Wrist-type wireless instrument and antenna apparatus

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BACKGROUND OF THE INVENTION

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.

SUMMARY OF THE INVENTION

The present invention has been made with a view toward solving the above problem. It is an object of the present invention to provide an improved antenna structure for a wrist-type wireless instrument or antenna apparatus of the type in which an antenna body, formed unitarily with the wrist band, functions as a slot antenna, wherein the space inside the wireless instrument casing is effectively utilized, thereby providing a simple structure which helps to obtain an antenna gain larger than that in the prior art.

To solve the above problem, the present invention provides a wrist-type wireless instrument that includes a casing containing a circuit board; a wrist band connected to either side of the casing; and a conductive plate having a slot and provided in the casing and in the wrist band, wherein a capacitive element is loaded across conductive sections of a conductive plate that are defined by a slot, wherein the circuit board is arranged on the inner side of a loop for wrist wear that is formed by the casing and the wrist band, and wherein the capacitive element is arranged approximately at the center of the circuit board.

More specifically, it is desirable that the wrist band be comprised of two (first and second) wrist band members that are respectively joined to each side of the casing. It is also desirable that the conductive plate be comprised of two (first and second) conductive plates and two electric paths provided in the casing, the two electric paths and the two conductive plates being respectively provided for the two wrist band members, a slot being formed in each conductive plate so as to extend longitudinally from the end edge of the casing to which the associated wrist band member is joined, thereby dividing each conductive plate into first and second sections.

Additionally, it is desirable that, on the casing side, the ends of the respective first sections of the first and second conductive plates be electrically connected to each other through one of the two electrical paths, the ends of the respective second sections of the first and second conductive plates being electrically connected to each other through the other of the two electrical paths, with the capacitive element being loaded between the two electric paths.

Further, it is desirable that the capacitive element be arranged between a display element for displaying information of the wrist-type wireless instrument and the circuit board. Alternatively, the capacitive element may be surface-mounted on the circuit board.

Further, it is possible for the capacitive element to be comprised of two varactor diodes that are connected in series.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are described hereafter, with reference to the drawings, in which:

FIG. 1 illustrates the construction of a conventional, prior art wrist-type wireless instrument antenna apparatus;

FIG. 2 is a graph showing the radiation directivity characteristic of a conventional, prior art wrist-type wireless instrument antenna apparatus;

FIG. 3 illustrates the construction of a wrist-type wireless instrument antenna apparatus according to the present invention;

FIG. 4 illustrates the operational principle of the wrist-type wireless instrument antenna apparatus shown in FIG. 1;

FIGS. 5(a) and 5(b) are diagrams illustrating antenna characteristics, which depend on the position and arrangement of the capacitive element, of which FIG. 5(a) is a plan view of a wrist-type wireless instrument antenna apparatus and FIG. 5(b) is a side view thereof;

FIGS. 6(a) and 6(b) are diagrams illustrating the operation of the present invention (the antenna characteristic depending on the position of arrangement and the capacitive element), of which portion 6(a) is a plan view of a wrist-type wireless instrument antenna apparatus according to the present invention and FIG. 6(b) is a side view thereof;

FIG. 7 is a schematic perspective view of a wrist-type wireless instrument equipped with a wrist-type wireless instrument antenna apparatus according an embodiment of the present invention;

FIGS. 8(a) and 8(b) are diagrams showing a wrist-type wireless instrument antenna apparatus according an embodiment of the present invention, of which FIG. 8(a) is a cutaway top view showing the structure of a near-casing portion thereof and FIG. 8(b) is a cutaway side view thereof;

FIG. 9 is a cutaway top view showing the structure of a near-casing portion of a wrist-type wireless instrument that is equipped with a wrist-type