**1. Introduction**

G-band electromagnetic wave provides availability for the design of terrestrial and satellite radio communication networks according to the radio regulations of International Telecommunication Union.

The European Commission Horizon 2020 ULTRAWAVE, "Ultra-capacity wireless layer beyond 100 GHz based on millimeter wave Traveling Wave Tubes", aims to exploit portions in the millimeter wave spectrum for creating a very high-capacity layer [1].

However, there are two problems. One problem is the atmospheric attenuation, which directly influences using these frequencies for long range communication [2]. The high atmosphere attenuation and the lack of enough transmission power limit the range to a few tens of meters, even by using high gain antennas [3].

Another problem is that there is a frequency range called "Terahertz Gap" between the highest frequency of microwave technology and the lowest frequency of photonic technology [4].

Vacuum electronic devices (VEDs) exhibit the advantages of high average power, high operation frequency, and high efficiency. Traveling wave tubes (TWTs), one of the most widely used VEDs, exhibit the incomparable advantage of wideband, which is much higher than that of the available solid-state devices [5]. In THz regime, TWTs are the most widely used VEDs.

In recent years, great progress has been made in the development of G-band TWT. Some TWTs operating at G-band have been demonstrated [6–11], and the performances of the TWTs are shown in Table 1.

**Citation:** Feng, Y.; Bian, X.; Song, B.; Li, Y.; Pan, P.; Feng, J. A G-Band Broadband Continuous Wave Traveling Wave Tube for Wireless Communications. *Micromachines* **2022**, *13*, 1635. https://doi.org/ 10.3390/mi13101635

Academic Editors: Lu Zhang, Xiaodan Pang and Prakash Pitchappa

Received: 5 September 2022 Accepted: 26 September 2022 Published: 29 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).


**Table 1.** Some G-band TWTs performances in recent years.

The bandwidth of most of the TWTs have narrower bandwidth (≤10 GHz), and half of them cannot operate at continue wave (CW) mode. These performances of these devices limited the applications in wireless communications, which require wider bandwidth (3 dB bandwidth ≥10%), higher operational duty cycles (100%), higher total efficiency (≥5%), higher gain (≥30 dB), and lower gain ripple (≤10 dB in band).

In order to solve the problems of the above G-band TWT and fit the requirements of terahertz communication applications, a G-band wideband continuous wave TWT is designed by Beijing Vacuum Electronics Research Institute (BVERI) and described in this article. The device is developed according to the following design routes, which is different from normal.


By the above design routes, a G-band TWT with a continuous wave output power of 8 W and a gain of 30.5 dB with 27 GHz bandwidth is realized. The maximum output power is 16 W and the bandwidth of 10 W output power is 23 GHz. The 3 dB bandwidth is greater than 12.3% of fc (center frequency). The gain ripple is less than 10 dB in band.
