Directional modulation (DM) is an emerging technology for securing wireless communications at the physical layer. This promising technology, unlike the conventional key-based cryptographic methods and the key-based physical layer security approaches, locks information signals without any requirements of keys. The locked information can only be fully recovered by the legitimate receiver(s) priory known by DM transmitters. This paper reviews the origin of the DM concept and, particularly, its development in recent years, including its mathematical model, assessment metrics, synthesis approaches, physical realizations, and finally its potential aspects for future studies.

This paper analyses the effects of receiver non-linearity on the performance of the most commonly utilized signal detectors in cognitive radio systems. The analysis covers both self-modulation products of a single orthogonal frequency-division multiplexing (OFDM) signal and intermodulation (IM) products of two OFDM signals, and also their contribution to the probability of false detections. As a result, this work presents the linearity requirements for the spectrum sensor receiver front-end as a function of the sensitivity of the signal detector. Furthermore, we show that the cyclostationary feature detectors are more robust than the energy detectors against IM products of multiple interferers. Theoretical results are verified in measurements with a cyclostationary feature detector using digital video broadcasting – terrestrial (DVB-T) signals as an example.

Wireless network has been emerged as one of the most promising technologies for sensor-integrated applications so far. The ever-increasing demand in long-term chronic monitoring of vital or essential signals is driving a technology revolution in dealing with critical issues of clinical, healthcare, fitness, and wellness, creating multidisciplinary collaborations to benefit mankind. The voltage-controlled oscillator has been one of the important building blocks in this regard. In this paper, we report on study concerning the design and implementation of such a key circuit, with particular emphasis on a μW-level low-power design. All the aspects regarding the oscillator are detailed. The proposed circuit structure and experimental results justifying our work are given as proof of concept.

A novel microstrip low-pass filter (LPF) using radial patched resonator and modified T-shaped resonators is designed to achieve ultra-wide stop-band. Also, the designed LPF has successfully addressed the problems of compactness and insertion loss, simultaneously. LC equivalent circuit of the proposed filter is calculated and equation for the sharpness in transition band is obtained based on LC equivalent circuit and transfer function. The proposed filter not only exhibits a very wide stop-band of 168%, but also is able to suppress the 11th-harmonic response in conjunction with a small size of 0.147 × 0.052λg, where λg is the guided wavelength at 1.47 GHz. It is shown that the simulated results are highly consistent with the measured data.

A novel ultra-wideband Microstrip bandpass filter using Radial Stub Loaded Resonator (RSLR) and interdigital coupled Lines is presented in this paper. The RSLR and decagonal patch form a resonator named M to create tuneable multiple notches in the passband for the suppression of Wireless Local Area Network systems’ interference. To realize sharp roll-off, two adjustable transmission nulls are located at the lower and upper frequencies out of the passband. Even and odd-mode analysis is presented to calculate modal resonance frequencies contributing to create the ultra-wide passband. Bessel function is employed to estimate the input impedance of the radial stub which is used in formulation of even and odd mode admittances. Simulated and measured results show a passband from 3.4 to 12 GHz (135% bandwidth around 6.39 GHz) with a rejection notch at 4.99 GHz is achieved with return losses better than 14 and 12 dB in the first and second passbands, respectively. The insertion loss is lower than 0.9 dB in the 90% of passband and rejection level is greater than 19 dB from 12.25 to 17.45 GHz.

In this paper, a novel multi-mode resonator is presented, which is formed by cascading several open-circuited transmission line sections with a coupled-line section. Owing to its symmetry, even- and odd-mode analysis methods are applied to analyze its resonance characteristic. Based on this resonator, a microstrip ultra-wide bandwidth (UWB) bandpass filter is designed, fabricated, and measured. The simulated and measured results show that its bandwidth can cover the desired UWB. Return loss in passband is better than −14 dB. This filter is featured by good selectivity and wide stopband. Stopband suppression as low as −40 dB can be achieved within frequency range from 12 to 16 GHz.

In this paper, by using composite right and left handed transmission lines and non-radiating CRLH-TLs, a dual-band 3-way series power divider (SPD) is presented. The proposed dual-band divider is designed and fabricated on an inexpensive FR4 substrate of thickness h = 0.79 mm and a relative dielectric constant of εr = 4.4 to work at 0.915 and 2.440 GHz. Also, in order to improve the isolation characteristics of this divider, three isolation resistors are implemented among output ports. Derived formulas are presented in details. The dual-band nature of the designed SPD, good input return loss, as well as equal power split among the three output ports, flexibility of spacing the output ports arbitrarily apart, and easy integration with planar devices make the proposed divider well suited for feeding many dual-band microwave circuits, such as multipliers, mixers, and planar array antennas. In addition, due to its excellent return-loss at the output ports and its high isolation, the proposed divider in this paper can be used as a power combiner.

Phase shifters are indispensable microwave components. In this paper, a dual-frequency, passive, analog, and reciprocal phase shifter is proposed, deploying the phase-delay characteristics of complementary split-ring resonator (CSRR). A transmission line is loaded with a pair of CSRR in the ground plane and the phase variations are compared with an ideal transmission line. The proposed phase shifter operates in the industrial, scientific and medical (ISM) and wireless local area network (WLAN) bands, providing a phase of 180° at 2.4 GHz and 90° at 5.4 GHz for beam steering applications.

In this paper, a novel design of generalized dual-band quadrature hybrid with arbitrary power division ratios is presented and investigated. This coupler has a simple structure and can cover a wide range of power division ratios at the two designated frequencies. Exact closed-form equations are derived based on the even–odd-mode analysis method. The proposed structure offers some special features, encompassing: (1) a generalized structure of some recently published papers; (2) isolation in vertical and horizontal directions in the two-dimensional structure; (3) the same sign of the output phase difference for the structure with horizontal isolation and the opposite sign for the structure with vertical isolation; (4) large power division ratios for both vertical and horizontal isolation conditions; (5) large frequency band ratio for both vertical and horizontal isolation conditions; (6) capability of the bandwidth improvement by exploiting coupled lines with small coupling coefficient. For demonstration, a microstrip implementation of a 16:1 dual-band coupler working on 2.45/5.8 GHz is developed. Consistency between simulation and measurement results justifies the proposed design method and its equations.

The gain properties of an axially slotted cylinder coated with two lossless layers and embedded in a certain lossless background medium have been investigated analytically. The effects of different types of coating layers, i.e. dielectric, magnetic, and Single Negative materials upon the gain characteristics of an axial slot have been discussed. It is observed that if inner layer is either of dielectric or magnetic type and outer layer is an Epsilon Negative type having its relative permittivity of negative one and slotted cylinder is embedded in the free space then the gain has directive pattern. It is also studied that if both layers are dielectric or magnetic types and embedded in a Mu Negative medium then its gain pattern becomes more directive with the maximum gain in the forward direction.

A novel design of a compact ultra wideband antenna with triple band-notched characteristics is proposed. Much wider impedance bandwidth (from 2.63 to 13.02 GHz) is obtained by using a star like-shaped radiator and a defected rectangular ground plane and band-notched functions are obtained by attaching L- and I-shaped structure on the ground and a capacitive-loaded loop (CLL) resonator on the patch. The triple band-notch rejection at WiMAX, WLAN, and ITU bands are obtained by attaching I-shape strip, CLL resonator, and flip L-shape, respectively. The parametric study is carried out to study the influence of varying dimensions on the antenna performance. To validate simulation results of the design a prototype is fabricated on the commercially available FR4 material. The measured results reveal that the presented triple band-notch antenna offers a very wide bandwidth of 10.41 GHz (2.63–13.04 GHz) with triple band-notched characteristics at WiMAX (2.94–3.7 GHz), WLAN (5.1–5.9 GHz), and ITU (7.4–8.7 GHz).

This paper presents design, fabrication, and testing of a palm tree structured monopole antenna for wideband applications. The proposed antenna has a wide impedance bandwidth (−10 dB bandwidth) from 4 to 10.4 GHz. Palm tree antenna of compact size 23 mm × 20 mm is designed and fabricated on an FR4 substrate of thickness 1.6 mm. To validate the design, a mathematical relationship between the parameters of the palm tree geometry and the lower cut-off frequency has been established. Parametric study has been carried out to obtain optimum wideband characteristics. The prototype is experimentally validated for the band 4–10.4 GHz within ultra-wideband operations. Transfer function, impulse response and Group delay has been plotted in order to address the time domain characteristics of the palm tree antenna with fidelity factor values. The possible applications cover 5.2–5.8 GHz WLAN, C-band operations, 5.5 GHz WiMAX, and Wireless USB.

A wideband orthogonally slot cut annular ring microstrip antenna fed by L-shaped probe is investigated using modal expansion cavity model and circuit theory approach. Simulation of the proposed antenna is performed using Ansoft HFSS and simulated results are compared with the measured and theoretical results. The impedance bandwidth of about 37.46% is observed at resonant frequency 3.15 GHz. The proposed antenna realizes an improvement in bandwidth of 13.46% and miniaturization in physical dimension about 10% from earlier reported structures. An improvement in bandwidth and miniaturization is due to thick substrate, L-probe feed, and orthogonally loaded slots. The measured results of fabricated antennas are in good agreement with simulated and theoretical results.

A novel compact asymmetric coplanar strip fed planar antenna with defected ground structure and defected microstrip structure for dual band application is presented. The proposed antenna is composed of defect in both ground plane and radiating strip. The antenna has an overall dimension of 21 × 15.35 × 1.6 mm3 when printed on a substrate with dielectric constant of 4.4 and loss tangent of 0.02. The antenna resonating at two different frequencies of 3.5 and 5.5 GHz is covering worldwide interoperability microwave access and wireless local area network bands. The planar design, simple feeding technique, and compactness make it easy for the integration of the antenna into the circuit board. Details of the antenna design, simulated, and experimental results are presented and discussed. Simulation tool, based on the method of moments (Mentor Graphics IE3D version 15.10) has been used to analyze and optimize the antenna.

A novel asymmetric “+” shaped fractal slotted circularly polarized microstrip antenna with a Yagi–Uda structure is proposed. Four asymmetric plus shape slots are embedded symmetrically in the center of all four quadrants of a square patch. To suppress undesirable higher modes, dumbbell-shaped defected ground structure (DGS) is introduced at the ground layer of the antenna. We introduce a method to compensate the reduction in gain occurring due to the presence of DGS, without changing in the overall size of the antenna. A 3 dB axial ratio bandwidth of 4 MHz at center frequency of 862 MHz, 10 dB impedance bandwidth of 13.20 MHz and a gain of 4.25 dB is achieved with the proposed antenna. A laboratory prototype of the proposed antenna is made to cross-verify the simulation results. Very good agreements between the two are obtained. The proposed antenna may prove useful for International Mobile Telecommunication application for designing high-gain arrays.

Since a conventional microstrip patch antenna is inherently a narrowband radiator, stacked-patch antennas are commonly used either to enhance the bandwidth or to achieve multi-band characteristics. However, the stacked patch structure has a number of geometrical variables which need to be optimized to achieve the desired characteristics. The conventional design procedure involves repeated costly and time-consuming simulations on an electromagnetic simulator to optimize the various geometrical parameters to arrive at the desired radiation characteristics. In this paper, the task of stacked patch antenna design has been approached as an optimization problem. In order to make a faster CAD module for the stacked-antenna design problem, the simulator has been replaced by a trained artificial neural network (ANN) and embedded in a particle swarm optimization algorithm (PSOA). The ANN is helpful in constructing the “function mapping black-box”, which can relate the frequencies and associated bandwidths of the antenna with its dimensional parameters. The role of the PSOA is to decide the geometrical parameters of the antenna, in response to the designer-specified frequencies and bandwidths. In order to validate the authenticity of the proposed method, a number of antennas have been designed, fabricated, and tested in the laboratory. Simulated and measured results have been compared which establish the accuracy of the proposed technique.

A novel tri-band asymmetric slot with defective ground structure (DGS) microstrip antenna for dual circular polarization (CP) is proposed. By placing a slot with 45° across diagonally at the center of the patch, excites resonant bands with narrow bandwidth and dual CP operation. The main design challenge is to introduce optimized defected element of circular dumbbell shape to enhance the impedance bandwidth with frequency shift operation at all bands. DGS enabled antenna is practically fabricated and simulated. Antenna is compact in size and shows a good quality of CP at two resonant bands and linearly polarized at one band. Structure displays the impedance bandwidth of 8% (2.26–2.45 GHz), 5.45% (3.41–3.6 GHz) and 3.07% (5.12–5.28 GHz). The design also shows 40 and 11 MHz, 3-dB axial ratio bandwidth at lower and middle bands, respectively. Simulated gain for each band is 4.72, 6.2 and 4 dB. Performance of antenna with and without DGS is also studied and compared. This single probe feed proposed antenna remains consistent with good radiation patterns and sufficient antenna gain over the operating bands. Excellent agreement was obtained between measurements and simulations.