A compact, frequency-reconfigurable filtenna with sharp out-of-band rejection in both its wideband and continuously tunable narrowband states is presented. It is intended for use in cognitive radio applications. The wideband state is the sensing state and operationally covers 2.35-4.98 GHz. The narrowband states are intended to cover communications within the 3.05-4.39 GHz range, which completely covers the Worldwide Interoperability for Microwave Access (WiMAX) band and the satellite communications C-band. A p-i-n diode is employed to switch between these wide and narrowband operational states. Two varactor diodes are used to shift the operational frequencies continuously among the narrowband states. The filtenna consists of a funnel-shaped monopole augmented with a reconfigurable filter; it has a compact electrical size: 0.235λ _L × 0.392λ _L , where the wavelength λ _L corresponds to the lower bound of its operational frequencies. The measured reflection coefficients, radiation patterns, and realized gains for both operational states are in good agreement with their simulated values.

A concentric arc meander line (CAML) slow wave structure (SWS) was proposed for applications on low voltage and high efficiency Ka-band traveling wave tube (TWT). Due to the CAML formed with a series of concentric arcs, the electric field in the radial direction may be more uniform, which may make the beam-wave interaction more sufficient and the electron efficiency improved. The metal shielding structure with symmetrical ridges was employed for impedance matching to obtain a wide bandwidth. Simulated results demonstrate that the proposed CAML-SWS TWT has a good transmission performance in Ka-band. When the initial beam voltage and current is set to 720 V and 0.2 A, respectively, the output power of the TWT can reach 44.15 W at 35 GHz with the gain and the electron efficiency of 18.6 dB and 30.66%, respectively. Furthermore, the 3-dB bandwidth can cover 24-42 GHz and the electron efficiency is over 23.2% in Ka-band.

A concentric arc meander line slow wave structure (CAML-SWS) for low voltage, high efficiency, and wide bandwidth V-band traveling wave tube (TWT) with dual sheet beam (DSB) is proposed. As a nonperiodic SWS, the CAML-SWS is utilized for obtaining low operational voltage and wide bandwidth performance. The DSB is employed for efficiency enhancement. The simulated reflection loss (| S₁₁|) is smaller than -30 dB covering the whole V-band. The 3-D particle-in-cell (PIC) simulation predicts that the output power of the proposed TWT can reach 58.6 W at 60 GHz, when the voltage is set at 940 V and the current of each sheet beam is 0.1 A. The corresponding peak gain and electron efficiency is 19.6 dB and 31.1% at 60 GHz, respectively. Furthermore, the 3-dB gain bandwidth is about 17 GHz from 51 to 68 GHz, where the output signal is steady and the frequency spectrums are quite clean.

A sheet beam electron gun with high compression ratio and long transmission distance performance for $W$ -band traveling-wave tube (TWT) is presented. A beam focusing electrode (BFE) with two pairs of unequal length outstretched structures is designed to provide equal positions of beam waist in both horizontal and vertical planes, which is beneficial to obtain large compression ratio and keep the beam cross section shape over a long distance. The operational voltage is set as 30 kV. The beam current is 0.505 A, and the corresponding cathode current density is about 5.59 A/cm ² . The simulation results show that the electron gun can provide a stable sheet beam under a 0.85-T magnet field. The compression ratio of the obtained sheet beam can be over 35, and the sheet beam can be transported through a 126-mm beam tunnel without interception with a 0.6 fill factor. The parameter sensitivities of the gun are analyzed and discussed as well.

A design method of the non-periodic meander line slow-wave structure (ML-SWS) and folded-waveguide slow-wave structure (FW-SWS) for traveling-wave tubes (TWTs) is proposed based on effective phase velocity synchronization. For the non-periodic ML-SWS and FW-SWS, the unified effective axial phase velocity seen by electrons, the dispersion equation, and the ideal beam-wave synchronization voltage are derived. The theoretical results demonstrate that the non-periodic ML-SWS and FW-SWS reported to date are consistent with the proposed method. Furthermore, two examples are presented as applications for the proposed method to the non-periodic SWS design, including the concentric arc FW-SWS and the zigzag ML-SWS. In addition, the simulated results of the two examples are in good agreement with the theoretical ones.

A beam–wave resynchronization (BWRS) method is proposed by changing the structural parameters of the nonperiodic (NP) slow wave structure (SWS) to reduce the phase velocity of the wave for traveling-wave tubes (TWT) application. The mathematical model and analysis for the BWRS method are introduced to investigate what condition the NP meander line SWS (ML-SWS) and the NP folded-waveguide SWS (FW-SWS) should satisfy so that the corresponding TWT can have a good output performance. As the applications, the NP zigzag ML-SWS and the concentric arc FW-SWS are designed according to the method, which is also compared with that designed according to the phase velocity synchronization (PVS). The simulated results show that, by using the BWRS method, the max output power (efficiency) of the zigzag ML-SWS TWT can be improved from 56.7 (18.9%) to 76.26 W (25.4%), and the bandwidth can be expanded from 6 to 10 GHz in the Ka-band. Furthermore, the max output power (efficiency) of the concentric arc FW-SWS TWT can be improved from 199 (27.6%) to 237 W (32.9%), and the bandwidth can be expanded more than twice in the W-band as before. These results indicate that the BWRS method is beneficial for the improvement of the output power and electron efficiency and may be beneficial for wider bandwidth.