Design of miniaturized slot antennas

摘要

In this paper a procedure for the design of a novel small slot antenna is presented. Using this method enables us to achieve a perfectly matched antenna with a fairly high efficiency for a given arbitrary small size. The miniaturization is achieved by terminating the short slot by an inductor. Inductive loading it realized by coiling the shortened slot line with a length of less than a quarter wavelength. Although the choice of antenna size it optional, it is subject to limitations such as antenna bandwidth and gain/efficiency. However, the directivity of this small dipole-slot antenna, in me limit, is equal to that of the infinitesimal Hertzian dipole, and its gain depends on the substrate material specifications and antenna size. Also, the smaller the antenna is, the narrower bandwidth it has. This in fact is considered to be a fundamental limitation of the small antennas. Microstrip line is used to feed the antenna, which provides more stable feeding compared to coaxial feed. Also, it is a proper choice for compensating the reactive part of the input impedance introduced by inductive loading. The topic of small antennas has been of prolonged interest and goes back more than half a century. Using the area of the substrate more effectively in microwave Circuits, as part of a general trend in monolithic circuit integration and antenna invisibility for military applications, has been among the major motivations. On the other hand, in the radio communication, where the line of night communication is not generally possible, the UHF-VHF frequencies should be used. At these low frequencies, the size of even a single half wave dipole antenna in preclusive in many mobile and wireless applications. Normally, there is a compromise between the size, efficiency and bandwidth of the antenna. Chu [1] addressed this subject by expanding fields of an arbitrary small antenna enclosed in a sphere, using spherical eigen-functions expansion. The Q of the antenna, which is by definition the ratio of the stored energy to the radiated power, can be related to the Q of each eigen-mode. This approach introduces a lower bound on the antenna's Q. The calculated Q is a function of radius of the sphere or correspondingly the largest dimension of the antenna. On the other hand, a lower bound on Q in some senses is an indication of an upper limit on the antenna bandwidth [2].