This paper proposes a general characteristic mode-based design procedure of simple three steps for wideband circularly polarized (CP) antenna design. First of all, the characteristic mode analysis is carried out to understand the different modes of a proposed antenna geometry without feeding network. Second, modal currents and their corresponding modal fields (radiation patterns) are studied for choosing modes to shape the required radiation pattern. Finally, a suitable feeding structure is chosen to excite the desired modes at the same time owns a good impedance matching. As an example, a CP patch antenna fed with cross-shaped aperture is proposed and designed following the design procedure. Patch consisting of H-shaped unit cells is used as the radiator. Characteristic mode method is applied to analyze the modes of the proposed antenna and explains the property of wide band circular polarization. It is revealed that higher order modes of the patch can be used to achieve circular polarization over a wideband of frequency. The antenna is fabricated using printed circuit techniques. The return loss and radiation properties are measured and compared with simulation results. With the highly coupled units, a wide impedance bandwidth of 38.8% is obtained. Besides, a wide 3-dB axial ratio bandwidth of 14.3% is achieved.

A power dependent frequency selective surface (FSS) is presented in this paper. The FSS is able to provide a passband for low power microwave signals and a stopband for high power microwave signals. Diodes play an important role to achieve this behavior due to their nonlinear property. The operation of such an FSS is studied by analyzing the equivalent circuit model, followed by simulations using circuit element and Spice model of diodes, respectively. The proposed FSS is also demonstrated experimentally. The low power experiment is carried out using free space measurements while the high power experiment is carried out using a rectangular waveguide. The experiments confirm a rapid reduction of transmitted power at power levels higher than 34 dBm and 18 dBm, respectively, with two different PIN diodes. Such a characteristic offers a great potential for use as a protection screen for electronic devices from high power microwave signals.

A Ka-band planar helix slow-wave structure (SWS) which is suitable for microfabrication is proposed and its design is described in this paper. A wideband design is achieved by using dispersion control techniques. The design shows an $S_{11}$ better than −20 dB over a 42.8% cold-test bandwidth with discrete input and output ports. Cold-test results for coplanar waveguide ports are also presented. The hot-test parameters of the SWS with discrete ports are investigated using CST Particle Studio. For an elliptical cross-sectional electron beam with the beam voltage of 3.72 kV and the current of 50 mA, a 3-dB small-signal gain bandwidth of 48.5% and a maximum gain of 42 dB are achieved. Saturated power of 26.5 W is achieved by using pitch tapering. Hot-test performance of the proposed SWS including a sever for high gain applications is also presented. Thermal simulations using CST have been carried out showing that the proposed SWS can have very good thermal dissipation properties. RF electric field values at some critical locations in the SWS have also been examined from the point of view of dielectric breakdown.

A modified double staggered grating waveguide (DSGW) slow wave structure (SWS) is proposed for millimeter-wave and sub-THz traveling wave tubes (TWTs). Gratings with a cross Section outline of exponential curves are applied instead of the traditional rectangular cross section. The proposed SWS owns a higher interaction impedance compared to DSGW, resulting in higher output power and gain. With proper design and optimization of the input–output couplers, the proposed SWS can have a wide −15 dB impedance bandwidth, extending from 192 to 230 GHz. Particle-in-cell (PIC) simulations are carried out for a TWT with sheet electron beam (SEB) and the main SWS having 70 periods. Such a TWT provides a maximum power of 116.64 W which indicates a significant improvement compared with the traditional DSGW.

The rapid development of information technology has promoted the transformation of traditional networks into intelligent networks. Huge data traffic is generated by various types of traffic services in the intelligent networks, which can easily lead to network congestion, system instability, and other problems. These problems may incur great requirements and pose challenges for queue management algorithms. Most traditional active queue management (AQM) algorithms judge the congestion level of the network based only on the size of the average queue length while ignoring the network traffic variations. This makes these algorithms difficult to achieve effective improvement of congestion control efficiency. To address this problem, a novel network congestion control algorithm, namely the average queue length and change rate-RED (AC-RED) is proposed in this paper. AC-RED can better relieve network congestion by reconfiguring the packet loss strategy model based on the average queue length change rate. The simulation results of NS2 show that in complex and dynamic network environments, the performance of AC-RED algorithm in the average queue length, packet loss rate, delay, and delay jitter is improved in most load conditions compared with the other five algorithms.