Bandwidth enhancement of a compact antenna by parasitic elements.

摘要

Antenna structures are playing a major role in wireless systems, including communication systems, radars, satellite systems, information networks and medical diagnostic systems. In these systems, external antennas are widely used, especially for devices with metallic enclosures. One of the common requirements for the external antennas is the need for achieving small dimension while keeping the antenna performance optimized. In this respect, electrically small antennas (ESAs) are employed. Realizing the design requirements of an ESA has been a challenge for antenna designers because some of the parameters such as large bandwidth and reasonably high input impedance are in conflict with the small antenna dimensions. A compact antenna with parasitic elements for bandwidth enhancement is proposed to address these issues for use with mobile devices. Loading the known antenna topologies with other structures could improve the design parameters such as gain, bandwidth, impedance, and bandwidth for antenna topologies which are impossible to achieve with the conventional antennas. In this respect, a parasitic element with a smaller scale is placed within the structure to obtain a second resonance close to the main antennas resonance for an increased bandwidth without increasing the overall dimensions of the antenna. This composite antenna is expected to have relatively high input impedances for each of the resonances, thus minimizing the need for an input matching network. A review of related prior work of antenna structures is presented to gain insight into the recent developments and methods in the field. The minimum quality factor (Q) and maximum gain of the ESA designs and their theoretical limitations are then discussed. The features of the antenna designs discussed in the review are used as foundation of the developed antenna structures. The properties and a fabrication method of the proposed antenna are then discussed. A commercially available software package based on finite element method is employed to aid in the antenna design. Several antenna prototypes are constructed to verify the design and the accuracy of the simulations. The prototypes are then tested using network analyzers and an RF anechoic chamber with the aim of characterizing the antenna performance in terms of antenna bandwidth and input impedance.