摘要:

A novel impedance matching technique is presented in which the reflected signal from a mismatched load is recombined, with appropriate phase and amplitude weightings, with the source signal so that, in principle, all the power from the source is delivered to the load. The method may be particularly suitable for transmit antenna matching units, especially those employing electrically small antennas and superconducting components. Results are derived to establish both design criteria and circuit performance, in terms of loss and bandwidth, in the presence of dissipative losses in the matching network.

DOI:10.1049/ip-h-2.1992.0070

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The authors present a method of applying fast Fourier transform (FFT) techniques to the solution of the electric field integral equation (EFIE) on finite periodic bodies using the methodof- moments (MoM). These techniques are used to speed matrix–vector multiplications in an iterative solution of the discretized EFIE, and to save memory space required for storing the impedance matrix. An individual array element can have arbitrary three-dimensional structure and be electrically connected to its neighbors. The method is independent of the type of basis functions used, of the method of surface patching, and of the field excitation (which need not be periodic). The algorithm can be applied both to scattering and to antenna problems. The authors demonstrate improvements in speed-of-execution and memory required.

DOI:10.1049/ip-h-2.1992.0071

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The paper discusses a technique for fitting quasiparabolic layers to measured vertical electron density profiles. The method adopted is to represent individual ionospheres by the appropriate combination of E, E-F valley, F1and F2layers connected by joining layers. The method produces quite accurate representations of measured ionospheric profiles. The advantage of using quasiparabolic layers is that analytical solutions of various ray parameters can be obtained, including the effect on signal strength of ray tube divergence. Furthermore, tilted ionospheres can also be considered. The methods are illustrated by application to two vertical electron density profiles obtained from the same ionogram. The primary difference in the profiles is that one contains an E-F valley. Results are presented showing not only the effects of the profile differences, but also the effects of tilts, antenna patterns and ionospheric absorption.

DOI:10.1049/ip-h-2.1992.0072

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The moment method of analysis as applied to conventional broadwall slot radiators in rectangular waveguide has been extended to accommodate the presence of an external parallel plate polariser. This has been achieved by imaging the half space Green's function, which is used in the conventional analysis, in the ‘perfectly’ conducting parallel planes. The results obtained are shown to be in good agreement with vector network analyser measurements on typical geometries.

DOI:10.1049/ip-h-2.1992.0073

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The frequency-dependent characteristics of shielded asymmetric microstrip lines on uniaxial and biaxial substrates are presented in this memorandum. The permittivity tensor of the supporting medium is assumed to have off-diagonal elements due to the misalignment between the waveguide co-ordinate system and that of the material. New data for the dispersive properties of the asymmetric structure are provided for frequencies up to 40 GHz.

DOI:10.1049/ip-h-2.1992.0074

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The attenuation function for radio waves propagating paraxially over a two-segment piecewise linear profile is derived, using path integration. It is shown that the path integral model yields results for an infinite conducting wedge, which are continuous across the shadow boundary. These results also agree with those obtained using a UTD wedge diffraction coefficient. An engineering extension for a finiteconductivity, rough-surface wedge is also presented. This finite conductivity model is then applied as an approximation to a practical irregular terrain to predict path loss for an experimental radio link, and the computations are compared with measurements.

DOI:10.1049/ip-h-2.1992.0075

出版商:IEE    

年代:1992

数据来源: IET

摘要:

A numerical method is presented for the problems of transverse electric (TE) and transverse magnetic (TM) scattered fields from homogeneous lossy dielectric cylinders. The numerical solution uses fictitious filamentary sources to simulate the field scattered by the cylinders. The currents of the fictitious sources are solved for, subject to the impedance boundary conditions. Numerical results are given and are compared with available analytical and numerical solutions.

DOI:10.1049/ip-h-2.1992.0076

出版商:IEE    

年代:1992

数据来源: IET

摘要:

An optimisation approach is presented for the problem of reconstructing the permittivity and conductivity profiles of a dielectric slab from the reflected and transmitted field data. The problem is treated as an optimal control problem where the norm of the difference of measured and calculated boundary data is minimised subject to the state equation governing the system. The original constrained optimisation problem is reduced to the evaluation of stationary points of an augmented functional which is obtained by the method of Lagrange multipliers. To find the necessary conditions for optimality, a variational approach is used which leads to a coupled system of four equations. The first two of these are differential equations named as state and costate equations, and the remaining two expressions are obtained by equating the gradients of the augmented functional, with respect to the permittivity and conductivity, to zero. Profile reconstruction is carried out by descent methods. At every iteration the state and costate equations are solved by the time domain finite element method. New estimates for the permittivity and conductivity profiles are obtained by a one dimensional search in a suitable descent direction.

DOI:10.1049/ip-h-2.1992.0077

出版商:IEE    

年代:1992

数据来源: IET

摘要:

The sense of rotation of the polarisation direction of an electromagnetic wave propagating in a chiral medium depends on the handedness of the medium. At certain frequencies, a phenomenon called the Cotton effect (in optics) occurs when the angle of rotation rapidly increases or decreases or even changes sign. This effect has been observed in artificial chiral composites fashioned by embedding chiral inclusions (miniature metallic helices) in an epoxy host medium in the 8–40 GHz frequency range. The ellipticity and rotation angle of the elliptically polarised transmitted wave and the power absorbed in nine samples (three volume concentrations with left, right and racemic samples containing helices of the same size) are measured using a free space set up. The handedness of the microstructure of a chiral material can be experimentally determined by studying the Cotton effect curves. The Cotton effect is observed in the frequency region where maximum electromagnetic power absorption occurs. At maximum power absorption, the characteristic frequency is 28.45 GHz and the ratio of the one turn length of the helix to average wavelengthL/λcis 0.6037, an average value obtained for all nine samples. This is a useful parameter in the design of microwave chiral absorbers.

DOI:10.1049/ip-h-2.1992.0078

出版商:IEE    

年代:1992

数据来源: IET

摘要:

In this paper, a groove guide has been analysed using the mode matching technique employing three domains in place of the two domains used in the conventional mode matching formulation. This approach has been shown to be computationally more accurate. The computed results match very well with the experimentally measured values. This method of domain decomposition can be applied to waveguide structures whose cross section can be subdivided into rectangular domains.

DOI:10.1049/ip-h-2.1992.0079

出版商:IEE    

年代:1992

数据来源: IET