Eva Antonino-Daviu

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.

Modal Analysis and Design of Band-Notched UWB Planar Monopole Antennas

The theory of characteristic modes is proposed for the analysis of simple band-notched UWB planar monopole antennas, since the information provided by these real modes (i.e. resonant frequency and radiating bandwidth) provides a very interesting physical insight into the radiation phenomena taking place in this type of antennas. Such an in-depth understanding paves the way for the proposal of novel designs of UWB antennas with increased functionality, through the control of the excitation and resonance of specific modes. Modal analysis of band-notched UWB antennas illustrates the existence of resonant modes linked to the embedded narrowband slot structure (slot modes), whose location within the planar geometry of the monopole determines the effect over the rest of the radiating modes of the wideband structure, and consequently, over the behavior of the antenna. By electronically controlling the excitation of the first slot mode, an UWB antenna with switchable band-notched behavior is proposed. Similarly, a tunable band-notched UWB antenna can be proposed by controlling the resonance of this slot mode. Prototypes for both the switchable and tunable band-notched antenna have been fabricated and measured, demonstrating the expected behavior. The understanding of the radiating behavior of this type of antennas can be easily extrapolated to other configurations, such as coplanar band-notched UWB antennas, which are more compact than the designs presented here, and may help to propose other antennas of this type.

A Broadband Pattern Diversity Annular Slot Antenna

An annular slot antenna suitable for pattern diversity in wideband applications is presented. Pattern diversity is obtained by feeding the antenna simultaneously with two microstrip lines printed on the back of the substrate, which generate orthogonal radiation patterns. Two shorts placed at 45 degrees between the microstrip lines and in the opposite direction ensure an isolation above 15 dB according to measurements. The antenna operates in the range of frequencies from 3 GHz to 12 GHz (1:4 bandwidth) and covers many different standards (Wireless-LAN, HIPERLAN2, WIMAX, UWB). Simulations and measurements for return loss, isolation and radiation patterns are presented. Furthermore, measurements have been carried out in a real multipath environment in order to evaluate pattern diversity performance. Results of measurements of the correlation coefficient and diversity gain over the entire operating bandwidth show that the antenna is suitable for antenna diversity systems.

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.

Planar Wideband Polarization Diversity Antenna for Mobile Terminals

This letter presents a planar wideband polarization diversity antenna for mobile terminals. The antenna consists of two orthogonally oriented quasi-complementary antennas (QCA) formed by the combination of an electric dipole and a magnetic slot and located at the separate ends of a mobile ground plane. The studies with a single quasi-complementary antenna element show that the electric dipole and the magnetic slot are partly compensating each other providing large bandwidth and offering good radiation properties. The measured -6-dB impedance bandwidth of the individual QCA is from 1.8 to 4.6 GHz, corresponding to an 87.5% relative bandwidth. The measured S21 is less than - 18 dB within the - 6-dB impedance bandwidth. The average measured total efficiency at the aforementioned bandwidth is - 0.95 dB. The measured radiation patterns are presented at 2 and 4 GHz with orthogonal polarizations. Finally, the measured envelope correlation and effective diversity gain (EDG) as a function of frequency are presented, with a maximum correlation less than 0.01 and EDG better than 8.0 dB.

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.

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.