This paper studies the excitation of a physical leaky mode in a covered microstrip structure at low frequencies. We calculate the current excited in the line by a delta-gap voltage source via a full wave analysis based on a mixed potential integral equation scheme. The current in the line is decomposed into its bound mode and continuous spectrum components. The bound mode component is associated with the propagation effects whereas the continuous spectrum component is associated with reactive and/or radiative effects and contains the contribution of the leaky mode. Our analysis also includes a detail study of the dispersion relations of the bound and leaky modes along with their corresponding electric fields. At low frequencies, in the covered microstrip structure with a low top cover height, we have found that the bound mode role is superseded by the leaky mode, in the sense that it is the leaky mode which partially or totally carries the signal energy. Therefore, the spurious effects associated with the excitation of a leaky mode, which usually appear at high frequencies in open microstrip lines, appear here in the low frequency range. This effect may have very relevant practical consequences in the performance of such systems.

Power transformers in service are subjected to a wide variety of electrical, mechanical and thermal stresses capable of producing insulation faults. This type of failure figures amongst the most costly faults in distribution networks since it produces both machine outage and electrical supply interruption. Major research effort has therefore focused on the early detection of faults in the insulating systems of large high voltage power transformers. Although several industrial methods exist for the on-line and off-line monitoring of power transformers, all of them are expensive and complex, and require the use of specific electronic instrumentation. For these reasons, this paper will present the On-Line analysis of transformer leakage flux as an efficient alternative for assessing machine integrity and detecting the presence of insulating failures during their earliest stages. An industrial 400 kVA-20 kV/400V transformer will be used for the experimental study. Very cheap, simple sensors, based on air core coils, will be used to measure the leakage flux of the transformer, and non-destructive tests will also be applied to the machine in order to analyse pre- and post-failure voltages induced in the coils.

This paper proposes a novel approach for the spatial microwave power combining. Anisotropic metamatierials are employed to trim the combined electrical fields and form a single beam radiation pattern. The radiation characteristics of a binary horn antennas array are investigated both numerically and experimentally at 12 GHz. The results show that much higher combining efficiency can be achieved. Given a designed combining efficiency, the strict relative position requirements in each transmission unit are reduced in this scheme.

Recent research into very large, regularly shaped, geological structures has shown that in the 100 kHz to 10 MHz frequency range electromagnetic waveguide behaviour is observed when the material forming the structure is not too lossy (conductivity σ<0.0001). While mode formation and modal behaviour in electromagnetic waveguides is very well understood, much of the literature describes high frequency structures for which it can generally be assumed that the loss tangent of the wave guiding medium (tanδ) is very much less than unity. In this case wave attenuation is small and can generally be considered to be insignificant. This is not true for large low frequency waveguides, such as those formed by geological strata, and little seems to have been reported in the literature on the nature of modes in waveguides of this description. The paper takes the form of a parametric study aimed at ascertaining the limitations to modal formation in waveguides, for which tand is greater than unity, by revisiting the basic equations describing electromagnetic wave propagation in lossy media. The theoretical predictions are supported by modelling studies on large waveguide strata formed from material layers with dimensions typical of a geological structure such as a coal seam or oil-wet, strata-bound, petroleum reservoir.

Great accuracy and reasonable computational time are desirable in a deterministic ray tracing model for an efficient radio frequency planning. An algorithm to speed up a ray tracing engine is described which allows to select arbitrary areas around transmitters and receivers by dividing the scene in a voxel chessboard. The reliability of the algorithm has been evaluated by comparing measured and predicted path losses in real urban scenarios while the algorithm performance is expressed in terms of computational time reduction.

We experimentally characterize the pulse-shortening phenomena in traveling-wave field effect transistors (TWFETs). When a decreasing voltage pulse is applied to the gate line and an increasing voltage pulse is simultaneously applied to the drain line, a sinusoidal wave supported by an exponential edge is developed in the drain line. In order to make the velocity of the sinusoidal wave coincident with that of the exponential edge, the wavenumber of this sinusoidal wave must be considerably increased. Moreover, the small-amplitude parts of the wave disappear with the shorter propagation while the large-amplitude ones remain in the drain line. The input pulse is considerably shortened owing to these two properties. This paper validates the pulse-shortening phenomena by performing several measurements using a breadboarded TWFET.

A printed broadband planar inverted-F antenna (PIFA) with zeroth-order resonator (ZOR) loaded is proposed whose shorting strip of the PIFA is replaced by an inter-digital capacitor and thin inductive strip in series. The loaded inter-digital capacitor and thin inductive strip act as a shorting strip at the 3.1 GHz, which allows the antenna to maintain its regular performance. Around 2.0 GHz, the antenna with the inter-digital capacitor and thin inductive strip works on the zeroth-order resonance mode, which makes the physical size be independent of the wavelength. By merging the two modes, a broadband performance can be achieved. The size of the antenna is only 12.5 mm×7.81 mm×1.6 mm with single layer. The measured antenna bandwidth is 1.63 GHz (about 65%), total gain is above 2.5 dBi and the simulated radiation efficiency is over 90% in the working band. Especially the antenna has same direction of the radiation patterns in the broadband. In the end, the antenna with lumped elements loading is also discussed.

The dielectric pyramid loaded and the dielectric cone loaded diagonal horn working at 150 GHz are investigated by using Gaussian beam mode analysis. With extremely low cross-polarized and axially symmetrical field distribution in the horn aperture, the calculated fundamental Gaussian mode coupling achieves about 98%. The far field radiation patterns of the two antennas are analyzed using fundamental Gaussian mode aperture-field distribution model whose results agree with high-accuracy CST^TM software computations, indicating that the dielectric loaded horns radiate fine Gaussian beams. The dielectric loaded geometry may be used to modify the diagonal horns with distorted beam.

This paper presents a novel compact suspended double side millimeter bandpass filter (BPF) with wide stopband using Suspended Compact Microstrip Resonant Cell (SCMRC). For SCMRC exhibit slow-wave band-stop characteristics, two distributed SCMRC structures are designed to achieve wide stopband characteristic. By apply SCMRC structure and double side design, this novel BPF is size reduced. Back-to-back rectangular waveguide to suspended microstrip probe transitions at different millimeter bands are designed and fabricated to verify transmission characteristics of novel SCMRC bandpass filter. Experimental results show low insertion loss (< 1.2 dB) in the passband and sharp, wide rejection in the stopband with about 150% bandwidth (below -15 dB, from 50 GHz to 100 GHz), make good agreement with simulated results.

Metamaterial covers exhibit inimitable electromagnetic properties which make them popular in antenna engineering. A traditional metamaterial cover has an identical transmission phase for a normally incident plane wave regardless of its polarization state. The purpose of this research is to show the possibility of using a polarization dependent metamaterial cover to change the polarization state of the incident plane wave. Novel polarization-dependent metamaterial (PDMTM) covers, whose transmission phases for two principal polarizations are different, are presented. A full-wave FDTD numerical technique developed by the authors is adopted for the simulations.

A simple and efficient joint algorithm of finite difference time domain (FDTD) and periodic boundary condition (PBC), called as the direct spectral FDTD method, has been investigated to study three-dimensional (3D) periodic structures with oblique incidence, where both the azimuth angle φ and the elevation angle θ are varying. The number of sampling points for the horizontal wave number can be determined by using an adaptive approach. As numerical results, the transmission and reflection coefficients from split-ring resonators (SRRs) and a dielectric grating slab are computed to validate the accuracy and efficiency of the direct spectral FDTD method. The computed results are in good agreement to the published ones obtained by other methods.

This paper presents a geometrically based method for the calculation of the node-to-node distance distribution function in circular-shaped networks. In our approach, this function is obtained from the intersection volume of a sphere and an ellipsoid. The method is valid for both overlapping and non-overlapping networks. Simulation results and comparisons with methods in the literature demonstrate the efficacy of the approach. The relation between networks geometric parameters and distance statistics is explored. As an application example, we model distance-dependent path loss and investigate the impact of channel characteristics and networks size on signal absorption. The aforementioned model is a useful and low-complexity tool for system-level modeling and simulation of mobile communication systems.

A highly birefringent four-hole fiber (FHF) with a pair of large air holes and a pair of small air holes are proposed for air/hydrostatic pressure sensing. The birefringence of the FHF can be up to 0.01 due to the rectangle-like fiber core surrounded by four air holes. Therefore, a FHF with a length of only several centimeters is required for high-sensitivity pressure sensing based on a Sagnac interferometer. Optical properties of the FHF such as effective index and birefringence are investigated. Pressure sensor based on the FHF depends on the pressure-induced refractive index change or pressure-induced birefringence. The stress distribution of the FHF subjected to an air/hydrostatic pressure is represented. Simulations show that the principal stress component parallel to the slow axis of the of the FHF under the air/hydrostatic pressure is greatly enhanced due to the existence of two large air holes, which consequently results in a high sensitivity of the FHF-based pressure sensor. Relationships between the pressure-induced birefirngence and the radius of the large air hole, the external diameter of the FHF, or the ellipticity of the elliptical FHF are investigated. The polarimetric pressure sensitivity of the FHF can be up to 607 rad/MPa/m.

Finite edge geometrical theory of diffraction (FEGTD) approach is a new and latest improvement in GTD technique. This FEGTD technique is applied to the H-plane and E-plane horn radiation problems with spherical source excitation. The horn patterns obtained with the FEGTD approach are found to be in good agreement with measured results.

Planar multiband bandpass filters are implemented based on the versatile multimode stepped-impedance resonators (SIRs). The resonant spectrum of a SIR can be calculated as functions of the length ratios for various impedance ratios of the high- and low-impedance sections. Thus, by properly selecting the geometric parameters and designing the input/output coupling structure, the SIRs are feasible to realize multiband multimode filters. Using a single SIR, a dual-mode dual-band, a dual-mode triple-band or a hybrid dual-/triple-mode dual-band bandpass filter can be realized. Emphasis is also placed on designing specified ratios of center frequencies and fractional bandwidths of the passbands. To extend the design flexibility, extra shunt open stubs are used to adjust the ratio of the passband frequencies. In addition, sharpness of the transition bands is improved by designing the input/output stages. Simulation results are validated by the measured responses of experimental circuits.

A compact printed and planar Multiple-Input Multiple-Output (MIMO) for Ultra Wideband (UWB) communications is presented. Two circular disc monopole antenna elements constitute the proposed UWB-MIMO antenna operating over the frequency band of 3.2-10.6 GHz. The isolation between the antenna ports has been enhanced to the value of more than 15 dB throughout the frequency band of interest. This enhancement is achieved by taking the advantage of an inverted-Y shaped stub that is being inserted on the ground plane of UWB-MIMO antenna. The insertion of the stub has also facilitated reduction of the size of the antenna, i.e., overall dimensions of the antenna are 40×68mm². The proposed antenna is investigated both numerically and experimentally.

The double and triple Langmuir probe diagnostic systems with their necessary driving circuits are developed successfully for the characterization of laboratory built low pressure inductively coupled nitrogen plasma, generated by 13.56 MHz radio frequency (RF) power supply along with an automatic impedance matching network. Using the DC properties of these two probes, the discharge plasma parameters like ion saturation current (I_io), electron temperature (kT_e) and electron number density (n_e) are measured at the input RF power ranging from 250 to 400 W and filling gas pressures ranging from 0.3 to 0.6 mbar. An increasing trend is observed in electron temperature kT_e and n_e with the increase of input RF power at a fixed filling gas pressure of 0.3 mbar, while a decreasing trend is observed in kT_e and n_e with the increase of filling gas pressure at a fixed input RF power of 250 W.

The planar metal particles, consisting of a multi-turn spirals, are studied with the aim of using them to realize high impedance surfaces or as an elementary cell to create an artificial material. These spirals present a resonant behaviour in a certain frequency band. To obtain miniature devices, a compromise between the surface and the efficiency of the resonance must be found. The compactness of the particles can be increased by using s spirals. However, the accuracy on resonant frequency of existing models is not sufficient for our applications. We present a simple analytical model that determines the resonant frequency from the geometric dimensions of the approximated model. This model is verified by electromagnetic simulations and by measurements.

Reflection from a planar DB interface placed in chiral and chiral nihility medium is studied. No difference between the two cases, regarding reflection chracteristics, is noted. No reflected backward wave is produced for DB interface placed in chiral nihility metamaterial. In this regard, DB interface may be considered as first known perfect reflector interface which yields non zero power when placed in chiral nihility medium.

A novel compact multi-band design of planar C-shaped monopole antenna with inverted L-shaped parasitic strip is proposed for IEEE 802.16m WiMAX system. The obtained impedance bandwidth across the 2.6/3.5/5.5 GHz operating bands can reach about 240/570/4470 MHz, respectively. Only with the antenna size of 15×30×0.8 mm³, the proposed monopole antenna has the compact operation with more than 50% antenna size reduction. The measured peak gains and radiation efficiencies are about 2.1/2.2/2.9 dBi and 91/96/94% for the 2.6/3.5/5.5 GHz operating band, respectively, with nearly omni-directional pattern in the XY-plane.