An ultra broadband all-dielectric transmitarray antenna (TA) is designed based on genetic algorithm optimization (GAO) and 3-D print fabrication technology. The array element is composed of all-dielectric pillars with different sizes distributed on the two sides of a substrate. The element is operating in the out-of-band resonance (OBR) zone, whose transmission amplitude is nearly one and the phase can be easily controlled. Afterward, the geometric parameters of the elements on different positions of the TA surface were optimized using generalized Snell's Law-based GAO, which achieved broadband phase compensation and good transmission performance. As the structure is all- dielectric, it can be easily manufactured using 3-D printing technology. The TA was fed by a ridged horn, and the antenna gains are enhanced by 7-12 dB in the measuring band (9-20 GHz). Moreover, the 1 dB steady gain ranges from 14.5 to 20 GHz (up to 29%).

The realization of electromagnetically induced transparency (EIT) on metamaterials has special properties, such as strong slow-light, frequency-selection and so on, which have allowed EIT to be widely used in the fields of slow-light, optical storages and filters. In this paper, a metamaterial with two pairs of split ring resonators and one cut-wire is designed to achieve dual-band EIT effect at 0.5–2.14 GHz and 0.4–2.10 GHz with independently tunable bandwidths of 1.64 GHz and 2.7 GHz, respectively. The coupled Lorentz model is adopted to principally study the coupling characteristics between dark and bright modes. It is shown that the coupling strength between the dark and bright modes could be modulated by the coupling distance, which make the dual-band transparent window could be independently modulated by only changing the coupling distance between the bright and dark mode. The group delay and energy storage are also simulated by setting the Gaussian pulse signal passing through the EIT structure. The results show that the group delay of the designed EIT structure is 16.9 times that of the same thickness of dielectric material. The manufactured metamaterial is tested in a microwave anechoic chamber. The experimental and theoretical results are well consistent. These results could be beneficial for the development of EIT research toward some up-and-coming novel slow-light, optical storage, sensor and optical filter applications.

A lens antenna (LA) with the characteristics of ultrawide band and high aperture efficiency is proposed in this letter. Moreover, the proposed lens is automatically designed by the genetic algorithm (GA) and finite-different time-domain method, which provides a convenient solution for lens designing. Compared to the feeding ridged horn, the measured gains of the LA are enhanced by 4.4–7.4 dB in the 6–18 GHz band, and the measured aperture efficiencies are also increased by 10%–33% in the band. The maximum aperture efficiency is 92.4% at 6 GHz. Moreover, the 1 dB steady gain band is 13–18 GHz (up to 32.3%). The all-dielectric structure can be easily manufactured using three-dimensional printing technology, which endues the lens with the advantages of low cost, light weight, and easy fabrication.

Low‐profile and wideband gain enhancement Fabry–Perot cavity antennas (FPCA) is proposed using gradient Partially Reflective Surface (PRS) and gradient Artificial Magnetic Conductor (AMC). A wideband source antenna is designed with a −10 dB impedance bandwidth of 58.1% (6.1–11.1 GHz). The PRS is constructed by a complementary frequency‐Selective Surfaces (FSS) with a square hole and four small patches in a unit, which realizes phase increasing with frequency. The PRS and the AMC are gradient alone x ‐direction to achieve wideband phase compensation. While they are uniform in y ‐direction to realize better gain enhancement. Compared with the source antenna, the gain enhancement of the proposed FPCA is about 4–7 dBi. Moreover, the simulated and measured 3‐dB gain bandwidths are 7.26–10.33 GHz (34.8%) and 6.8–10.8 GHz (46%), respectively. The gain enhanced bandwidth is about 52.3% (6.5–11.1 GHz). The impedance bandwidth is 6–11.1 GHz (59.65%). By using the gradient size AMC, the profile of the FPCA is reduced from 0.5 λ ₀ to 0.2 λ ₀ , where λ ₀ is the wavelength of 8.5 GHz. The proposed FPCA has the characteristics of wide impedance bandwidth, wide 3‐dB gain bandwidth, wide gain enhanced bandwidth and low‐profile.

A dual-band gyrotron traveling wave amplifier (Gyro-TWT) can reduce the size, cost, and weight of a transmitter in dual-band radar and communication systems. In this paper, a dual-mode input coupler for K/Ka dual-band gyrotron traveling wave amplifier (Gyro-TWT) is designed. This structure is composed of two different types of input couplers, one is the coaxial input coupler for the Ka-band ${\mathrm{TE}}_{2,1}$ Gyro-TWT and the other is a Y-type input coupler for the K-band ${\mathrm{TE}}_{1,1}$Gyro-TWT. For reducing the backward wave of the${ \mathrm{TE}}_{2,1}$ mode reflecting into the Y-type input coupler to influence the operating bandwidth, a Bragg reflector with a strong mode selective characteristic is inserted between these two couplers, which can make the reflection coefficient of the ${\mathrm{TE}}_{2,1}$ mode better than −1 dB and the phase matched in the whole bandwidth, and the transmission coefficient of the ${\mathrm{TE}}_{1,1}$ mode can reach better than −1 dB. Based on the simulation results, the −1 dB bandwidth of the Ka-band ${\mathrm{TE}}_{1,0}^{⬜}$-${\mathrm{TE}}_{2,1}^{◯}$ mode input coupler reaches 3.32 GHz and the −1 dB bandwidth of K-band ${\mathrm{TE}}_{1,0}^{⬜}$-${\mathrm{TE}}_{1,1}^{◯}$ mode input coupler reaches 3.15 GHz. The designed dual-mode input coupler has the advantages of broad bandwidth and low loss and can be well used in dual-band Gyro-TWTs.

The lack of terahertz sources, especially the high-power ones, is one of the main obstacles in the developments of terahertz science and technology. A gyrotron, based on the electron cyclotron masers, is one of the most promising sources to generate high-power terahertz radiation. Therefore, a 0.42-THz gyrotron with a gradually tapered complex cavity operating at the second harmonic is designed and experimentally tested in this paper. Experimental results show that the designed gyrotron can generate high-power THz radiation of 19.2 kW at 0.42 THz with the beam voltage V₀ of 51 kV, beam current I₀ of 4.4 A and the external magnetic field B₀ of 8.03 T, corresponding to an efficiency of 8.6%. By comparison with theoretical results, it can be shown that the operating mode in the designed gyrotron is confirmed as the TE_17.4 mode. At the same time, the excitation of the fundamental TE_0.5 mode is also observed in the experiment when the beam current is increased to 7.3 A, where the operating frequency is 216.143 GHz. Through extensive experimental efforts, it is found that the designed gyrotron can operate stably at the desired TE_17.4 mode when the beam current is in the range of 3-7 A.