The present invention relates to an antenna of elongated shape for an instrument of small volume, in particular a telephone-watch, capable of receiving and transmitting radiobroadcast messages at at least two frequencies of high and low value, this antenna being formed, from a feed point, of a first radiating element the length of which is tuned to the high frequency and at least a second radiating element, following the first, the length of this second element added to that of the first having a total length tuned to the low frequency, the first and second radiating elements being connected to each other by a resonant circuit whose resonance frequency is chosen to limit the length of the antenna to its first element when the high frequency is active and to use the total length of the antenna when the low frequency is active.
An antenna answering the generic definition above is known from the state of the art. It is described, in particular, on page 17-6 of the xe2x80x9cARRL Handbook 1989xe2x80x9d and is illustrated in FIG. 1 accompanying the present description. Another example of such an antenna is, for example, disclosed in U.S. Pat. No. 2,282,292. It is a dipole antenna powered by a feeder 25. From a feed point 2, each strand of the antenna includes a first radiating element 3, then a resonant circuit 5, and finally a second radiating element 4. The antenna is intended to be tuned to two different frequencies, for example 28 and 21 MHz. The length L1 of first radiating element 3 is matched to the frequency of 28 MHz (or more exactly to the quarter wavelength of this frequency). The length L2 of second radiating element 4 added to length L1 of the first element leads to a radiating element of length L3 matched to the frequency of 21 MHz (or as above, to the quarter wavelength of this frequency). Resonant circuit 5 is an oscillating circuit including a coil 6 and a capacitor 7 connected in parallel. The values of these components are chosen to resonate at 28 MHz. Since the impedance of the resonant circuit is at a maximum at this frequency, the resonant circuit will act as a xe2x80x9ccapxe2x80x9d for said frequency and thus limit the length of the strand of first radiating element 3. However, at 21 MHz, the resonant circuit has very low impedance, such that the total length of the strand is used. Thus, via relatively simple means a section L1 or the whole L3 of the antenna can be made to resonate.
At the frequencies considered hereinbefore (the short-wave range) the antenna is made by means of tubes forming radiating elements 3 and 4, these tubes being joined by a sleeve containing resonant circuit 5 made by means of discrete components namely a coil or inductor 6 and a capacitor 7.
The frequencies implemented in these instruments of small volume, for example a mobile telephone or even a telephone watch are much higher than those referred to above. If the principle of matching the antenna to at least two different frequencies can remain the same as that described hereinbefore, the technique used for these short wavelengths will have to be matched to the antenna employed. This antenna has to be able to operate at least with the official frequencies standardised for example by the GSM (Groupe Spxc3xa9cial Mobile) which provides a high frequency fh equal to 1.9 GHz and a low frequency fb, equal to 900 MHz.
The idea of the present invention is to propose an antenna capable of being matched to the aforementioned frequencies. For this purpose, in addition to satisfying the definition given in the first paragraph of this description, the antenna is characterised in that the first and second radiating elements each have a conductive strip of substantially rectangular shape and in that the resonant circuit includes the combination of an inductor and a capacitor, said inductor being a narrow substantially rectilinear band formed integrally with at least one of said strips and connected to the strip by one of its ends.
It will be noted that European Patent document No. 0 470 797 discloses an antenna capable of being matched to several frequencies. All the embodiments envisaged in this document nonetheless rely on inductors formed of discrete components which have thus to be bonded via their ends to the various radiating elements of the antenna.
It will also be noted that International Patent document No. WO 99/03168 discloses a compact antenna capable of being matched to at least a low frequency and a high frequency, the antenna being intended, in particular, to be fitted to mobile telephone apparatus. According to an embodiment described with reference to FIG. 1 of this document, the antenna has two radiating elements connected to each other by a resonant circuit which can schematically represented as the parallel arrangement of a capacitor and an inductor. It is proposed to realise this resonant circuit and particularly the inductor in the form of relatively wide printed strip having the shape of a meander. The capacitance value of the resonant circuit is determined here by the stray capacitance present between the xe2x80x9cturnsxe2x80x9d or meanders of the inductor.
One drawback of this solution lies in the fact that the adjustment of the resonant frequency of the resonant circuit is difficult to carry out. Indeed, if one wishes to modify the inductance value of the resonant circuit, the width and/or length of the meander has to be modified. By carrying out such an operation, the stray capacitance value of the resonant circuit is also thereby affected.
The solution of the present invention has the advantage of being able to easily adjust the resonance frequency of the resonant circuit by acting independently on the inductance value or on the value of the capacitor. In particular, the inductor formed of a substantially rectilinear narrow path does not substantially affect the capacitance value of the resonant circuit. Furthermore, a narrow path for the inductance has the advantage of higher inductivity for equal dimension with respect to the solution envisaged in International Patent document No. WO 99/03168.