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In this paper, a study of the coupling between a rectangular waveguide and circular waveguide is presented, aimed at developing a design procedure for a rectangular to circular waveguide directional coupler. Since radio-astronomical receivers often exploit circular horns, a coupler sharing the same waveguide type is at a premium, and furthermore, it reduces the feeding network complexity in dual polarization applications. A novel approach for obtaining traveling wave modal amplitude coefficients is presented, suitable for modes of any order. This approach is used to analyze the behavior of a single hole coupler between a rectangular and circular waveguide. From this starting point, a four holes directional coupler with good performance is designed in the 22-GHz band. Measurements and full-wave numerical simulations show excellent agreement with the proposed theory. Index Terms--Directional couplers, polarization, waveguide couplers, waveguide theory.

In this paper the electromagnetic coupling between a corrugated horn and a reflector dish is analyzed with a new hybrid perturbative technique. This coupling may produce significant modifications especially in the return loss and in the crosspolar pattern of the feed. The hybrid perturbative approach combines three different techniques: a mode matching (MM) technique for the propagation inside the horn, a method of moments (MoM) technique for the radiation of the horn taking into account the external part of the horn itself, and a fast Gaussian beam asymptotic physical optics (FGBAPO) technique for the interactions between the feed and the reflector. Some measurements and the comparison with rigorous numerical results demonstrate the accuracy of the new hybrid technique and the effects of this electromagnetic coupling on the feed performance. Index Terms--Gaussian beams (GBs), integral equations, electromagnetic coupling, horn antennas, reflector antennas, perturbation methods.

A rigorous Green's function analysis of rectangular microstrip patch antennas enclosed in a rectangular cavity is described. The formulation makes use of a frequency-independent preprocessing (image extraction) on the spectral domain representation of Green's function. Measured and simulated results are shown for both single and stacked-patch antennas enclosed in a cavity. Index Terms - Microstrip antennas.

Broad-band stripline circulators are studied by means of a mixed numerical technique which employs both Boundary Integral and Segmentation methods; this technique allows the analysis of planar circuits where the substrate is constituted by several regions with arbitrary shapes and different electrical properties. It is known that tracking circulators require matching structures because they present a low-gyrator impedance (real and almost constant in an octave frequency band). The matching structures (generally tapers or multisection's transmission line transformers) must be realized on a reciprocal substrate. The overall device (circular disk on ferrite substrate and matching structure on dielectric substrate) constitutes a planar circuit with an inhomogeneous medium. The method of study presented here allows the determination of the overall impedance matrix of the planar circuit constituted by the nonreciprocal disk with sections of striplines connected to each port; in this way, the discontinuities between reciprocal and nonreciprocal medium are included in characterization of the overall device. Moreover the accuracy of the representation is increased. In fact, the coupling ports of the overall device may be located at a suitable distance from the disk boundary where higher-order modes excited by the discontinuities have been sufficiently attenuated and only the TEM mode is present on the striplines (which is the only one considered in the design of the matching structures). Index Terms - Boundary elements, planar circulators, stripline devices.

The application of the incremental inductance rule is described for the computation of losses in quasi-TEM coupled transmission lines. It is shown that the well-known formula valid for the single-line case can be extended to the case of normal modes travelling in general coupled structures providing one uses the power-current modal characteristic impedance definition. Numerical results are shown for several coupled lines systems.

Some results on the application of the Boundary-Element Method to the computation of the scattering parameters of planar circulators also working in the region [Mu][subscript eff] < 0 are presented. Spurious solutions are first pointed out, and two different ways to eliminate them are discussed. The analysis supports Schroeder's explanation of the appearance of spurious modes in integral equation techniques. Extensive numerical simulations are presented to validate the formulation proposed.

The application of a new boundary-integral technique to the study of planar circulators in stripline/microstrip technology is described. The technique employs Green's second identity with plane waves as weighting functions, and leads to a very efficient algorithm for the analysis of planar devices in anisotropic media. Extensive comparison is made with results from the standard Contour-Integral formulations with very good agreement between the two techniques. The new technique is more stable in the critical region ([[mu].sub.eff] close to 0) and is somewhat faster.

A new method to analyze shielded planar transmission lines with finite metallization thickness is described. It is based on a mixed spectral-space domain representation of Green's function. The method allows the accurate determination of the static parameters of the transmission line with great efficiency as compared to methods available in the literature. Several numerical examples are shown on single and coupled planar transmission lines.

An original generalized-admittance-matrix (GAM) approach for the analysis of multiport junctions in rectangular waveguide having an arbitrary circular-rod inset is presented in this paper. The method is based on an efficient computation of the GAM for the blocks that compose the structure. The efficiency is due to the fact that, for each block, the full three-dimensional GAM is obtained by solving a set of uncoupled two-dimensional problems. The accuracy and efficiency of the method developed have been assessed by comparing the results obtained with some measured data and with data obtained by the finite-element method. Index Terms--Combline filters, generalized admittance matrix.

A comprehensive analysis of electromagnetic (EM) inversion applied to pavement profiling by using a monostatic ground penetrating radar (GPR) is presented. Since in GPR systems using transfers EM (TEM) horns, the antenna is positioned close to the investigated medium and a strong EM interaction occurs. This effect is taken into account by modeling the antenna with equivalent sources placed on the aperture section and by using an accurate EM modeling within the inversion loop in order to account for the contribution of near-field effects. Experimental results on the antenna modeling procedure and on a pavement profile estimation validate the feasibility of both the calibration and the inverse problem proposed. Index Terms--Electromagnetic scattering of inverse problems, electromagnetic scattering by nonhomogeneous media, radar applications.