1. Field of the Invention
This invention relates to a wrist-type wireless instrument in which an antenna body, unitarily formed with a wrist band, functions as a slot antenna and, in particular, to an antenna apparatus for attaining an improvement in terms of antenna gain.
2. Description Of Related Art
FIG. 1 shows the structure of an antenna body 1, which is an example of the wrist-type wireless instrument in which an antenna body, unitarily formed with a wrist band, functions as a slot antenna. A strip-shaped conductive plate 2 is fastened to a wrist band, and a narrow slot 3 is formed so as to extend along the length dimension of the wrist band, thereby dividing the conductive plate 2 into two sections 2a and 2b. In FIG. 1, the wireless instrument is shown in the state in which it is placed on a person's wrist, i.e., the conductive plate 2 is looped. A capacitive element 4 is electrically connected across the sections 2a and 2b of the conductive plate 2. By appropriately adjusting the capacitance value of this capacitive element 4, it is possible for the antenna to be tuned to a desired frequency. It is desirable that the capacitive element 4 be electrically connected approximately in the central portion with respect to the length dimension of the conductive plate 2 because in this portion the current distribution can be well balanced and a large antenna gain can be obtained. Feeders 5a and 5b, connected to the conductive plate sections 2a and 2b respectively, are connected to a wireless instrument circuit section (not shown).
FIG. 2 shows the radiation pattern of this antenna. The drawing shows the vertical polarization component in the X-Y plane, which is normalized in maximum antenna gain. The radiation is maximum in the direction of the X-axis, which corresponds to the loading direction of the capacitive element 4. This fact indicates that the antenna is functioning as a slot antenna.
However, in this slot antenna for wrist-type wireless instrument, which is unitarily formed with the wrist band, the antenna is restricted to rather small dimensions, so that it is rather difficult to obtain a sufficient level of antenna gain. Thus, with the conventional antenna, it has been difficult to maintain a satisfactory radio communication quality.