Traveling wave tubes (TWTs) play important roles in civilian, defense, industrial, and scientific application, including communication systems, radar systems, electronic countermeasure systems, environmental monitors, and deep space exploration^[1-4]. Microstrip-based slow-wave structure (SWS) TWTs are particularly promising due to their advantages including wide bandwidth, lightweight, compact size, and suitability for mass production^[4-7]. The first meander-line TWT was assembled in a microwave oven by A. W. Scott in 1969. Later, this group designed S- and X-band meander-line TWTs in 1973^[8-9]. These early developments laid the groundwork for modern designs.

Recently, many new microstrip meander-line TWTs have been proposed, such as: the angular log-periodic microstrip meander line ^[5,10-11], the S-shaped microstrip meander line^[12], the two-dimensional annular microstrip meander line^[13], the two-dimensional ring-rod planar microstrip meander line and the W-band parallel double segment microstrip meander line TWTs^{[6, 14-15]}. In Singapore, researchers at Nanyang Technological University have studied a symmetric configuration of two V-shaped microstrip meander-line slow-wave structures (MLSWSs) in a TWT. This TWT can deliver 28 W at 32 GHz with a gain of 28 dB, driven by a 3.6 kV, 50 mA sheet beam. They also developed a planar helix SWS with straight-edge connections (PH-SEC), and test results demonstrate a strong correlation with the simulation outcomes^[16-17]. At Saratov State University in Russia, researchers have microfabricated a V-band meander SWS and conducted tests on a W-band meander-line SWS using magnetron sputtering and laser ablation processes^[18-19]. In China, a novel dielectric supported staggered dual-meander-line (DS-SDML) SWS for E band TWT has been proposed, and simulation results indicate that this TWT can produce 283 W at 75 GHz driven by 11.8 kV, 0.2 A sheet beam. Additionally, a diamond rods-supported angular log-periodic meander-line has been manufactured and tested, with transmission parameters measuring approximately −10 dB^{[10, 20]}. In USA, some zigzag meander-line SWSs have been analyzed and measured^[21]. In India, a 3D folded meander-line (FML) SWS has been manufactured and tested, demonstrating an S₁₁ value greater than −15 dB in the 90−100 GHz range^[22].

A dual-channel TWT driven by a pencil beam, featuring a U-shaped microstrip SWS, is proposed in this paper. The electromagnetic characteristics, the transmission characteristics and the particle-in-cell (PIC) simulations of this TWT are studied in section I, II, & III respectively. Section IV presents the S-parameters measurement, and section V concludes this study.

The single period of the dual-channel SWS is illustrated in Fig.1, which comprises opposing U-shaped microstrip meander-line within a cylindrical metal shell. Key geometric parameters include cavity diameter D, substrate dimensions s_y and s_x, period length p, microstrip thickness b, straight segment length of the U-shaped microstrip h, and microstrip width a.

The dispersion is the most important electromagnetic (EM) characteristic for a SWS. Dispersion analysis using CST Studio Suite reveals that the microstrip thickness b and the substrate dielectric constant have a dominant effect on the phase velocity and bandwidth, as shown in Fig.2 and Fig.3. Here, the dielectric constants of the diamond substrate, the BN substrate and the Rogers substrate are set to 5.68, 4.0 and 2.2 respectively. It can be seen in the figures, reducing b narrows the cold bandwidth and depresses the phase velocity, while, increasing the dielectric constant narrows the cold bandwidth, but promotes the phase velocity. The soild purple line signifies the beam voltage line in the Fig.3. The beam velocity is slightly faster than the phase velocity of the EM wave at operating frequency about 80 GHz.

The optimized geometric parameters are D=1.5 mm, s_y=0.254 mm, s_x=0.9 mm, p=0.21 mm, b=0.1 mm, h=0.3 mm, and a=0.035 mm.

The coupling impedance of the SWS is another critical parameter, as shown in Fig.4. The impedance increases with s_y due to the reduction in beam-SWS gap.

The dual-channel TWT （Fig.5） employs two SWSs (the upper & the lower SWSs), one beam tunnel and 2.4 mm coaxial interface. Impedance matching between the microstrip line SWS and the 2.4 mm coaxial interface, is achieved by tuning the terminal diameter D_o and D_i, as shown in Fig.6.

Next, the transmission characteristics are simulated, as shown in Fig.7. The reflection coefficient S₁₁ ＜ −15 dB is achieved over 75−84 GHz.

The PIC simulations are carried out with CST Particle Studio^[23]. The parameters are shown in Table 1. The operation voltage is determined by the phase velocity as shown in Fig.3. The operating current is balanced based on the output power, the beam diameter and the focusing magnetic field. The input power is 720 mW which is achievable with a solid source. A magnetic field B_z of 0.5 T is used to focus the pencil beam. The number of periods the SWS is 130.

Fig.8 shows an input/output signal and the fast Fourier transform (FFT) result for identical input signals at both input ports. Fig.9 shows the output power and the gain at different frequencies. The maximum output power is 18 W at 80 GHz, corresponding to a gain of 14 dB. Fig.10 shows the input power sweep.

Secondly, the output signals for dual frequency inputs (720 mV/76 GHz-port1, 720 mV/77 GHz-port3) are showed in Fig.11, which evidently demonstrates independent transmission and amplification.

The experiment system consists of a metal sleeve, two 2.4 mm coaxial interfaces, and two three-way connections, as shown in Fig.12. The three-way connections are assembled to the port positions of the sleeve and connected the SWS to the 2.4 mm coaxial interface. The 2.4 mm coaxial line must be converted into 1.0 mm coaxial interface first, and then connected to WR10 standard waveguide (1.27 mm×2.54 mm), as shown in Fig.13. At last, the vector network analyzer (VNA) with standard waveguides is used to measure the S-parameters.

Three substrates are tested. Rogers 5880: the covered copper layer is removed using a picosecond laser machine and subsequently tested^[24-25]. The SWS is shown in Fig.14. The cold test results exhibit poor transmission performance as shown in Fig.15.

Next, diamond substrate utilizing a molybdenum alloy microstrip is assembled and tested^[25]. Due to the high melting point of the molybdenum alloy, which makes it challenging to process with the picosecond laser machine, a similar result is obtained.

Finally, the quartz substrate is processed and tested. The lithography technique demonstrates high precision and is used to manufacture the microstrip SWS. Although the maximum microstrip achievable thickness is only 3 µm, the test result is relatively favorable, as shown in Fig.16. The test results are better than the simulations in some frequencies, because the copper conductivity is 5.8×10⁷ S/m instead of 4×10⁷ S/m in the simulation.

A W-band dual-channel TWT driven by a pencil beam (18.8 kV, 0.1 A) is proposed and experimentally investigated in this paper. The maximum output power in PIC simulations reaches 18 W, corresponding to a gain of 14 dB. The experiment validates that quartz substrate with lithography technique is an optimized choice for the microstrip TWTs. Future work will focus on assembly and hot testing. We anticipate that the microstrip TWT will facilitate research in high-speed communication systems.