The design, based on theory, simulation, and cold as well as hot tests, is proposed for a compact GW-level multibeam relativistic klystron amplifier (MB-RKA) with a low beam confining magnetic field in the $\textit{S}$ -band, which can operate repeatedly and stably up to 8 min. For this purpose, graphene cathodes and a microchannel water cooling system were used. For the sake of compactness, the input section of the MB-RKA was taken as a cylindrical conformal sectorial waveguide and the output section as an inner cylindrical coaxial waveguide. The experiments were performed with a magnetic field of 0.39 T. In the single-pulse operation, the output power, pulsewidth, gain, and efficiency of the device were experimentally found as 1.047 GW, 68 ns, 56 dB, and 37% respectively, when the beam voltage, beam current, and injected power were taken as 530 kV, 5.30 kA, and 2.40 kW, respectively. In the multipulse operation at a pulse repetition rate of 20 Hz, over a period of 8 min, the MB-RKA experimentally delivered the mean output power of 0.889 GW of 42-ns pulsewidth, taking the mean beam voltage and current as 551 kV and 5.14 kA, respectively.

A W-band slot array antenna based on a substrate integrated waveguide (SIW) for microwave power transmission (MPT) is proposed in this paper. By size optimization, the transition from the rectifier element to the antenna is limited to a small size. It realizes a compact arrangement of the radiating slots, which not only improves the aperture efficiency of the antenna but also makes it easy to integrate into a large-scale array. For antenna testing, a coplanar waveguide–SIW–rectangular waveguide transition structure is added at the end of the antenna, and an antenna with this transition structure is processed by PCB printing technology and measured. The measured reflection coefficient is less than −10 dB at 90–96 GHz, the aperture efficiency is greater than 60% at 93.5–94.5 GHz, the maximum gain is 13.2 dB at 93.5 GHz, and the aperture efficiency is 79%. The test results of the antenna show that the antenna has a good performance and can be applied to the MPT system as a rectenna.

A Ka-band high-power waveguide wide-slit antenna array with high efficiency and high-power capacity is proposed in this paper. The antenna uses a waveguide vertical transition structure as a part of the feed network, which improves the compactness and power capacity of the antenna. A 2 × 8 antenna array was simulated, fabricated by 3D printed technology, and measured. The measured reflection coefficient is less than −10 dB in the range of 30.4–30.9 GHz, the tested gain at 30.8 GHz is 25.8 dB, and the corresponding aperture efficiency is larger than 80%. This antenna can be applied in high-power millimeter-wave systems.

In this article, a human posture-enabled OAM-mode-reconfigurable wearable array antenna is proposed. Using the flexibility of human action, the layout of the array antenna is reconfigurable through three groups of different human postures, and the mode reconfiguration of the OAM array antenna is realized without an additional phase shift feed network. The three groups of human posture are defined as dual-arm interactive human posture, ipsilateral limb interactive human posture, and heterolateral limb interactive human posture. The simulated and measured results show that the wearable OAM array antenna based on three groups of different human postures can switch between the modes ${l} = -1$ , ${l} =0$ , and ${l} = +1$ at 2.45 GHz. The simulated mode purity is greater than 80%, and the measured mode purity is greater than 70%. The mode switching is accomplished through simple human action, reducing the complexity of the feeding network, which provides a novel technical approach to address the issues of security communication and anti-interference in the wireless body area network (WBAN).