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Folded waveguide (FW) traveling-wave tubes (TWT) is a typical kind of vacuum electron device (VED) with some unique characteristics of high frequency, high power, and moderate bandwidth, which are widely used in radars, communication systems, electronic countermeasure, and so on. For the application to the demonstrations of ocean communication with the frequency of over 100 GHz and continuous-wave power with 15 W, an FWTWT operating at the frequency of 108 GHz and the CW power with 50 W is designed. The optimizations of dispersion characteristics and structural parameters of this FWTWT are optimized through a detailed theoretical analysis and a particle-in-cell (PIC) simulation, respectively. For the improvements of communication quality by the increase of output power, a novel method of phase-velocity tapered (PVT) FW slow wave structure (SWS) is used. Comparing with the conventional FWSWS, the output power with the proposed scheme is enhanced 1.5 times. Moreover, the effects of angle and kinetic spreads of electron beam velocity on output power are first investigated using PIC simulations. On the other hand, for obtaining the optimum signal transmission, a tapered and modified pill-box window is presented and used as the transmission structure, which is the characteristics of the broad bandwidth and low attenuation of transmission. With the help of optimized parameters, the CW FWTWT operates at the frequency of 108 GHz, and the maximum output power, gain, and bandwidth are obtained at 78.6 W, 31.9 dB, and 11 GHz, respectively. The optimum results are beneficial to the development of CW FWTWT. [ABSTRACT FROM AUTHOR]

Multiloop retrodirective cross-eye jammers offer the possibility of both simplifying system implementation and improving system performance. However, the signals for each jammer loop propagate along different paths, leading to potentially detrimental effects on system performance. Static and dynamic compensation for path-length differences are introduced and analyzed. Of the two, static compensation is simpler but is only effective for limited engagement geometries. Dynamic compensation is more general but requires an accurate estimate of the engagement geometry. [ABSTRACT FROM AUTHOR]

We present the concept and the design of an antenna-based device for restoring the reflection from a ground plane over a broad incidence angular range, when an arbitrarily large object is placed on it. The proposed device is based on fully passive antenna arrays, whose elements are properly connected to each other by following a mirrored version of the Van Atta configuration used for realizing retroreflective arrays. Thanks to its self-adaptive behavior, the device allows faithfully emulating the reflection from a flat metallic plane, regardless the incidence angle of the illuminating field. However, the antennas reduce the amount of reflected energy due to their intrinsic limits in absorption. Except for this aspect, it acts in a similar fashion as a carpet cloak, restoring the signal reflection from the ground, even in the presence of an extremely electrically large object. The concept is numerically and experimentally verified by covering an electrically large metallic triangular bump ( 2.8\lambda 0\times 5.6\lambda 0 ) with a pair of equal antenna arrays. The proposed design may find application in radar and antenna systems as a thin, lightweight, and easy to design and fabricate solution for concealing electrically large objects over a wide frequency and angular bandwidth. [ABSTRACT FROM PUBLISHER]

An equivalent circuit based method for synthesizing miniaturized frequency selective surface (FSS) using lumped reactive components has been proposed. Compared with similar research, the proposed method allows us to determine FSS structural parameters and lumped reactive component values from the desired frequency response, systematically. Details and equations of the proposed method are discussed and provided. For verification, a miniaturized FSS composed of gridded square loop has been designed. By loading lumped inductors and capacitors, the designed FSSs can provide a first-order pass-band operating at 2 GHz with a bandwidth of 0.5 GHz. The designed FSS has been analyzed by both full-wave simulations and experimental verifications. Good agreements between the simulated and the measured results can be observed. [ABSTRACT FROM PUBLISHER]

This paper presents the results of a study, implemented for the equivalent circuit model (ECM), concerning the bandwidth in square-loop frequency-selective surfaces (FSSs). For the square-loop ECM, there is no formulation to quantify the effects of dielectric supporting layers, geometric features, and incident wave properties on the FSS bandwidth. Hence, it is not possible to control or evaluate the bandwidth of the FSS in the design from the knowledge of these parameters. In this paper, we addressed this issue using the ECM as an analysis method. In the first part of this study, we considered the square-loop FSS with thin dielectric supporting layers, where the bandwidth was expressed as a function of geometric parameters, incidence angle, and polarization of the incident wave as well as a factor that takes into account the effective permittivity of the dielectric support. Particle swarm optimization method was used to synthesize square-loop FSSs via the ECM with a specific resonant frequency and bandwidth requirements. In the second part, the behavior of bandwidth as a function of dielectric thickness was obtained by using a matrix cascading technique (MCT), implemented with ECM and a model for the dielectric effective permittivity. This approach takes into account the modifications on bandwidth due to multiple reflections on dielectric boundaries and allows calculating its value for any dielectric thickness and configuration. It is remarkable the low computational cost and acceptable accuracy obtained with the proposed approaches. All simulation results using ECM and MCT were compared with Ansys-HFSS commercial software simulations. [ABSTRACT FROM AUTHOR]

In this work, a single chip integrated architectural solution for a standalone electronic attack (EA) system based on DRFM repeater is proposed without the need for a separate IFM receiver. This integrated architecture proposes to employ bandpass sampling based subnyquist discrete time transceiver front-end to estimate the carrier frequency and bandwidth of the incoming pulse radar signals. The proposed system is verified for linear frequency modulated, constant carrier frequency and phase coded pulse radar signals and they are classified on the basis of Wigner–Ville distribution. The proposed system is validated from 1 to 10 GHz with instantaneous bandwidth of 300 MHz, dynamic range from −60 dBm to −20 dBm and system gain from 30 dB to 60 dB. The minimum delay achieved between reception and retransmission is 13 pulse repetition intervals (PRI). To the best of authors' knowledge, this work reports the first subnyquist bandpass sampling based single chip architectural solution for a standalone EA system where RF front-end and digital controller are integrated into the system. [ABSTRACT FROM AUTHOR]

Forward looking sonars (FLS) are nowadays popular for many different applications. In particular, they can be used for Automatic Target Recognition (ATR) in the context of Mine Countermeasures. Currently, ATR techniques are applied to raw data which generates many false positives and the need for human supervision. Mosaicing FLS data increases target contrast and thus reduces false positive rate. Moreover, it implies a considerable data size reduction which is important if one thinks of exchange of data in real time through an acoustic channel with very limited bandwidth. Results of applying a real-time mosaicing algorithm to FLS data generated during Mine Countermeasures missions are shown and discussed thoroughly in this article. [ABSTRACT FROM AUTHOR]

Distributed Denial of Service (DDoS) attacks exhaust victim's bandwidth or services. Traditional architecture of Internet is vulnerable to DDoS attacks and an ongoing cycle of attack & defense is observed. A recent attack report of year 2013--'Quarter 1' from Prolexic Technologies identifies that 1.75 percent increase in total number of DDoS attacks has been recorded as compared to similar attacks of previous year's last quarter. In this paper, different types and techniques of DDoS attacks and their countermeasures are surveyed. The significance of this paper is the coverage of many aspects of countering DDoS attacks including new research on the topic. We survey different papers describing methods of defense against DDoS attacks based on entropy variations, traffic anomaly parameters, neural networks, device level defense, botnet flux identifications, application layer DDoS defense and countermeasures in wireless networks, CCN & cloud computing environments. We also discuss some traditional methods of defense such as traceback and packet filtering techniques, so that readers can identify major differences between traditional and current techniques of defense against DDoS attacks. We identify that application layer DDoS attacks possess the ability to produce greater impact on the victim as they are driven by legitimate-like traffic, making it quite difficult to identify and distinguish from legitimate requests. The need of improved defense against such attacks is therefore more demanding in research. The study conducted in this paper can be helpful for readers and researchers to recognize better techniques of defense in current times against DDoS attacks and contribute with more research on this topic in the light of future challenges identified in this paper. [ABSTRACT FROM AUTHOR]

Predicting the propagation path of a network worm is highly beneficial for taking appropriate countermeasures in advance. Traditional worm propagation models mainly deal with the total number of infected hosts during a period of time, which cannot indicate a worm's track. We choose the worm using random scanning to study, as it was the basic type and all the others were derived from it. A novel model proposed in this paper locates the subnets going to be infected at a given time based on the infection measurement of the subnet. The time and frequency for victims in the subnet to increase were calculated according to common characteristics of worm diffusion and the relationship between malicious traffic and bandwidth usage. Taking the two factors above as input, fuzzy reasoning was adopted to deduce the real-time infection situation for each subnet. The bigger the value of infection situation, the more likely the corresponding subnet would be attacked in a short time. Simulation experimental results show that the model estimates the worm's track dynamically with acceptable accuracy. Furthermore, the increase interval of victims in subnet is much longer for worm with slower spread speed, which provides sufficient time to carry out pertinent response. [ABSTRACT FROM PUBLISHER]