Marta Cabedo-Fabres

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.

Analysis of the coupled chassis-antenna modes in mobile handsets

Due to the development of electronics in the last decade, mobile terminals are becoming smaller and smaller. In the miniaturization process of terminals, the antenna is a critical element. The objective of this paper is to apply the characteristic mode theory, defined by Harrington and Mautz (1971), to identify the modes mentioned in Vainikainen et al. (2002) and consequently improve the design process of the antenna-chassis structure. The theory of characteristic modes brings a clear insight into the physical phenomena taking place in the structure and provides very useful information for the design.

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.

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.

Improved Planar Wideband Antenna Element and Its Usage in a Mobile MIMO System

A simple antenna structure for a multiple-input-multiple-output (MIMO) system in a mobile terminal with spatial diversity is presented. A single antenna element consists of a combination of an electric dipole and a square magnetic slot. The studies with a single quasi-complementary antenna element show that the electric dipole and the square-shaped magnetic slot are partially compensating the effect of the electric conductor closely spaced to the electric dipole (0.0037-0.01λ) . Simulations also show that by scaling antenna structure, a 0.7-2.1-GHz frequency bandwidth can be achieved. The measured -6-dB impedance bandwidth of the MIMO antenna prototype is from 2.0 to 5.6 GHz, corresponding to a 95% relative bandwidth. The measured S21 is less than - 19 dB within the - 6-dB impedance bandwidth. The average measured total efficiency at the aforementioned bandwidth is - 0.85 dB. The measured radiation patterns are presented at 2 and 5 GHz with a maximum total gain of 2.8 and 5.7 dBi, respectively. The measured envelope correlation of the MIMO antenna is less than 0.04 with 90% MIMO efficiency within the -6-dB impedance bandwidth.

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.

A Novel Low-Profile High-Gain UHF Antenna Using High-Impedance Surfaces

A novel wideband low-profile planar antenna design based on the use of a high-impedance surface (HIS) is presented for digital television (DTV) reception in the UHF band. The proposed design is based on a wideband monopole antenna in close proximity to an artificial ground plane, composed of an array of square metal loops at the top, an air gap and a ground plane at the bottom, without grounded vias. Low-cost substrates are employed in the design: rigid PVC for the antenna and foamed PVC for the artificial ground plane. The advantages of the proposed design for DTV applications are high gain, low profile, light weight, low cost and large bandwidth. Details of the proposed antenna design are described, and measurements together with a comparison to a conventional antenna with PEC ground plane are presented.

Periodic Leaky-Wave Antenna on Planar Goubau Line at Millimeter-Wave Frequencies

A periodic leaky-wave antenna on a planar Goubau line is presented. This transmission line is formed by a planar single-wire waveguide on a thin dielectric substrate. Leakage is produced by adding dipoles along the line on the bottom face of the substrate. A coplanar waveguide is used to feed the antenna, which acts as a smooth transition between the input coaxial cable and the planar Goubau line. The advantage of using this line lies on its losses, lower than those of typical microstrip lines due to the absence of a ground plane. As a result, a higher radiation efficiency than in microstrip-fed antennas can be obtained while keeping similar advantages, e.g., low profile or low production cost. A prototype of the antenna at 40 GHz has been fabricated. Measurements of this prototype are presented in this letter.

Creation of a Magnetic Boundary Condition in a Radiating Ground Plane to Excite Antenna Modes

The creation of a magnetic boundary condition into the plane of symmetry in a radiating ground plane of a portable device is proposed to enhance its radiation efficiency and bandwidth. This magnetic boundary condition is achieved by exciting the antenna through symmetrical feeding. A novel antenna consisting of a symmetrical folded dipole is presented in order to exemplify how a broad bandwidth and very good radiation properties can be obtained with the proposed technique. The Theory of Characteristic Modes is used to analyze and compare a folded dipole with the symmetrical folded dipole. This study shows how the magnetic boundary condition favors excitation of antenna modes in a broad bandwidth while avoiding excitation of transmission line modes. Two different prototype antennas, a monopole and a symmetrical folded dipole with an LC-balun, were implemented and measured. The measured -6 dB impedance bandwidth of the monopole extends from 0.95 GHz to 2.15 GHz. In turn, the bandwidth of the symmetrical folded dipole extends from 0.90 GHz to 1.96 GHz. The average of the measured total efficiencies at the aforementioned bandwidth is -1.6 dB for both prototypes. The measured maximum total gain at 0.95 GHz is 2.6 dBi for the monopole and 1.9 dBi for the symmetrical folded dipole.

Systematic study of elliptical loop antennas using characteristic modes

We make use of characteristic modes to analyze and optimize elliptical loop and other wire structures. This work introduces some preliminary results to point out that characteristic modes may help in antenna design, since they give clear physical insight into the radiation characteristics. Relevant information about antenna resonant behavior can be obtained by studying characteristic mode variation with frequency. Since mode excitation is proportional to current amplitude at the feed point, antenna properties can be improved by properly combining and exciting the desired modes. Characteristic modes and their associated eigenvalues are obtained from the symmetric generalized impedance matrix of the structure. In this work the Galerkin method of moments formulation has been applied for computing the generalized impedance matrix.