325.0 GHz, 33.8% fractional bandwidth) was measured, over which the measured reflection and isolation coefficients were smaller than –15.3 dB and –11.6 dB, respectively. Compared with other previously reported input–output coupling structures for SB-TWTs, this structure is highlighted by its superior comprehensive performance, including compact geometry, insensitive to the manufacturing tolerance, and good electrical performance over a wide frequency bandwidth, which makes it especially applicable in the terahertz-band operation.

A novel extended interaction oscillator (EIO) capable of continuous multimode operation is proposed to make the leap from the single-mode operation and continuous dual-mode operation to the continuous multimode operation and overcome the continuous electric tuning range limitation of the conventional EIO. The “cold cavity” simulation and theory demonstrate the possibility of a stable continuous multimode operation. A −3 dB continuous electric tuning range of 4.15 GHz (4.3% relative bandwidth) with a center frequency of 95.74 GHz is predicted by the “hot cavity” simulation. Meanwhile, a comparative study on the uniform-gap and stagger-gap circuit explains the working mechanism of the conventional stagger-gap circuit with a single-mode ( $\pi $ mode) operation.