EP0954051B1 - Slot antenna in particular for a timepiece - Google Patents

Description

The present invention relates to a slot antenna intended to be connected to an unbalanced output of a antenna circuit, and more particularly, the adaptation and the excitement of such a slot antenna in horological applications. However, it will be understood that the invention is not limited to these applications.

This type of antenna includes a conductive element disposed on a dielectric substrate and defining a radiant slit with parallel edges. Such an antenna is already described in document EP 0 766 152 in the name of the Applicant. Other examples of slit waits are also described in documents GB 2,304,465 and EP 0 643 437. Figure 1 shows the antenna described in the document EP 0 766 152. This slot antenna 11 consists in a conductive element in printed circuit 12 fixed on a dielectric substrate 13. This substrate is preferably a flexible material allowing the accommodation of this antenna in a mounting groove of a watch case so that it is wrapped around the housing. A radiant slot 14 is formed in the conductive element. This antenna is connected to an antenna circuit comprising a micro receiver-transmitter 15 located in the housing watchmaker. The connection between antenna 11 and the microphone receiver-transmitter 15 is assigned at two points excitation 16 and 17, normally by a line of transmission such as a coaxial cable. This coaxial cable can be considered as the output of the antenna circuit. As the output of the antenna circuit is asymmetrical, it is necessary to adapt the symmetrical antenna to this exit. Thus, it is mandatory to have a conversion mounted between the balanced antenna and the output asymmetric. Such a device is known to the person skilled in the art. trade under the English term "balun" (for balanced-to-unbalanced). This "balun" is usually done using discrete elements, such as a capacitor and / or a line section. In document EP 0 766 152 mentioned above, a line 18 connects an excitation point 16 via an adaptation capacitor C (see figure 1) at the microphone receiver-transmitter 15 of the antenna circuit, and the other excitation point 17 is connected to ground. The value of capacitor obviously depends on the frequency used, but may be of the order of several pico-farads in the applications described in this document. We understand as such a "balun" is impractical, bulky and expensive.

However, for reasons of aesthetics, size, price etc., (criteria which are particularly crucial in watchmaking technology), we are looking for solutions which reduce costs and simplify the editing, excitement as well as adaptation of such antenna.

Thus, the invention aims to provide an antenna slot which avoids the use of discrete components for excitement and adaptation by offering an antenna with slot having an integrated excitation, which serves at the same time time of "balun" realizing the transition between the exit of the micro receiver-transmitter and the antenna.

The invention therefore relates to a slot antenna according to claim 1.

Thanks to these characteristics, antenna structure optimized for excitation in a timepiece. In addition, this antenna allows obtain a gain / bandwidth ratio at least equal to that of known antennas, but with great simplification, saving space and eliminating components while being cheaper to manufacture.

• la figure 1 déjà décrite est une antenne à fente de l'art antérieur,
• la figure 2 est une vue de dessus d'un premier mode de réalisation d'une antenne à fente selon l'invention,
• la figure 3 est une vue de coupe de l'antenne de la figure 2, et
• la figure 4 montre une vue de dessus d'un deuxième mode de réalisation d'une antenne à fente selon l'invention.

Other characteristics and advantages of the invention will appear during the description which follows, given solely by way of nonlimiting example and made with reference to the appended drawings in which:

• FIG. 1, already described, is a slot antenna of the prior art,
• FIG. 2 is a top view of a first embodiment of a slot antenna according to the invention,
• FIG. 3 is a sectional view of the antenna of FIG. 2, and
• Figure 4 shows a top view of a second embodiment of a slot antenna according to the invention.

Figures 2 and 3 show a first mode of realization of the slot antenna according to the invention and suitable for a timepiece. The structure general of the antenna according to the invention is similar to that of the antenna of the aforementioned document EP 0 766 152. Thus, we see a slot antenna 21 which consists of a conductive element in printed circuit 22, for example copper, fixed on a dielectric substrate 23. This substrate is preferably of a thin flexible material, for example example of perstorp or kapton epoxy, allowing the housing of this antenna in a mounting groove of a clock case so that it is wrapped around of the case, as explained in more detail in the above-mentioned document. Preferably, this substrate 23 has a thickness of the order of 0.1 mm.

The conductive element 22 is rectangular in shape and defines a radiating slot 24 with parallel edges. In effect, the conductive element 22 has, in this example, two long parallel edges 22a and 22b, and two short edges parallel 22c, 22d. The slot 24 is therefore also of rectangular shape.

In this embodiment, we see that the element conductor 22 does not completely close slot 24, but leaves a central opening 26 on a 22b of its edges long. Of course, the other edge 22a could remain open instead of this edge 22b.

The antenna also has a central line printed 27 which is arranged in the slot 24 at the place of the opening 26. This central line 27 is of the same material and is attached to substrate 23 in the same way as the conductive element 22. In this example, the line central 27 has a rectangular shape having a width "d", but such a form is not mandatory. The side bottom of the center line is aligned with the edge long 22b, and the upper side advances in the slot in approaching the other long edge 22a. However, the line central 27 does not touch this edge 22a, but is distant there from a distance ∂. The central line 27 is also distant by a distance s from each of the two ends of the long edge 22b of the conductive element 22.

The antenna 21 is connected to an output 29 of a circuit antenna comprising a micro receiver-transmitter 25 which is found for example in a watch case. The connection between the antenna 21 and the micro receiver-transmitter 15 made by a transmission line 31 such as a cable coaxial or a microstrip line having a conductor of excitement and mass. The antenna is excited by means of a signal applied to the central line 27. At this effect the line excitation conductor transmission 31 is connected to an excitation point 28 located on the center line 27. The ground conductor and the conductive element 22 are grounded, by example by means of a connection point, not referenced, on the long edge 22b. In the example shown, each of the two ends of this edge 22b are connected to the mass.

Thanks to the central line 27, the excitement is integrated into the antenna and in the same plane. This line central serves at the same time as "balun", that is to say this central line allows direct connection to the microphone receiver-transmitter without the need for elements coping skills such as adaptability. It is therefore the central line 27 which makes the transition between the asymmetrical output 29 of the antenna circuit and the antenna 21. In fact, this structure of the antenna according to the invention can be compared to a coaxial cable which has also a central line (the conductor) surrounded by a grounded item. This structure provides an antenna with very good gain / bandwidth ratios compared to the antennas of the prior art.

Indeed, the Applicant has found that by varying the distance ∂, you can modify the antenna adaptation 21. Thus, by playing on the distance ∂, we can obtain a correct adaptation of the antenna to the output 29 of the antenna circuit. The value of this distance corresponds therefore the value of an antenna's adaptability according to the prior art.

la longueur totale de l'élément conducteur 22, c'est-à-dire d'un bord long 22a, 22b est environ 45 mm, sa largueur est autour de 2 mm, la longueur de la fente est de l'ordre de 36,4 mm, et sa largeur est autour de 4 mm, la distance s est environ 0,27 mm, la largeur d de la ligne 27 est autour de 12 mm, et la distance ∂ est environ 0.1 mm.

For example, the antenna can be used to operate at 1.9 GHz. The approximate dimensions for this frequency are as follows:

the total length of the conductive element 22, that is to say of a long edge 22a, 22b is approximately 45 mm, its width is around 2 mm, the length of the slot is of the order of 36 , 4mm, and its width is around 4mm, the distance s is about 0.27mm, the width d of line 27 is around 12mm, and the distance ∂ is about 0.1mm.

Of course, these dimensions depend on the frequency, and can be changed as required.

It is thus understood that the shape of the antenna 21 allows optimal use of the available space, and that miniaturization of the antenna is possible while keeping an optimal gain / size ratio. This therefore allows to use this antenna, which has a structure suitable for optimizing excitement, in a portable device such than a wristwatch.

Figure 4 shows a second embodiment of the antenna according to the invention. We can see that, compared to the first mode of Figures 2 and 3, the conductive element 22 ' is closed and completely surrounds the slot 24 '. Line central 27 'is inside the slot 24' and is arranged in the same way as in the first mode, that is to say that it is at a distance ∂ from the edge long 22a 'of the conductive element 22'. The central line 27 'is not completely rectangular here, but it has, at its side near the long edge 22b 'a projection 30 which advance towards this edge 22b '. The long edge 22b 'is continuous, but it has a notch to receive at least partially the projection 30 of the central line 27 '.

The antenna according to this second mode can also be connected directly via transmission line 31 to antenna circuit comprising the micro receiver-transmitter 25 without the need for discrete components.

Thanks to the antenna according to the invention, it is possible to incorporate it into a timepiece likely to be worn on the wrist. The antenna is therefore able to receive and / or emit electromagnetic fields carrying radio messages. In a known way, the piece of watchmaking has organs for displaying the time, for example hands or a display digital, and an antenna circuit including the microphone receiver-transmitter to review messages received by the antenna,

An antenna shielding can be provided according to the invention, that is to say it is possible to provide a ground plane placed behind the substrate such that described in more detail in European patent application EP 0 766 152 above.

Claims (7)

1. Slot antenna (21) arranged to be connected to an asymmetrical output (29) of an antenna circuit (25), comprising

   a dielectric substrate (23),

   a printed circuit conducting element (22; 22') attached to said substrate and defining a radiating slot (24; 24') having parallel edges, characterised in that the antenna further comprises a printed central line (27; 27') arranged in said slot (24; 24') for exciting the antenna and for serving at the same time as a balun allowing a direct connection to said output (29) in such a manner that said central line (27; 27') realises the transition between said output and the antenna, central line (27; 27') being arranged at a distance δ from a first long edge of said conducting element (22; 22'), said distance δ being representative of the adaptation of the antenna to said output (29).
2. Antenna (21) according to claim 1, characterised in that said substrate is of a flexible material.
3. Antenna (21) according to any of claims 1 or 2, characterised in that said conducting element (22) is rectangular and is discontinuous and that it comprises in a second long edge (22b), an opening (26) which is connected to said slot (24).
4. Antenna (21) according to claim 3, characterised in that said central line (27) is arranged in said opening and enters said slot (24) by approaching said conducting element (22), but that said central line remains removed by a distance δ from the first long edge (22a) of said conducting element (22).
5. Antenna (21) according to any of claims 1 or 2, characterised in that said conducting element (22') is rectangular and is continuous and that it completely surrounds said slot (24').
6. Antenna (21) according to claim 5, characterised in that said central line (27') comprises a projection (30), and that a second long edge (22B') of said conducting element comprises a notch for receiving said projection (30).
7. Timepiece susceptible of being worn on a wrist, and comprising time indicating means, an antenna capable of picking up and/or emitting an electromagnetic field carrying radio transmitted messages and an antenna circuit for receiving messages picked up by the antenna, characterised in that said antenna is a slot antenna (21) according to any one of the preceding claims.

Images