Surface impedance of thin graphite films with metallic properties is evaluated by a waveguide technique based on measuring reflection and transmission coefficients of thin film membranes at operating frequencies in rectangular waveguides. One- and two-layer membranes of finite thickness, completely filling the waveguide cross-section, are investigated. Formulas allowing analytical estimates of surface impedances for nonmagnetic films made of amorphous carbon are derived. Simulation results for graphite films at frequencies from 5 to 10 GHz are analyzed.

In this paper, the correlation coefficients of skewed dipole arrays for antenna diversity are analyzed theoretically for each polarization characteristic and in various propagation environments. The correlation is not simply increased by two closely located antennas with different polarization characteristics and it is not decreased by increased antenna distance. This result is interpreted from the correlation analysis of two skewed dipoles with different polarization characteristics. The embedded beam patterns of the two skewed dipoles are calculated using the mutual impedances derived using the effective length vector (ELV) method; then, the mutually coupled correlation coefficients for θ, ϕ, and total polarizations are effectively derived. The correlations are also analyzed for various realistic propagation environments using statistical channel models with angular density functions and cross polarization discriminations (XPDs). Finally, this paper provides an effective correlation analysis for two dipoles and proposes optimal geometries for the two skewed dipoles in various propagation environments for each polarization characteristic and with environmental variables.

This letter presents an omnidirectional printed dipole antenna array with a wide bandwidth. The array is composed of four dipole units etched on a thin substrate, which is simple in structure and easy to be processed. By modifying the triangle-shaped radiation dipole units and gradually increasing the width of microstrip feeding transmission line, the performance of the dipole antenna array is greatly improved. Simulation results show that this omnidirectional antenna has a peak gain greater than 7.39 dBi, and the impedance bandwidth is 16% (VSWR<1.6), ranging from 2.3 to 2.7 GHz.

This paper proves that the use of conventional diagnostic methods of rotor crack and local demagnetization based on the harmonic analysis of the output voltage or counter-electromotive force is effective only with a certain ratio of the number of slots and poles. This statement was proved experimentally. The diagnostic method of the rotor cracks and local demagnetization which is universal for all types of windings and the number of slots of 2-pole synchronous electric machines with permanent magnets is proposed. The mathematical apparatus for the implementation of the proposed method is developed and verified with the help of FEM and experimental studies. All the experimental studies have been carried out for various rotor magnetic systems and a different number of stator slots.

In this paper, we are interested in the design of a new Ultra-Wideband (UWB) directional Vivaldi antenna with narrow beam, in the frequency range of 1.17 to 4.75 GHz. The simulation of the designed antenna is carried out on Computer Simulation Technology Microwave Studio (CST-MWS). The mutual coupling effect reduction is considered. The designed antenna is tested for Ground Penetrating Radar (GPR) and Through the Wall applications. The emitted waveform is a Stepped Frequency Continuous Wave (SFCW) signal, generated by a Vector Network Analyser (VNA). The acquired raw data are focused by using back projection algorithm.

A dual-port reader antenna based on magnetic coupling in near-field (NF) and linear polarization in far-field (FF) is proposed for UHF RFID multiservice applications. The prototype consists of four straight dipoles fed by double-side parallel stripline structure with two feed ports. The proposed antenna can operate at different modes by feeding each corresponding port. In NF mode, a strong and uniform magnetic field can be generated over the interrogation zone. In FF mode, a linearly polarized performance can be obtained. The antenna prototype is printed onto a piece of FR4 substrate with an overall size of 180 × 180 × 1.6 mm³. The measured tests on reading range are carried out, and the results exhibit 100% reading rate for near-field tags within 100 mm and reading distance of far-field tags is 120 cm. Both simulated and measured results show good capability for near- and far-field applications.

In this paper, a novel stabilization scheme to prevent parametric oscillations in power amplifiers is presented. Based on a new oscillation detection approach, the inductive degeneration technique was used, for the first time, to successfully stabilize a high-power amplifier and prevent parametric oscillations. A 0.15 um AlGaN/GaN Microwave Monolithic Integrated Circuit high power amplifier operating at 5.8 GHz with 10% fractional bandwidth was designed and successfully stabilized using this approach. The proposed (4.7 x 3.7) mm² three-stage amplifier achieves a saturated output power of 35 W with 29% power added efficiency and a large-signal gain of 26 dB.

Four wave mixing (FWM) in optical fiber is unwanted effect to an optical transmission system, which can severely limit the wavelength division multiplexing (WDM) and lower the transmission efficiency. In this work, the robustness of normal Non-Return-to-Zero (NRZ), Return-to-Zero (RZ) and Modified-Duobinary-Return-Zero modulation (MDRZ) to FWM have been evaluated. Furthermore, the system performance is evaluated with the effect of fiber length tuning and applying 160 Gb/s data rate. The findings show that the RZ modulation offers a lower FWM power of -44 dBm at 700 km fiber length than -30 and -38 dBm of NRZ and MDRZ respectively at the same fiber length. In terms of system performance at the first channel and 700 km distance, the minimum BER is observed in normal RZ modulation, equal to 1.2×10^-23. It is also noticeable that if NRZ and MDRZ modulations are applied, the system performance will be quickly changed and get worse, where the BEARs are increased to 1.3×10^-6 and 1.3×10^-8 consecutively at same channel and parameters.

A dual-polarized broadband single-layer reflectarray antenna based on square spiral element is presented in this paper. Two designs with similar square spiral structures but different construction processes and transition patterns are investigated and compared. The comparison results show that the design utilizing the sweeping method of varying the length of the first stub of one-arm square spiral is more suitable for the building the reflectarray. Several 441-element reflectarray antennas fed by horn with different offset angles are also simulated and compared to show the broadband characteristic. A reflectarray with offset angle of 15 degrees is then fabricated and tested. The measured results show good radiation performances, and the 1-dB gain bandwidth of 34.7% is obtained. The measured gain is 27.1 dB at the center frequency of 15 GHz, which is equivalent to 45.6% aperture effifficiency.

This paper presents a novel Y shaped fractal defected ground structure (FDGS) for the mutual coupling (MC) reduction between coplanar closely spaced microstrip antennas. The proposed FDGS has band-gap characteristic, which induces the current distribution on the antenna patch. This will contribute to achieving 25 dB MC reduction. When realizing the MC reduction, the antenna efficiency is increased. Moreover, the envelope correlation of the MIMO system is decreased, which helps to increase the MIMO system capacity.

A new microstrip lowpass filter using radial stub loaded resonators is presented. A microstrip high impedance main transmission line loaded with five radial stub loaded resonators by five high impendence transmission lines is introduced in the design of the filter. Owing to the introduction of the radial stub structure, the filter achieves compact size and ultra-wide stopband. A demonstration filter with 3 dB cutoff frequency at 0.8 GHz has been designed, fabricated and measured. Results indicate that the proposed filter is able to suppress the 17th harmonic response referred to a suppression degree of 15 dB, together with a small size of 0.060λ_g×0.080λ_g, where λ_g is the guided wavelength at 0.8 GHz.

A broadband low-profile circularly polarized (CP) antenna with a stepped sequential feeding structure is proposed in this letter. The CP antenna consists of four dipoles and parasitic structures. A novel feeding network using parallel transmission lines is introduced to realize phase difference required for circular polarization. Four dipoles are excited by the feeding structure to achieve CP radiation, and parasitic elements are loaded to broaden 3-dB axial ratio bandwidth (ARBW). To verify this design, the proposed antenna is fabricated and measured. Measured results show that a 10-dB impendence bandwidth of 42% (1.82 to 2.8 GHz) and 3-dB ARBW of 36.4% (1.89 to 2.73 GHz) are achieved, respectively. Moreover, the designed antenna has a low profile of 0.13λ (λ is the free-space wavelength at the center frequency) and a gain of average 9.5 dBi with less than 1.5-dB variation within the 3-dB ARBW.

In this paper, design of a profiled conical horn using piecewise biarc hermite polynomial interpolation is proposed. In order to improve the performance of the horn, a novel concept of `targeting the Region of Interest (RoI)' is used. The target specifications are decided based on the applicability of the horn in a reflectometry system for plasma diagnostics (D-band). Apart from the design details, the simulated results of the proposed horn are covered.

An optimized design of Microstrip Log Periodic Dipole Array (MLPDA) antenna with non-cross feed structure is reported in this paper. Particle Swarm Optimization (PSO) is utilized to reduce the size and enhance the bandwidth of proposed antenna. Proposed design employs an improved feed structure of non-cross feed array antenna to avoid complexity of conventional feeding with long coaxial line and creating Co Planar Waveguide (CPW) feed. A simple FR-4 substrate with thickness of 1 mm is utilized for simulation using CST. A fitness function based on S₁₁ parameter is used to achieve the optimization goal. A prototype of proposed PSO optimized antenna is developed to validate the simulation results. The proposed antenna offers higher bandwidth and significantly smaller size than cross feed LPDA antennas, with less complexity and low cost through parameter optimization, while maintaining the log periodic nature and gain.

A low-profile and broadband slot antenna with artificial magnetic conductor (AMC) surface is designed for X and Ku communications. Loaded with evolved C-shaped branches, the proposed coplanar waveguide (CPW)-fed slot antenna, which consists of two radiating slots, exhibits wide impedance frequency band performance. The presented AMC, the unit cell of which is made up of two central hexagonal circles with six rectangle branches, operates in a wide in-phase reflection frequency band ranging from 6.0 to 13.94 GHz (79.64%) at the reference plane 4 mm above the AMC surface. An AMC surface composed of 8×10 AMC unit cells is located under the slot antenna with a distance of approximately 0.107λ (λ denotes the free-space wavelength at 8.0 GHz), which improves the radiation and impedance match properties of the broadband slot antenna while maintaining low profile. A prototype of the proposed slot antenna with AMC surface is fabricated and measured. Measured results show that the composite antenna achieves a wide impedance bandwidth from 7.64 to 14.58 GHz (62.47%). The measured peak gain is up to 10.26 dBi, and the maximum cross-polarization level is -17.5 dB for both E and H planes. Good agreements between the measured and simulated results validate good performance of the presented slot antenna within the desired frequency band.

A small-sized loop antenna loaded with the series-connected capacitor and inductor for wireless local area network (WLAN) operation is proposed. The main radiator of an L-shaped, loop structure and its small ground were constructed on a single-layered FR4 substrate of thickness 0.8 mm and occupied a miniature size of 5 mm × 20 mm only. It was found that by loading the series-connected capacitor and inductor between the antenna feed port and the loop radiator, the quarter-wavelength loop resonance can be easily excited together with the controllable half-wavelength resonance. The design prototype was able to operate in the 2.4-GHz (2400-2484 MHz) and 5.8-GHz (5725-5825 MHz) WLAN bands. The proposed antenna was simple in structure and yet provided dual-band operation and good radiation performance.

This paper presents an extension and update of a theoretical procedure developed by the authors for the determination of the electromagnetic waves scattering at interfaces between dielectric waveguides in cascade. The theoretical core of the problem is based on the generalized scattering matrix concept, together with the generalized telegraphist equations formulism and the modal matching technique. The new version includes the following updates: a) possibility of using any material as waveguide cover, b) inclusion of alternating microchannels with optical waveguides, and c) possibility of analyzing periodic structures of segmented optical waveguides for sensing applications. The spectral results obtained for modulus and phase of the reflection and transmission coefficients have shown the potentiality of the new proposal in the scientific topics of photonic crystals, refractive index sensors and optical biosensors.

A three-dimensional fully interlaced woven microstrip-fed substrate integrated waveguide has been designed, manufactured and experimentally validated. The waveguide has been conceived based on the conventional substrate integrated waveguide (SIW) technology and works in a range of frequencies between 7.5 GHz and 12 GHz. The SIW structure is suitable to be translated into different equivalent woven structures depending on the characteristics of the employed threads, as it has been presented in previous works. In this work, a structure based on rigid weft threads has been employed with the aim of translating both the waveguide and the corresponding SIW to microstrip transitions, into woven patterns and, therefore, achieving the main purpose of a complete integration of the circuit into the textile, avoiding the use of external transitions for its validation. Consequently, three prototypes, using three different lengths, have been manufactured and experimentally characterised, and the theoretically predicted behaviour of the prototypes has been experimentally verified.

This paper proposes a harmonic efficiency selectivity circuit (HESC) for achieving a broadband Class-J/F^-1 continuum mode power amplifier (PA) with enhanced efficiency. Design equations are derived through continuum mode condition analysis and are used in implementing the HESC. The implemented HESC topology is then used in attaining the broadband Class-J/F^-1 continuum mode PA. A theoretical parameter termed harmonic-alpha (ρ^h) acting as a sub-unit structure in HESC is introduced. Considering harmonic losses, ρ^h possesses a lookup table containing information on the harmonics. ρ^h operates in unison with the HESC in selecting the suitable harmonics with the best efficiencies. With ρ^h, the relationship among the HESC, the optimal impedance at the device's drain, and the terminal load impedance is defined for a greater freedom of harmonic impedance solutions space, efficiency improvement, and bandwidth extension, thus, indicating an increased flexibility in the design of broadband continuum mode PAs. This method is validated with a realized PA prototype operating from 1.3 to 2.4 GHz corresponding to a fractional bandwidth of 59.5%. The experimental results under continuous wave signals indicate that 79% peak efficiency, 42.68 dBm peak output power, and 16.96 dB peak gain are recorded. Moreover, at 1.7 GHz, when being tested with modulated signals at an average output power of 34.83 dBm, the lower and higher adjacent channel power ratios (ACPRs) without digital predistortion (DPD) are -34.9 dBc and -33.9 dBc, respectively, and a drain effifficiency (DE) of 45% is recorded. With DPD, -50.8 dBc and -50.3 dBc are respectively obtained at lower and higher ACPRs at an average output power of 34.6 dBm, and a DE of 44% is achieved.

Wireless communication is very valuable in underground mines, in which channel characterization plays an important role. In this paper, both narrowband and wideband measurements at three typical ultra-high frequencies of 433, 900 and 2400 MHz in a real mine shaft are performed. To our knowledge, this is the first work focusing on radio propagation in the mine shaft environment. Important channel characteristics, such as the path loss, delay spread and the number of multipath components were extracted from the measured data and compared with that in tunnel channels. The effects of frequency and antenna position on the path loss were investigated. The relationship between the root-mean-square (RMS) delay spread and the transmitter-receiver distance was also analyzed. The results will deepen our understanding of the mine shaft channel and help to design shaft wireless systems.