In this article, a 1-THz traveling-wave tube (TWT) based on flat-roofed sine waveguide (SWG) slow wave structure (SWS) has been studied theoretically and experimentally, and parts of the component have been fabricated. First, the flat-roofed SWG SWS has been optimized for working at 1 THz, the normalized phase velocity is around 0.287, and the interaction impedance is over 3.5 Ω in the frequency range of 1.02-1.04 THz. Meanwhile, the performance of the designed TWT has been simulated, whose saturated output power is 720 mW and the corresponding gain is 28.57 dB. What is more, to decrease the transmission loss of the high-frequency structure, the input and output structures have been designed and a transition structure that transforms the waveguide to standard rectangular waveguide has been proposed. The simulation results show that the reflection coefficient of the novel proposed transmission system is less than -20 dB, and the insertion losses of the transition structure with the input and output waveguides are 2.45 and 1.68 dB, respectively. Finally, the designed SWS has been fabricated by deep reactive-ion etching (DRIE) and tested; S ₁₁ is generally less than -10 dB in the frequency range of 1020-1040 GHz.

An innovative complementary electric split-ring resonator metamaterial (MTM) structure applied as the slow-wave circuit for a cascade backward-wave oscillator (CBWO) operating in C-band is studied in this paper. The idea of a drift tube in a multiresonant cavity extended interaction klystron is borrowed to design a novel backward-wave oscillator (BWO). The construction of this device features two BWOs separated by a short cutoff waveguide for permitting the flow of the electron beam and stopping the electromagnetic wave. The high-frequency characteristics are analyzed and optimized by using a high-frequency structure simulator and computer simulation technology (CST). Meanwhile, the S-parameter retrieval approach is used to retrieve the effective permittivity and permeability. In addition, the CST code is adopted to investigate the performance of the MTM-based CBWO. The particle-in-cell simulation results show that the novel CBWO is capable of achieving over 51.77% electronic efficiency from 4.8344 to 4.8687 GHz. Meanwhile, the maximum electronic efficiency can reach 82.44%, corresponding to a peak output power of 14.51 MW at 4.8466 GHz. These results indicate that the MTM-based CBWO proposed in this paper has the characteristic of miniaturization, manufacturability, and high electronic efficiency.

The $W$ -band flat-roofed sine waveguide (SWG) slow wave circuit was designed and processed in this article. Considering the limitations on present practical fabrication level, sensitivity analysis was performed on the potential influencing factors for the performance of the flat-roofed SWG slow wave structure (SWS). As indicated by the calculation results, the gap between the SWSs and the side-wall strips could have a significant impact on the dispersion, interaction impedance, and the transmission of the entire slow wave circuit. Based on the theoretical analysis, two types of $W$ -band flat-roofed SWG slow wave circuits were fabricated and tested. The side-wall strips of the two circuits are processed with/without rounded corner. The circuit without rounded corner may cause a large gap and severe reflection. In comparison, the other one with rounded corners can make the parts fit tighter, thus minimizing the potential gap. The test results demonstrate that the $W$ -band flat-roofed SWG traveling-wave tube (TWT) possesses a voltage standing wave ratio less than 1.8, which is basically consistent with the simulation results.

To improve the performance of efficiency bandwidth product (EBP) and power capacity for G-band continuous-wave traveling-wave tube (CW TWT), a novel staggered U-shaped groove sine waveguide slow-wave structure (SUGSWG SWS) is proposed in this article. The theoretical analysis of high-frequency and beam–wave interaction characteristics indicates that the SUGSWG TWT has low insertion/reflection loss, broadband (3-dB power bandwidth of over 19 GHz), and high maximum output power (over 66 W), of which the EBP could reach whopping 103% $\cdot$ GHz (about three times the state of art). In the merit of its special geometry, the power capacity of this novel SWS is also about 1.6 times its existing peers. A prototype of a two-section SUGSWG TWT is designed, fabricated, and experimented for verifications. The hot test demonstrated that the peak power of 56 W, the 3-dB power bandwidth of 16 GHz, and the EBP of 75.4% $\cdot$ GHz were achieved, with the transmission rate practically tuned around 93%. Those measurement results are consistent with the theoretical predictions under the same transmission conditions. Those studies validate the remarkable potential of our proposed novel slow wave structure (SWS) in TWTs for THz applications.

In this paper, a switchable filter based on the microstrip line (ML) with reflectionless-bandpass-to-reflectionless-bandstop responses is designed, theoretically validated, and fabricated. This single-port reflectionless bandpass filter (R-BPF) consists of a BPF and a shunt-connected bandstop section with terminated absorption resistors. The single-port reflectionless bandstop filter (R-BSF) is made of a BSF and a parallel bandpass circuit with terminated absorption resistors. These two reflectionless operational modes, namely R-BPF and R-BSF, are allowed to reconfigure the multifunctional filtering device using PIN diodes. In addition, a theoretical analysis of terminal impedance is performed to illustrate the working mechanism of the reflectionless response. To demonstrate the application of the proposed designs, a prototype of the switchable filter for R-BPF to R-BSF responses is fabricated and measured. For the R-BPF mode, the 3-dB fractional bandwidth (FBW) is 36.75% (1.67–2.42 GHz) with a 10-dB reflectionless bandwidth (RBW) of 1.36–2.58 GHz (i.e., FBW of 61.9%). For the R-BSF mode, the 10-dB bandwidth is 13% (1.85–2.11 GHz) with a 10.7-dB RBW of 1–3 GHz (i.e., FBW of 100%). An acceptable agreement between the measured and simulated results has been achieved.

The classical Pierce’s 1-D small-signal theory of backward-wave gain, generalized to allow axially varying the double-graded radius and pitch circuit characteristics, is applied. Numerical calculations demonstrate that the double gradient circuit can effectively suppress the backward-wave oscillation (BWO). Furthermore, the hot test experiment is carried out to predict the performance of the tube. The hot test results show that this helix traveling-wave tube (TWT) based on the double-graded pitch and radius circuit has achieved over 5.06 kW of output power, and the corresponding device gain achieves more than 34.1 dB in the frequency range of 8-18 GHz.