Abstract:
An antenna comprises a first antenna element, which has a first helix, and a second antenna element, which has a second helix. The first and the second antenna elements each have a feed point at an outer end of the corresponding helix and an open end at an inner end of the corresponding helix. A symmetrical helix antenna according to the invention can be integrated in a comparatively simple manner in an existing system, for example in a hearing aid. By integrating the antenna in a plastic housing, the antenna cannot be seen at all from the outside. The antenna is comparatively small in relation to conventional antennas.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2006/001335 filed Feb. 14, 2006 and claims the benefits thereof. The International Application claims the benefits of German application No. 10 2005 008 063.4 filed Feb. 22, 2005, both of the applications are incorporated by reference herein in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates in general to an antenna, in particular to an antenna for wireless data transmissions to a hearing aid. 
     BACKGROUND OF THE INVENTION 
     There are presently numerous portable devices from which and to which data is to be transmitted by wireless means. One possibility which suggests itself here is to realize the data transmission by electromagnetic coupling. In doing this, particular difficulties arise if the devices used are very small, because in such a situation there are problems in integrating an antenna structure into the device concerned. An important example of a very small device for which wireless data transmission is required is a hearing aid. 
     In accordance with the prior art, the transmission of data to a hearing aid is often realized in practice by inductive transmission links. For this purpose, an induction loop is integrated into the hearing aid. However, this type of inductive transmission of speech or data, as applicable, to a hearing aid requires special installations in the area concerned, where the wireless transmission of speech or data is to take place. 
     In another form of embodiment of wireless radio transmission systems, magnetic antennas are used in the hearing aids. These essentially couple into the magnetic components of an electromagnetic field, and are generally designed as conductive loops. Radio transmission systems of this type generally work at frequencies which are significantly lower than the frequencies used in mobile radiocommunications, e.g. in the VHF band at 174 MHz. 
     European patent application EP 1 326 302 A2 describes a fractal antenna structure which is realized on an integrated circuit, and which can be used in a hearing aid. However, the fractal antennas described in the document cited can only be considered for significantly higher frequencies. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to devise an antenna, which can be integrated into a portable device, and which has smaller maximum geometric dimensions than a dipole antenna for the corresponding frequency. 
     This object is achieved by an antenna as claimed in the claims. 
     The present invention creates an antenna with a first radiator, which has a first spiral, and a second radiator, which has a second spiral, where the first radiator has a first feed point at an outer end of the first spiral and has an open end at an inner end of the first spiral, and where the second radiator has a second feed point at an outer end of the second spiral and has an open end at an inner end of the second spiral. 
     The central thought behind the present invention is that the maximum dimensions of a linear antenna can be reduced by making both radiators in the form of a spiral. In this case, each of the two radiators has one feed point which is sited at the outer end of the spiral concerned. On the other hand, the inner ends of the two spirals are open circuits. In contrast to a simple shortening of the two radiators, curling up the two radiators results in an antenna with an input impedance which can without problem be matched to the transmission powers or to the transmitting and receiving stages, as applicable, used in practice. 
     An antenna design in accordance with the invention thus enables it to be fully integrated into a mobile device which has wireless data transmission. Because of its small dimensions and the flexibility of its geometric layout, the antenna structure in accordance with the invention can then be integrated into a plastic housing. It is thus possible to design an antenna which is completely invisible from outside. It should further be emphasized that on its feed points, an antenna in accordance with the invention has essentially symmetrical electrical characteristics in relation to a fixed external reference potential. The feed to the antenna can have a symmetrical layout, enabling interference in a receiving section to be reduced. An antenna layout in accordance with the invention also enables the antenna structure to be realized as a slot antenna in a metal surface. This is possible because of the duality principle, and allows the maximum possible flexibility in the design of an antenna. 
     In a preferred form of embodiment of the antenna in accordance with the invention, the gap between the first feed point and the second feed point is at least 0.005 times the freespace wavelength at an operating frequency for which the antenna is designed. Such a gap between the feed points ensures that the input impedance of the antenna lies in a technically advantageous range, so that impedance matching can be effected by simple means. Furthermore, a gap between the feed points of more than 5*10 −3  times the freespace wavelength ensures good reproducibility of the antenna structure. 
     With a preferred form of embodiment, the gap between the center of gravity of the first spiral and the center of gravity of the second spiral is greater than the hypotenuse of a right-angled triangle in which the first cathete has a length equal to half the diameter of the first spiral and in which the second cathete has a length equal to half the diameter of the second spiral. Here, the center of gravity of a spiral is defined as the geometric center of gravity of a line which follows the course of the spiral. The diameter of a spiral is defined as the maximum distance between any two points which lie on the spiral. An appropriate design of the antenna ensures that the first spiral and the second spiral have an adequate gap between them, and that no excessively strong direct coupling exists between the two spirals. Because, specifically in the case of very small geometries, a strong coupling between the two spirals reduces the effectiveness of the radiation and leads to an unfavorable feed point impedance. 
     In a further exemplary embodiment, the antenna is so designed that a parallel projection of a first spiral substrate area in the direction of the first spiral axis misses a second spiral substrate area, and that a parallel projection of the second spiral substrate area in the direction of the second spiral axis misses the first spiral substrate area. Here, a spiral substrate area is defined as the area bounded by the outermost spiral turn of a spiral, forming one single contiguous area with the minimum possible area. In other words, a spiral substrate area is an area with an approximately circular shape which is suitable for carrying a spiral. A spiral axis can be constructed by approximating the spiral section by section by a circle, and by then forming a normal vector which is perpendicular to the plane in which the approximating circle lies. Averaging these normal vectors for the various sections of the spiral then gives the direction of the spiral axis. If the spiral lies in a plane, then the direction of the spiral axis is simply that of the normal to this plane. On the other hand, if the spiral lies on a curved surface, then the spiral axis is approximately equal to the average of the normals to the surface over the area in which the spiral is located. Such a design for the antenna ensures that the antenna functions as an electric dipole with the ability to radiate, and that the two spirals are not arranged approximately parallel to each other. 
     In the case of a further preferred exemplary embodiment, the first radiator and the second radiator are electrically conductive structures. However, it is just as well possible that the first radiator and the second radiator are radiating slots, which are surrounded by a conductive structure. It is thus also possible to make an antenna arrangement in accordance with the invention in the form of a slot antenna, in accordance with the principle of duality. 
     In a manner in accordance with the invention, the radiators of an antenna are thus formed by coiling up the two arms of an extended linear radiator to form a first spiral and a second spiral. Here, the coiling up is to be regarded not in the physical sense of how the material is processed, but as a procedure in the designing of the antenna, so that as defined even a metallization layer, a flat metal foil, a wire or any comparable conductive material can be considered to be coiled up. The same applies for a slot in a conductive structure. The manufacturing technology of the processing can be, for example, coating in conjunction with photolithographic structuring, cutting, stamping or some other manufacturing method. It should further be emphasized that the two arms of the extended linear radiator are not coiled up jointly, but separately from each other. Hence, the two spirals, which form the first radiator and the second radiator, are not coiled together or intercoiled, as applicable, but are present as separate spirals. They are thus spatially apart. 
     The first spiral and the second spiral have, preferably, the same directional sense of coiling or circulation or rotation, as applicable. The result of this, at least approximately, is a point symmetry of the arrangement, leading to particularly advantageous radiation characteristics for the antenna. In order to determine the circulation sense, two spirals which do not lie within a plane are mapped by a parallel projection onto a plane, where the parallel projection rays all run in the same direction and have the same orientation. The sense of the circulation of the projection then represents the circulation sense of the two spirals. Two spirals in a plane then have the same circulation sense if, when the two spirals are followed from their inner ends to their outer ends, each of them has the same qualitative curvature characteristic (curved to the left or curved to the right). 
     It is further preferred that the design of the antenna is such that in relation to a reference potential it has essentially symmetrical electrical behavior at the first feed point and the second feed point. This enables the antenna to be fed symmetrically and, by comparison with asymmetric antennas, renders superfluous a large reference potential area. It is advantageous to avoid an extended reference potential area, in particular for very small devices, because in this case their dimensions are smaller than the wavelength of the transmission frequencies used, and because such devices often have no large metallic or metallized housing components. 
     It is further preferred that the first radiator and the second radiator are formed on the surface of a dielectric material. Namely, because it has been found that applying an antenna structure in accordance with the invention onto a dielectric substrate does not significantly degrade the antenna characteristics. The use of a substrate is advantageous because this not only improves the mechanical robustness of the antenna compared to a self-supporting metallized structure, but also makes manufacture easier. Namely because it is then possible, for example, to apply the metallic structures onto the surface of the dielectric material by a coating process (e.g. vapor deposition, lamination, bonding), followed by structuring them. So it is unnecessary to manufacture separately a metallized structure, which would be very difficult to handle and lacking in mechanical robustness. 
     It is further preferred that the surface of the dielectric material, on which the first radiator and the second radiator are formed, is domed. There is then no problem in being able to adapt the antenna structure in accordance with the invention to the topology of an existing surface. This is particularly important in the realization of an antenna on or in the housing of a device, where the shaping of the housing must generally take into account numerous criteria. 
     Apart from this, it is advantageous to integrate the first radiator and the second radiator into the housing of an electronic device which is made of a dielectric material and which houses an electric circuit. It is, indeed, not only possible to apply the antenna structure in accordance with the invention to the surface of a dielectric substrate, but it is also possible to integrate it into the substrate, i.e. the housing. Such a design can bring very major advantages with many applications, firstly because it protects the antenna against external influences and damage, and secondly because the antenna is no longer visible from outside. The radiation characteristics of the antenna are not significantly degraded if the housing is thin enough. 
     It has further been found that the antenna in accordance with the invention can with advantage be arranged on the surface of a housing which is part of a behind-the-ear hearing aid. Such a behind-the-ear hearing aid is typically designed to be worn behind the pinna of a person&#39;s ear. It has been found that the adaptation and radiation characteristics of an antenna in accordance with the invention are good even in this difficult operating environment. 
     Finally, it is preferred that the working frequency of an antenna in accordance with the invention lies between 500 MHz and 6 GHz. It is further preferred that the antenna has a maximum dimension of less than 10 cm. This enables the antenna in accordance with the invention to be used in portable devices. 
     It is further advantageous if the antenna has a maximum dimension of less than one fifth of the freespace wavelength at an operating frequency at which the antenna is operated. In this case, the spiral is tightly enough coiled to achieve a suitable field distribution. Incidentally, by comparison with a conventional dipole antenna the size advantage of an antenna in accordance with the invention comes most strongly to the fore when the antenna is small compared to the freespace wavelength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred exemplary embodiments of the present invention are explained in more detail below with reference to the attached drawings. These show: 
         FIG. 1  a schematic representation of an antenna in accordance with the invention, in accordance with a first exemplary embodiment of the present invention; 
         FIG. 2  a schematic representation of an antenna in accordance with the invention, in accordance with a second exemplary embodiment of the present invention, arranged on the housing of a hearing aid; 
         FIG. 3  a photographic image of a prototype of an antenna in accordance with the invention, in accordance with the second exemplary embodiment of the present invention, arranged on the housing of a hearing aid; 
         FIG. 4   a  a block diagram of an electrical test rig for determining the input reflection factor of an antenna in accordance with the invention; and 
         FIG. 4   b  a graph of the logarithm of the input reflection factor against frequency for an antenna in accordance with the invention, in accordance with one exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a schematic diagram of an antenna in accordance with the invention, in accordance with a first exemplary embodiment of the present invention. The antenna in its entirety is labeled  100 . It has a first radiator  110  together with a second radiator  112 . The first radiator  110  has a first spiral  120  together with a first feed point  122 . The first feed point  122  is located at the outer end  124  of the first spiral  120 . On the other hand, the inner end  126  of the first spiral  120  is open circuit. The second radiator  112  is constructed similarly to the first radiator  110 , and has a second spiral  130  together with a second feed point  132 . The second feed point  132  is arranged at the outer end  134  of the second spiral  130 . The inner end  136  of the second spiral  130  is open circuit. 
     The first radiator  110  and the second radiator  112  will preferably be an electrically conductive arrangement. However, it is also possible to use a radiating slot which is surrounded by a conductive structure, for example a metallization. If the radiator is formed by a conductive structure, this can be manufactured using numerous technologies. For example, the spirals  110 ,  112  can be formed from an appropriately shaped wire. Equally well, a processed foil of conductive material (e.g. copper foil) can be used to manufacture the conductive spirals. Further, the radiator structure can be formed by a thin conductive layer which has been applied to a substrate during manufacture and has then structured. 
     The conductive structure can either be self-supporting (i.e. only fixed at one or a few fixing points) or can be applied to a substrate. It is, incidentally, not necessary that the two radiators  110 ,  112  lie in one plane. Rather, they can be inclined to each other, or their track can be adapted to fit a curved surface, provided that the graph of the electrical and magnetic field lines does not basically change compared to the exemplary embodiment shown. 
     The two radiators  110 ,  112  can be connected to a transmission link or associated circuitry at the feed points  122 ,  132 . In the exemplary embodiment shown, these lie at the outer end  124  of the first spiral  120  and at the outer end  134  of the second spiral  130 . The connection can be made, for example, via a pair of wires which lie in the same plane or on the same material surface, as applicable, as the two radiators  110 ,  112  themselves. Apart from this however, it is also possible that the feed is made at right angles to the plane or surface, as applicable, in which the two radiators  110 ,  112  lie. For this purpose, there may for example be through-contacts (feedthroughs) at the outer ends  124 ,  134  of the two spirals  120 ,  130 . It is also possible to have hybrid solutions, in which some part of the feed structure lies in the plane of a radiator and another part of the feed structure is arranged outside this plane or surface. It is also entirely possible to have feed lines which are oriented at an angle to the plane of the antenna. Incidentally, the feed structure can incorporate matching circuits (e.g. wires with varying thickness, matching stubs or lumped elements). Apart from this it is possible that the spirals are not connected at their outermost ends, but at a distance from the end. It is possible by this means to effect any required impedance matching if this has not already been achieved by the geometry of the radiators. In relation to such a form of embodiment, the outer end of the spiral is not to be regarded in a narrow geometric sense as a point, but rather as a region which extends from the outermost end of the spiral towards the inner end of the spiral for about 1/10 of the freespace wavelength, measured along the track of the spiral. 
     If the radiator is in the form of a radiating slot, then the connection can be made via any desired arrangement which is suitable for the excitation of a slot antenna, where the feed structure is matched to the feed point impedance of the slot antenna, or is arranged to achieve impedance transformation to a preferred impedance. 
     It is furthermore possible that the width of the spirals varies from the outer end to the inner end. In particular it is possible, depending on the application situation, that the width of the spirals (i.e. the width of the conductive structure or the radiating slot) at the inner ends  126 ,  136  is greater than or smaller than the width of the spirals at their outer ends  124 ,  134 . By such means it is possible, for example, to improve the impedance characteristics or the bandwidth of the antenna. 
     In the case shown, of the exemplary embodiment  100  of an antenna in accordance with the invention, the two spirals  120 ,  130  have the same circulation sense. However, it is also possible that the circulation sense of one spiral is changed, so that the two spirals  120 ,  130  which form the antenna have opposing circulation senses. 
     On the basis of the structural description, the way that an antenna in accordance with the invention functions is described below. 
     The antenna in accordance with the invention is based on a dipole antenna, with the arms of a linear dipole antenna being coiled up into spirals  120 ,  130 . By this means, the maximum dimension of the antenna is reduced by comparison with an extended dipole antenna. Because the antenna in accordance with the invention is essentially based on a dipole antenna, it is a symmetrical antenna. The electrical characteristics at the feed points  122 ,  132  is thus essentially symmetrical with respect to a reference potential, whereby any geometric asymmetries which there may be do admittedly affect the electrical characteristics. 
     The way in which the present antenna works can be understood roughly by starting with a conventional dipole antenna with reduction coils. However, in the case of an antenna in accordance with the present invention, the entire dipole is coiled up. The coiling axis is here approximately perpendicular to the plane or the area in which the spiral concerned lies. By contrast, conventional reduction coils are constructed either as lumped elements or as a number of windings, and are mostly arranged close to the feed point, whereby the radiation essentially emanates from the remaining extended dipole. 
     On the other hand, in the case of an antenna in accordance with the invention, the split between a region which is coiled up for the purpose of geometric shortening and an extended radiator is eliminated. Rather, a complete dipole is coiled up. 
     If an antenna geometry in accordance with the present invention is used, the particularly favorable field distribution means that the effect thereby achieved includes, from the point of view of its efficiency, a matching of the antenna to conventional waveguide impedances. 
     By this means, in spite of the small geometric dimensions of the antenna, an adequate radiation efficiency can be achieved. It is furthermore possible to avoid a large part of the transmission power being lost in a matching network. 
     The antenna in accordance with the invention can be used self-supporting, can be applied to a substrate, or integrated into a plastic housing. In this case, it has been found that if the antenna in accordance with the invention is assembled in a plastic housing or on a plastic housing this does not involve any unacceptable deterioration in the electrical characteristics. Hence the antenna in accordance with the invention is well suited, for example, for use in small portable devices such as hearing aids, pagers and mobile telephones. 
       FIG. 2  shows a schematic diagram of an antenna in accordance with the invention, in accordance with a second exemplary embodiment of the present invention, arranged on the housing of a hearing aid. The entirety of the arrangement is labeled  200 . 
     The arrangement  200  shown includes a spiral antenna  210  which is applied to the hearing aid body  220  of a hearing aid  240 . Together with the ear mold  230  and the spiral antenna  210 , the hearing aid body  220  forms the hearing aid  240 . 
     The spiral antenna  210  consists of two radiators  110 ,  112 . Since the spiral antenna  210  corresponds in its components to the spiral antenna  100  described by reference to  FIG. 1 , the same elements in  FIG. 1  and  FIG. 2  are labeled with the same reference marks, and are not explained here in any more detail. 
     The arrangement  200  thus shows how a spiral antenna  210  in accordance with the invention can be built onto a hearing aid  240 . It is worth remarking about this that the two spirals  120 ,  130  can be adapted to the shape of the hearing aid body  220 . 
     In the case of the realization shown, the spiral antenna  210  is applied to the outer side of the hearing aid body  220 . However, it is equally well possible to form the antenna on the inner side of the hearing aid housing. It is also conceivable that the spiral antenna  210  is embedded between several layers of the hearing aid housing so that, for example, a protective layer protects the spiral antenna  210 . The protective layer can at the same time be used to adapt the appearance of the hearing aid  240  to the user&#39;s preferences. 
     The spiral antenna  210  in conjunction with the hearing aid  240  will preferably be designed to receive a speech or data signal which is transmitted wirelessly, and to pass it on to the electronics in the hearing aid. Here, a speech signal which is received can be output via the ear mold  230  to the auditory canal of a user of the hearing aid  240 . Data signals which are transmitted wirelessly can further be used to influence the settings of the hearing aid  240  and, for example, to adjust them according to the user&#39;s preferences. 
     The spiral antenna  210  can be used both for transmitting and also for receiving. For example, it may be desirable to transmit status data from the hearing aid to a receiver. Because of the reciprocity, the spiral antenna  210  can be used both as a transmitting antenna and also as a receiving antenna, where transmission and reception can take place simultaneously or in time multiplex. 
     For appropriate applications, it is preferred that the spiral antenna is designed for an operating frequency lying between 500 MHz and 6 GHz. For example, it is advantageous to use the ISM band at 868 MHz. It is also possible to use, for example, frequency bands which are reserved for medical applications. 
     When a spiral antenna  210  in accordance with the invention is used in conjunction with a hearing aid  240 , or with other mobile transmission and/or reception devices such as pagers and mobile telephones, the size of the complete spiral antenna structure is restricted to less than 10 cm. However, it has been found that the antenna structure in accordance with the invention has adequately good characteristics in spite of the small dimensions. It has furthermore been found that, when used in conjunction with a hearing aid, the overall size of the antenna structure should not be less than 1/16 of the freespace wavelength at an operating frequency of the antenna, if 1/16 of the freespace wavelength is less than 2 cm. If, at low frequencies, 1/16 of the freespace wavelength is greater than 2 cm (i.e. the freespace wavelength is greater than 32 cm), then the overall size of the antenna structure should preferably be at least 2 cm. The antenna must therefore in every case, even at low frequencies below 1 GHz, be smaller than the hearing aid. An overall size of antenna structure of about λ/5 has been shown to be especially advantageous because this gives the best possible compromise between the space occupied by the antenna and the radiation characteristics. 
       FIG. 3  shows a photographic image of a prototype of an antenna in accordance with the invention in accordance with the second exemplary embodiment of the present invention, arranged on the housing of a hearing aid. The entirety of the arrangement is labeled  300 . Since the arrangement is essentially the same as the arrangements  100 ,  200  shown in  FIG. 1  and  FIG. 2 , the same elements are here labeled with the same reference marks as for the arrangements  100 ,  200  described above, and are not explained here in any more detail. 
     The arrangement  300  shows a prototype of a hearing aid with a spiral antenna  210  affixed to it. The prototype has been simulated using an electro-magnetic field simulator, and cut out of self-adhesive copper foil and bonded to the hearing aid. The feed to the two radiators  110 ,  112  is worth noting here. The two feed points  122 ,  132  have feedthroughs at which the electrical connections from the outer ends  124 ,  134  of the two spirals  120 ,  130  are fed into the inside of the hearing aid. The gap d between the two feed points is about half the diameter of the two spirals. Hence, the gap between the two feed points is greater than would be expected with a conventional dipole arrangement. Apart from this, it should be noted that the minimum gap between the first spiral  120  and the second spiral  130  will preferably lie between 0.3 times the diameter of a spiral and 0.5 times the diameter of a spiral. This will ensure that a suitable coupling is guaranteed between the spirals, which is adequate to permit optimal radiation. 
     The gap d between the two feed points  122 ,  132  is typically less than the diameter of the first spiral  110 , and is also less than the diameter of the second spiral  112 . It is, for example, preferred that the gap d between the two feed points  122 ,  132  is in the range between 0.25×dMIN and 0.75×dMIN, where dMIN defines the diameter of the smaller of the two spirals  110 ,  112 , or is equal to the diameter of the two spirals if the two spirals  110 ,  112  have the same diameter. 
     It is further preferred that the two spirals  110 ,  112  are designed in such a way that a direction tangential to the first spiral  120  at the first end  124 , i.e. a direction which defines the alignment of the spiral at its first end  124 , and a direction tangential to the second spiral  130  at the second end  134 , enclose an acute angle which is not greater than 30°. In other words, at their outer ends  124 ,  134 , or in the region of their feed points  122 ,  132 , as applicable, the two spirals  110 ,  112  have approximately the same alignment. Hence in the region of the feed points  122 ,  132  the currents in the two spirals  110 ,  112  flow in approximately the same directions, with the effect that the radiation from the two spirals  110 ,  112  is maximized in the region of the feed points  122 ,  132 . 
     With a further preferred exemplary embodiment, the gap between the two feed points  122 ,  132  is in a range between 0.4 times the diameter of one of the two spirals  110 ,  112  and 0.6 times the diameter of the appropriate spiral  110 ,  112 . 
     An appropriate construction ensures that in other respects the two spirals  110 ,  112  function as the two arms of a dipole antenna. 
       FIG. 4   a  shows a block diagram of an electrical test rig for determining the input reflection factor of an antenna in accordance with the invention. The entirety of the test rig is labeled  400 . 
     The test rig includes an antenna  410  in accordance with the invention. At its feed points  412 ,  414 , this has approximately symmetrical electrical characteristics. For this reason, the antenna is coupled to a network analyzer  430  via a balun  420 . Here the balun  420  includes, for example, a balun transformer so that on the network analyzer side an asymmetrical signal  434  is available. Depending on the test data required, the network analyzer  430  can be a scalar network analyzer or a vector network analyzer. 
       FIG. 4   b  shows a graph of the logarithm of the input reflection factor (or return loss, as appropriate) against frequency for an antenna in accordance with the invention, in accordance with an exemplary embodiment of the present invention. During its manufacture, the prototype of the antenna in accordance with the invention which was tested was cut out from a self-adhesive copper foil, and bonded to a hearing aid. An example of a prototype of this nature is shown in  FIG. 3 . For the measurement of the return loss, i.e. the logarithm of the input reflection factor, the antenna  410  was connected to the network analyzer  430  via a discrete balun  420 , as per the test rig  400  (cf.  FIG. 4   a ). Furthermore, during the measurements the hearing aid  240  with the antenna  210  bonded on it was worn on the ear of a subject, in order to take into account also the effects of the human head or ear, as applicable, on the characteristics of the antenna. The results of the measurements are shown in the graph  510 . Here, the frequency in a range from 500 MHz up to 1200 MHz is plotted on the a bscissa  520 . The ordinate  522  shows the return loss in the range from −80 dB up to +20 dB. The measured return loss as a function of the frequency can be seen from the curve  530 . Here, the return loss shows a clear maximum at about 860 MHz, with a −10 dB bandwidth for the return loss amounting to about 35 MHz. The maximum achievable return loss amounts to about 12 dB. Away from the payload frequency, the return loss falls back to about 2 to 3 dB. This indicates a low radiation from the antenna  410 . 
     So, as expected the antenna only radiates effective power in a frequency interval around the design frequency. The −10 dB bandwidth of about 35 MHz corresponds to a relatively usable bandwidth of about 4 percent. 
     The present invention thus specifies a new type of antenna for wireless speech and data transmission. The antenna in accordance with the invention has been conceived in particular for very small devices such as hearing aids, which are worn behind the ear. It is especially well suited for mobile transmitting and receiving. A special merit of the symmetric spiral antenna in accordance with the invention consists in the fact that it can be integrated in a comparatively simple way into an existing system, for example a hearing aid. Because the antenna can be integrated into a plastic housing, it can be made so that it is completely invisible from outside. Furthermore, the antenna can be realized with a comparatively small size, and permits symmetric feeding. Apart from this, the antenna structure in accordance with the invention can also be integrated into a metal surface as a slot antenna. 
     The antenna in accordance with the invention is especially well suited for integration into a hearing aid. However, because of its small physical size and the ability to integrate it into a plastic housing, other application areas can be conceived for an antenna in accordance with the invention, such as for example pagers and mobile telephones.