Eigenmode analysis of dielectric loaded top-hat monopole antennas

Eigenmode expansions are used for computing the currents on a dielectric loaded top-hat monopole radiating above an infinite conducting ground plane. The approach is facilitated by adding a parallel ground plane above the antenna, thus allowing use of cylindrical harmonic field expansions in each of three regions. Expansion coefficients are found by enforcing boundary and continuity conditions at conducting surfaces and regional interfaces. Convergence and accuracy are assessed using comparisons with three other methodologies, namely an integral equation solution, a quasi-static analysis, and multi-octave experimental measurements     

An approximate but accurate analysis of the dielectric wedge antenna fed by a slab waveguide using the local mode theory and Schelkunoff equivalence principle

A computationally efficient method to obtain design parameters for tapered radiators is presented. The method uses a local mode theory in conjunction with the Schelkunoff equivalence principle. Radiation patterns of directive gain for dielectric wedge antennas of varying lengths and different dielectric constants are presented. Both the TE and TM cases are considered. The method is validated by comparison with data obtained from a recently developed more rigorous mode-matching method. Excellent agreement is obtained over the physically important angular range from endfire to broadside for the TE case and over the angular range spanned by the major lobe for the TM case.     

A millimeter-wave omnidirectional circular dielectric rod gratingantenna

A millimetre-wave omnidirectional circular dielectric rod grating antenna is investigated theoretically and experimentally. As an eigenvalue problem, the antenna is analyzed with the rigorous mode matching method. The radiation characteristics of the antenna are calculated and measured in the Ka-band frequency range. A comparison between the experimental results and the theoretical predictions is made and good agreement is found. The practicality of the omnidirectional antenna and the effectiveness of the theory are thus justified     

Congenital ball-and-socket anomaly of the ankle

We present the clinical and radiographic appearance of 11 patients with a ball-and-socket ankle joint with an average follow-up of 13.7 years. Ten patients demonstrated parallel symptoms of the femur-fibula-ulna syndrome (FFU) associated with a mean leg-length discrepancy of 4.3 cm (range, 2-20). In six patients, an annual follow-up of talar development was possible. Radiographically the patients showed an additional tarsal coalition at a mean age of 4.8 years (range, 3.2-5.9). This earlier fusion could be detected by an arthrographic examination. All patients had no clinical complaints except a slight reduction of hindfoot mobility. The ball-and-socket ankle joint is presumably not congenital itself but secondary to congenital conditions in the peritalar region.     

Benign and malignant cells in effusions: diagnostic value of image DNA cytometry in comparison to cytological analysis

Identifying tumor cells in body cavity fluids reliably is a well-known diagnostic problem. Since cytometric quantitation of nuclear DNA content appears to be a promising new tool in the diagnosis and prognostic evaluation of many solid human tumors, we examined its validity in detecting malignant cells in cytologically positive effusions. For this purpose, image DNA cytometric measurements, including the evaluation of DNA-ploidy and the calculation of the DNA index (DI), were performed in 80 body cavity fluids. The results were correlated with cytology, clinical course and final histological diagnoses. We used aneuploidy, as shown by interactive image DNA cytometry, as a marker for the malignancy of cells that occur in body cavity fluids with a 100% specificity and 94.8% sensitivity. Cytological investigation showed a 92.3% specificity and 95.4% sensitivity. Combining both methods raised the specificity to 100% and the sensitivity to 98.5% and had a positive predictive value of 100% and a negative predictive value of 93.8%. The DNA-index (DI) was significantly higher in malignant effusions than in benign effusions: 1.5 +/- 0.74 (mean +/- SD) versus 1.11 +/- 0.26 (p < 0.05). Along with the difficult cytological evaluation of malignant cells in body cavity fluids, image DNA cytometry can be a helpful additional method for evaluating these cells. Combining the two techniques results in a highly specific and sensitive prediction of malignant cells. We, therefore, suggest using these methods for the reliable identification of tumor cells in effusions.     

An array technique for reducing ground losses in the HF range

Ground losses of HF antennas situated near the earth's surface are reduced if single antennas are replaced by small arrays of, for example, two or four closely spaced radiating elements. It is shown both theoretically and experimentally that an array spacing ofsim 0.1 lambdais sufficient to achieve significant improvement in ground-to-ground transmission efficiency. In particular in the lower HF range and for poorly conducting ground, the improvement approaches a theoretical limit equal to the number of elements in the array.     

Diakoptic theory for multielement antennas

A theory is presented for the analysis of multielement antennas which consist of interconnected, conductive structure elements of electrically small dimensions. The theory is based on the retarded electromagnetic potentials which permit a diakoptic approach to the problem. The antenna is broken up into its individual structure elements. Each element is assumed to be excited by currents which are impressed at its terminals, i.e., junctions with adjacent elements (current coupling) and by the electric fields of the currents and charges on all the other elements (fieid coupling). Both excitations are treated independently. Each impressed current produces a "dominant" current distribution, a characteristic of the element, which can be readily computed. Current coupling is formulated by "intrinsic" impedance matrices which relate the scaler potentials at the terminals of an element, caused by its dominant current distributions, to the impressed currents of the element. Field coupling produces "scatter" currents on all the elements and is formulated by a "fieid-coupling" matrix which relates the scalar potentials at the terminals, caused by field coupling, to the impressed currents at all the terminals. Intrinsic and "field-coupling" matrices are combined to form the "complete" impedance matrix of the diakopted antenna. Enforcing continuity of the currents and equality of the scalar potentials at all the interconnections between the elements yields a system of linear equations for the junction currents and the input impedance of the antenna. Current coupling dominates field coupling. Fieid coupling is primarily affected by the dominant current distributions of the elements, and in general the scatter currents have negligible effect on it. Although detailed numerical investigations will be presented in another paper, a simple example is included here to demonstrate that the diakoptic theory yields very good results even if greatly simplified assumptions are made.     

Scattering by periodic metal surfaces with sinusoidal height profiles--A theoretical approach

A theory of scattering by periodic metal surfaces is presented that utilizes the physical optics approximation to determine the current distribution in the metal surface to first order, but modifies this approximate distribution by multiplication with a Fourier series whose fundamental period is that of the surface profile (Floquet's theorem). The coefficients of the Fourier series are determined from the condition that the field radiated by the current distribution into the lower (shielded) half-space must cancel the primary plane wave in this space range. The theory reduces the scatter problem to the familiar task of solving a linear system. For certain basic types of surface profiles, including the sinusoidal profile considered here, the coefficients of the linear system are obtained as closed form expressions in well-known functions (Bessel functions for sinusoidal profiles and exponential functions for piecewise linear profiles). The theory is thus amenable to efficient computer evaluation. Comparison of numerical results based on this theory with data obtained by recent numerical schemes shows that for depths of surface grooves less than a wavelength and for unrestricted groove widths, reliable and comparable, if not more accurate, data is obtained, in many cases at considerably cheaper computational cost.     

A transport theory of pulse propagation in a strongly forwardscattering random medium

The scalar time-dependent equation of radiative transfer is used to develop a theory of pulse propagation in a discrete random medium whose scatter function (phase function) consists of a strong, narrow forward lobe superimposed over an isotropic background. The situation analyzed is that of a periodic sequence of plane-wave pulses, incident from an air half-space, that impinges normally upon the planar boundary surface of a random medium half-space; the medium consists of a random distribution of particles that scatter (and absorb) radiation in accordance with the aforementioned phase function. After splitting the specific intensity into the reduced incident and diffuse intensities, the solution of the transport equation in the random medium half-space is obtained by expanding the angular dependence of both the scatter function and the diffuse intensity in terms of Legendre polynomials, and by using a point matching procedure to satisfy the boundary condition that the forward travelling diffuse intensity be zero at the interface. Curves of received power show that, at small penetration depths, the coherent (reduced incident) intensity dominates, whereas at large depths, the incoherent (diffuse) intensity is the strongest and causes the pulses to broaden and distort. The motivation for this study was to complement a test series, on mm-wave pulse propagation in vegetation, by a theory that provides understanding of overall trends and assistance in the interpretation of measured results. In the mm-wave region, all scatter objects in a forest have large dimensions compared to a wavelength and, therefore, produce strong forward scattering and a phase function of the type assumed in this paper     

Modified diakoptic theory of antennas

The diakoptic theory of antennas, suggested by Goubau, is an independent approach to the numerical analysis of multi-element antennas of complex configuration. Based on physical reasoning, the theory is designed to be amenable to efficient computer evaluation. However, as an intermediate step, the theory involves hypothetical current sources with one terminal only, which violate the current continuity condition, so that Maxwell's equations cannot be applied without precautions. Here, a modified version of the diakoptic theory is presented which avoids this complication by a simple change in the basic approach while otherwise it preserves most features of Goubau's original theory. In addition, the modified version starts with a well-established, rigorous impedance matrix formulation of the problem and allows one to readily establish the relationships with the method of moments. Thus, it may facilitate understanding the diakoptic method.