Alejandro Valero-Nogueira

The Theory of Characteristic Modes Revisited: A Contribution to the Design of Antennas for Modern Applications

The objective of this paper is to summarize the work that has been developed by the authors for the last several years, in order to demonstrate that the Theory of Characteristic Modes can be used to perform a systematic design of different types of antennas. Characteristic modes are real current modes that can be computed numerically for conducting bodies of arbitrary shape. Since characteristic modes form a set of orthogonal functions, they can be used to expand the total current on the surface of the body. However, this paper shows that what makes characteristic modes really attractive for antenna design is the physical insight they bring into the radiating phenomena taking place in the antenna. The resonance frequency of modes, as well as their radiating behavior, can be determined from the information provided by the eigenvalues associated with the characteristic modes. Moreover, by studying the current distribution of modes, an optimum feeding arrangement can be found in order to obtain the desired radiating behavior.

Experimental Demonstration of Local Quasi-TEM Gap Modes in Single-Hard-Wall Waveguides

This letter reports an experimental study of a gap waveguide which is able to support a plurality of degenerate local quasi-TEM modes. No sidewalls are needed to separate these modes, which is a very useful property in the upper microwave and millimeter-wave bands. The waveguiding media is formed in the gap between a conducting plate and a hard surface. The working principle is described first. Next a practical realization is explained and tested experimentally.

A New Fast Physical Optics for Smooth Surfaces by Means of a Numerical Theory of Diffraction

A new technique to compute the physical optics (PO) integral is presented. The technique consists of a blind code that computes the different contributions (stationary phase points, end points, etc.) numerically. This technique is based on a decomposition of the surface into small triangles and a fast evaluation of each triangle by means of a deformation of the integration path in the complex plane. This algorithm permits a fast and accurate evaluation of the PO integral for smooth large surfaces. The CPU time is almost independent of frequency.

Gap Waveguides Using a Suspended Strip on a Bed of Nails

This letter discusses the feasibility of a new type of waveguide consisting of a strip suspended on a bed of nails. The bed of nails enforces a stopband, while the strip opens a propagating path that can be used to design circuits and feeding networks for the microwave and millimeter-wave bands. Advantages and drawbacks with respect to existing ridge gap waveguide are discussed. Experimental demonstration is also given.

Optimization technique for linearly polarized radial-line slot-array antennas using the multiple sweep method of moments

This paper presents an effective full-wave procedure to optimize linearly polarized radial-line slot-array antennas. The procedure is based on a recursive method for solving large matrix equations known as the multiple sweep method of moments. As a result of this paper, new design suggestions have been introduced which greatly contribute to the performance improvement of this type of antennas. These elements are the arrangement of the slots in concentric ellipses instead of circles and a slight but crucial modification on the usual collocation of the reflection-canceling slots.

On the use of characteristic modes to describe patch antenna performance

This work introduces some preliminary results focused to point out that the theory of characteristic modes may help in the design of microstrip patch antennas, as it brings clear insight of the physical phenomena taking place in it, and presents no limitation over the height of the patch or the dielectric constant of the materials. Characteristic modes are defined as the real currents on the surface of a conducting body that depend on its shape and size, and are independent of the feed point. As characteristic modes form a close and orthogonal set of functions, they can be used to expand the total current. Another advantage of this method, is that for electrically small and intermediate size bodies, only a few modes are needed, and the problem can be dealt with only by considering two or three modes.

A novel band-pass filter topology for millimeter-wave applications based on the groove gap waveguide

In this paper a new type of band-pass filters for high frequency applications (40 GHz and beyond), with very good electrical performance and enhanced manufacturing flexibility, is demonstrated. These filter structures are based on the recently proposed groove gap waveguide, which is just made of two simple metal blocks separated by an air gap. This means that electrical contact between the two metal blocks is not needed, thus involving several advantages when compared to standard rectangular waveguide filters operating at millimeter-waves. A 4th order band-pass filter with a response centered at 40 GHz and a 2.5% fractional bandwidth has been designed, manufactured and verified with experimental results.

Design of microwave circuits in ridge-gap waveguide technology

This paper presents recent advances is a new waveguiding technology referred to as ridge gap waveguides. The main advantages of the ridge gap waveguides compared to hollow waveguides are that they are planar and much cheaper to manufacture, in particular at high frequencies such as for millimeter and submillimeter waves. In these waveguides there are no mechanical joints across which electric currents must float. The gap waveguides have lower losses than microstrip lines, and they are completely shielded by metal so no additional packaging is needed, in contrast to the severe packaging problems associated with microstrip circuits. The gap waveguides are realized in a narrow gap between two parallel metal plates by using a texture on one of the surfaces. The waves follow metal ridges in the textured surface. All wave propagation in other directions is prohibited (in cutoff) by realizing a high impedance (ideally a perfect magnetic conductor) through the textured surface at both sides of all ridges. Thereby, cavity resonances do not appear within the band of operation. The paper studies the characteristic impedance of the line and presents simulations and measurements of circuits designed using this technology.

Wideband radiating ground plane with notches

Self-resonant notch antennas, fabricated using microstrip technology are very compact structures. Typically, notch antennas consist of a quarter wavelength slot cut on the edge of a semi-infinite ground plane; they are fed from a coaxial cable without the need of any balancing system. However, when a notch is cut on a finite ground plane of resonant dimensions, the shape and size of the ground plane affect the performance of the notch antenna significantly. It can be demonstrated that the interaction between the resonances of the notch and the ground plane results in a filtering effect at some frequencies. The paper uses the theory of characteristic modes to perform a modal analysis of different configurations of notch antennas on a finite ground plane. By means of this theory, the current on the surface of the antenna is expressed as a sum of real, and orthogonal, eigencurrents, that are known as characteristic modes. The filtering effect due to the interaction of the resonances can be explained with the information provided by the current distribution of the modes and their associated eigenvalues. Using a double notched radiating ground plane with two feed points, the filtering effect is reduced, and the matching is improved.

Modal analysis of a radiating slotted PCB for mobile handsets

The purpose of this paper is to describe the procedure carried out to design a handset antenna using the Theory of Characteristic Modes. The antenna, which is based on the PCB resonance design concept, consists of a folded slotted PCB that is excited by means of a planar square monopole. This antenna is suitable for mobile terminals as it provides excellent wideband performance and omnidirecctional radiation patterns.