A High Interaction Impedance Microstrip Meander-Line With Conformal Dielectric Substrate Layer for a W-Band Traveling-Wave Tube

A U-shaped microstrip meander-line (MML) with an additional conformal dielectric substrate layer (CMML) is investigated for application in <inline-formula> <tex-math notation="LaTeX">${W}$ </tex-math></inline-formula>-band traveling-wave tubes (TWTs). The interaction impedance of such slow wave structure (SWS) is at least 29% higher than that of typical N-shaped and U-shaped MML circuits with the same dimensions. Furthermore, the conformal substrate layer probably results in less energy concentration in the dielectric substrate to reduce attenuation and also reduces the chance of electron striking and accumulation on the substrate. In addition, a customized waveguide housing with input–output coupler is optimized for the relatively wider substrate width to house the SWSs and facilitate measurements. The particle-in-cell (PIC) simulation results predict that it potentially could provide maximum saturated output power 31.4 W and 21.9 dB gain at 96 GHz, with a 3-dB bandwidth of 92–98 GHz when the operating voltage is 6550 V and beam current is 100 mA, respectively. If the thickness of conformal quartz layer increases to <inline-formula> <tex-math notation="LaTeX">$30 ~\mu \text{m}$ </tex-math></inline-formula>, the maximum output power can reach over 80 W. The measured S-parameters of the proposed entire 20-period structure match the simulated one well. The measured <inline-formula> <tex-math notation="LaTeX">${S}_{21}$ </tex-math></inline-formula> is better than −5.3 dB in the frequency range of 88–102 GHz. Attenuation is about 5.9–7.7 dB/cm in the <inline-formula> <tex-math notation="LaTeX">${W}$ </tex-math></inline-formula>-band, which is better than the measured results reported before.