摘要:

A hybrid technique which utilises the finite element method (FEM) and an expansion of the field in terms of the Floquet modes is presented for the analysis of the scattering from and through periodic structures, having arbitrary geometry, material properties and periodicity with respect to the wavelength. This formulation can play an important role in validating approximate methods and in the analysis of the interaction of an electromagnetic wave with complex periodic structures. The periodic boundary conditions which are imposed for applying the FEM procedure are discussed in detail. Numerical results for bothTEZandTMZscattering are presented.

DOI:10.1049/ip-h-2.1993.0011

出版商:IEE    

年代:1993

数据来源: IET

摘要:

A novel concurrent implementation of a global-optimisation algorithm is used to extract parameter values for MESFET equivalent-circuit models. The problem of the location of the global minimum of an error function is addressed by the use of an efficient stochastic global-optimisation program which does not require accurate parameter estimates and which operates on a multiprocessor array. The program has been used to estimate the parameter values of a process-based physical equivalent model of a MESFET under multibias small-signal operation. These results are compared with those derived from an electrical equivalent-circuit model.

DOI:10.1049/ip-h-2.1993.0012

出版商:IEE    

年代:1993

数据来源: IET

摘要:

A new quasistatic analysis of microstrip line structures with multidielectric layers is presented, using a hybrid approach that combined the Galerkin finite-element method and the conformal-mapping technique. Wheeler's mapping function is adopted, which overcomes both the problem of infinite field extension and the field singularity difficulty around the conductor edges. Numerical results for effective dielectric constants, characteristic impedances, charge distributions, and field distributions for various multilayer microstrip line structures are presented and discussed. Comparisons are also made of the computed results with the available quasistatic ones, and good agreements are obtained.

DOI:10.1049/ip-h-2.1993.0013

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The paper describes the establishment of an LF/VLF/ELF transmitting antenna at Homer tunnel near Milford Sound in the South Island of New Zealand. The antenna consisted of a 3.3 km-long PVC-covered aluminium conductor passing through the tunnel and over the mountain above, forming a large vertical loop antenna with a lossy core. Measurements of the input impedance of the antenna are presented and compared with theoretical estimates.

DOI:10.1049/ip-h-2.1993.0014

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The paper discusses how variations in the pattern of convective plasma flows should be included in self-consistent time-dependent models of the coupled ionosphere-thermosphere system. The author shows how these variations depend upon the mechanism by which the solar wind flow excites the convection. The modelling of these effects is not just of relevance to the polar ionosphere. This is because the influence of convection is not confined to high latitudes: the resultant heating and composition changes in the thermosphere are communicated to lower latitudes by the winds which are also greatly modified by the plasma convection. These thermospheric changes alter the global distribution of plasma by modulating the rates of the chemical reactions which are responsible for the loss of plasma. Hence the modelling of these high-latitude processes is of relevance to the design and operation of HF communication, radar and navigation systems worldwide.

DOI:10.1049/ip-h-2.1993.0015

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The design of an array of seven log-periodic dipole antennas is described: this is intended to generate an approximate plane wave in the near field. The array is used as an illuminating antenna for a compact range primarily intended for electromagnetic susceptibility or emission measurements, but is also potentially usable for antenna measurements. It is capable of operation at lower frequencies than reflector antennas, and achieves good-quality plane waves in the near-field region. The design procedure was based on a numerical near-field synthesis technique that produced a set of complex excitations for the seven elements. The magnitude-only synthesis technique was found to give better results than alternative methods. The resulting computed field distributions showed that acceptable approximations to a plane wave could be generated over a substantial quiet zone, and an experimental test of an array constructed in accordance with the design algorithm showed good agreement with the predicted performance.

DOI:10.1049/ip-h-2.1993.0016

出版商:IEE    

年代:1993

数据来源: IET

摘要:

Some path-independent integrals are shown to exist in the potential field of transmission lines in the TEM mode. These integrals have physical significance that can be used effectively in design, optimisation and inspection. In particular, they will be useful in sensitivity analysis of capacitance or impedance with cross-sectional dimensions, and the ordering of relative importance of cross-sectional dimensions. Each represents the rate of decrease of potential energy of the system with an increase in a dimension of the conductor. In the paper, the integrals are defined, and their relationship with line parameters such as capacitance or impedance are given. Examples are included to illustrate the validity of path independence. An alternative method, the stiffness derivative procedure, for a numerical calculation of the line integrals is also presented.

DOI:10.1049/ip-h-2.1993.0017

出版商:IEE    

年代:1993

数据来源: IET

摘要:

A moment-method calculation is described of the maximum mutual coupling (or minimum isolation) between two loop antennas mounted on the tail-cone of a helicopter at the low-frequency end of the aircraft HF band. The wire-mesh mathematical model of the aircraft is constructed with a high sampling density in an attempt to produce estimates of the required nearfield dependent parameters with sufficient precision to calculate the maximum mutual coupling with acceptable accuracy. The calculation is a fairly demanding test of the procedure adopted for the construction of the mathematical model, because it depends upon the accuracy with which both near-field and far-field parameters are determined. This mathematical model does not incorporate any empirically derived information, and the calculation was of necessity performed in advance of an aircraft being available for experimental purposes. The difficulty of assessing the credibility of calculated results under such circumstances is considered. NEC was used as the solution code for the wire-mesh model. An independent corroboration of the calculation was made possible by the siting and configuration of the proposed antennas. This allowed a simple equivalent circuit model of the arrangement to be constructed, values of the circuit elements to be estimated, and hence an independent approximate determination of the maximum coupling to be made. The degree of consistency between the results obtained by these two methods is considered to enhance the confidence which can be placed in the results obtained using momentmethod mathematical models of this type when applied to near-field aircraft HF antenna problems.

DOI:10.1049/ip-h-2.1993.0018

出版商:IEE    

年代:1993

数据来源: IET

摘要:

A newly developed method of design for a shaped reflector antenna is described. A conventional quadratic reflector configuration is assumed; the reflectors are then modified to yield the desired aperture shape and field distribution by introducing shaping functions. This method is useful for designing antennas with an arbitrary shaped beam, such as an elliptical-beam antenna, and has been verified through a 4.7 m × 2.3 m (equivalent to 3.3 m diameter) dual-band (30–20 GHz) earth-station antenna for the Japanese domestic satellite system CS. The measured aperture efficiency is more than 76%, and the ratio of the major and minor axes of the elliptical beam is 2:1.

DOI:10.1049/ip-h-2.1993.0019

出版商:IEE    

年代:1993

数据来源: IET

摘要:

The establishment of an LF/VLF/ELF transmitting facility at the Homer tunnel near Milford South in the South Island of New Zealand is described. The antenna consisted of a 3.3 km-long PVC-covered aluminium conductor passing through the tunnel and over the mountain above, forming a large vertical loop antenna. Measurements of the electric field radiated from the transmitter, recorded at both Lauder and Palmerston, 133 and 225 km from the transmitter site, respectively, are presented. The antenna radiation pattern derived from observations made onboard an Orion aircraft circling the antenna at a distance of 200 km is also presented. Theoretical models are derived which enable fits to the radiated field data to be made over a wide frequency range (0.5–40 kHz). With 1 kW of input power the antenna produced peak radiated fields at frequencies near 10 kHz equivalent to those of a vertical monopole source radiating 150 mW.

DOI:10.1049/ip-h-2.1993.0020

出版商:IEE    

年代:1993

数据来源: IET