Patent Publication Number: US-7218280-B2

Title: Antenna element and a method for manufacturing the same

Description:
The invention relates to a radiating antenna element intended particularly for small-sized radio devices. The invention also relates to a method for manufacturing an antenna element according to it. 
   BACKGROUND OF THE INVENTION 
   An internal antenna is generally used in small-sized radio devices, such as mobile phones, in order to avoid a part protruding from the cover of the device. Internal antennas are usually planar antennas, because they have relatively good electric properties. A planar antenna comprises a radiating plane and a ground plane parallel with it. The planes are generally connected to each other by a short-circuit conductor because of the matching of the antenna. The structure is dimensioned so that it functions as a resonator at the operating frequency, which is a prerequisite for effective radiation. In modern mobile stations it is a normal requirement that the antenna must operate on two different frequency bands, in which case two resonators are also required. This requirement is met by dividing the radiating plane into two branches of different lengths by means of a non-conductive slot or area. Together with the ground plane and a medium, each branch forms a resonator, the natural frequency of which is arranged at one operating band of the radio device. 
   The radiating plane can be a separate metal sheet, in which case its slot is formed by cutting while the whole plane is cut from a larger sheet. Saving of material is achieved by manufacturing the radiating plane of thin metal foil. Then the radiating plane cut from the foil is, for example, glued onto the antenna&#39;s dielectric frame or onto the inner surface of the cover of a mobile station. The difficulty is to make the shape of the foil element remain exactly right during fastening. Even a relatively small change in the dimensions of especially the non-conductive area of the plane impairs the characteristics of the antenna significantly. The risk of changing the shape of the foil element is avoided if a dielectric plate coated by a metal foil is used for manufacturing the antenna. The desired radiator pattern is formed on the surface of the plate by etching away the surplus parts from the coating. The resulting antenna element is then fastened at a certain distance from the ground plane. 
     FIG. 1  shows a radiating antenna element  100  manufactured by the known method described above. It comprises a dielectric substrate  110  and a radiating plane  120 , which is a conductor layer on the surface of the substrate. The radiating plane has an antenna feed point FP and a short-circuit point SP close to each other. From the latter, the radiating plane is directly connected to the ground plane when the antenna element is installed on place. The non-conductive area  130  starts from the same edge of the element beside which the feed point and the short-circuit point are, and divides the radiating plane into two conductor branches as seen from the short-circuit point SP. The first conductor branch  221  comprises the peripheral areas of the plane, forming a pattern resembling the letter C. The second, shorter conductor branch  222  comprises the inner area of the plane. The lower operating band of the antenna is based on the first conductor branch, and the upper operating band of the antenna is based on the second conductor branch. The antenna element has been cut to such a shape that it follows the inner space of the end part of the radio device in question.  FIG. 1  shows the outline COV of the end part. 
   The non-conductive area  130  of the antenna element  100  has been formed by removing part of the conductive coating of the substrate by etching. The chemicals needed in the process cause a considerable cost in production. This drawback is emphasized if the area between the conductor branches is made relatively wide in order to increase the bandwidths of the antenna. Besides, the chemicals used are environmental poisons, the disposal of which causes additional costs. In principle, it could also be used laser for removing the conductor material in the known manner. However, laser suits well for making very narrow slots only. Removing a relatively wide conductor area would thus be impractical, i.e. expensive, and it would also impair the mechanical and electrical characteristics of the dielectric plate used as a substrate. 
   SUMMARY OF THE INVENTION 
   The purpose of the invention is to reduce the mentioned drawbacks of the prior art. The antenna element according to the invention is characterized in what is set forth in the independent claim  1 . The method according to the invention is characterized in what is set forth in the independent claim  7 . Some preferred embodiments of the invention are set forth in the other claims. 
   The basic idea of the invention is the following: The radiating element of a multiband planar antenna is manufactured of a plate, which comprises dielectric substrate by one side coated with conductive material. The radiating conductor branches corresponding to the operating bands of the antenna are formed by removing the conductor coating narrowly from the border line of the area between the designed conductor branches. The conductor area confined by the created border groove can be used as a parasitic additional radiator. If needed, the conductor area confined by the border groove can also be split into a number of small conductor areas, in order to make sure that the conductor area does not radiate or have any substantial effect on the coupling between the radiating conductor branches. The removal of the conductive coating is preferably carried out by laser. 
   The invention has the advantage that a relatively wide area “invisible” at the operating frequencies of the radiating branches of the antenna can be formed between the branches by the customary laser technique. This means lower manufacturing costs compared to the use of the etching process. In addition, the cost of problem waste handling is avoided, which sort of wastes are the chemicals released in the etching process. The invention also has the advantage that the conductor area remaining between the radiating branches can be utilized as an additional radiator on the frequency range of 2.4 GHz, for example. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention will be described in more detail. Reference will be made to the accompanying drawings, in which 
       FIG. 1  presents an example of a prior art antenna element, 
       FIG. 2  presents an example of an antenna element according to the invention, 
       FIG. 3  presents another example of an antenna element according to the invention, 
       FIG. 4  presents a third example of an antenna element according to the invention, 
       FIG. 5  presents an example of a method according to the invention, 
       FIG. 6  presents an example of an antenna element according to the invention as installed in a radio device, 
       FIG. 7  presents another example of an antenna element according to the invention as installed in a radio device, 
       FIG. 8  shows an example of band characteristics of the antennas using an element according to the invention, and 
       FIG. 9  shows an example of the efficiency of antennas using an element according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  shows an example of a radiating antenna element according to the invention. The antenna element  200  comprises a dielectric substrate and a radiating plane  220  on its surface, divided into two conductor branches, like in the element of  FIG. 1 . The elements differ from each other with respect to the area separating the radiating conductor branches. In  FIG. 1 , the conductive coating has been entirely removed from that intermediate area  130 . In  FIG. 2  again, the original conductive coating is almost entirely left on the corresponding intermediate area  230 . The conductive coating has been only narrowly removed at the border line of the intermediate area. The line-like non-conductive area thus created is called a “groove”. So, the intermediate area  230  is confined by a border groove  231 . The conductor area remaining inside the border groove, which is slightly smaller than the intermediate area  230 , forms in the complete product, in principle, together with the ground plane and the other part of the radiating plane a resonator, in which it is possible to excite oscillation. The element according to  FIG. 2  has been dimensioned so that the frequency of said oscillation is considerably above the natural frequencies of the resonators corresponding to the first  221  and also the second  222  conductor branch of the radiating plane. Therefore, the conductor area  223  of the intermediate area does not significantly influence the function of the antenna on its operating bands. 
     FIG. 3  shows another example of a radiating antenna element according to the invention. The antenna element  300  is of the same kind as the element presented in  FIG. 2 . The only difference compared to  FIG. 2  is that the conductor area remaining inside the border groove  331  of the intermediate area between the radiating conductor branches is now split into smaller parts by grooves forming a lattice pattern. The lattice pattern comprises a set of parallel grooves, such as groove  332 , and another set of grooves perpendicular to those mentioned above, such as groove  333 . The grooves are here at even distances, and so the small parts of the conductive coating, or pads, separated by the grooves are square-shaped, except of course the pads cut by the border groove. Two pads, CA 1  and CA 2 , are marked in  FIG. 3  by reference lines. The pads in the intermediate area are made so small that they are entirely “invisible” at the operating frequencies of the antenna. In that way it has been ensured that the conductive coating of the intermediate area does not radiate or have any significant effect on the electromagnetic coupling between the radiating conductor branches. In this example, the pads are square-shaped. They could as well be rectangles, parallelograms or something else by shape, as long as they are sufficiently small. 
     FIG. 4  shows a third example of a radiating antenna element according to the invention. The antenna element  400  is also of the same kind as the element presented in  FIG. 2 . The only difference to  FIG. 2  is that in this example, two grooves  432  and  433  have been made in the conductor area  423  remaining inside the border groove  431  of the intermediate area of the radiating branches. Those two grooves are joined in the border groove  431  on the opposite sides of the intermediate area, whereby meanders increasing the electric length of the conductor area  423  are formed in it. In this way, the natural frequency of the resonator corresponding to the conductor area  423  can be tuned to the band used by some radio system, such as Bluetooth or GPS (Global Positioning System). The conductor area functions as a parasitic radiator on that band, and is thus utilized in this embodiment. 
   In all the embodiments of the invention, the conductive coating of the intermediate area between the radiating conductor branches of the antenna element remains almost entirely on place. In practice, removing the entire coating would require the use of the etching technique, which is attempted to be avoided. Etching can naturally also be used merely for forming the border groove and possible other grooves, in which case the resulting component is comformable to the invention. The grooves required can also be made by machining the surface of the element mechanically. However, the best result economically and electrically is achieved by the laser technique, which is thus the primary machining technique for the conductive coating. 
     FIG. 5  shows an example of a method according to the invention. In step  501 , preparations are made for machining the conductive coating of the antenna element. They include cutting the element to the right shape when a ready-coated substrate plate is used or cutting a mere conductor foil and fastening it to the antenna frame or to a part of the casing of the radio device. In addition, the right program is loaded to the laser machine tool. In step  502 , the antenna component is placed on the machining platform of the laser tool. The component can be placed either so that the laser beam falls directly on the conductive coating or the other way round, in which case the laser beam first penetrates the dielectric substrate. Each case requires its own, suitable laser frequency. In step  503 , the radiating branches of the antenna component are formed by machining the border groove of the area between them. The border groove is created when the laser beam evaporates the conductor material from a narrow area. In step  504  it is checked whether other grooves are intended to be made on the intermediate area. If so, those grooves are machined in the same way as the border groove (step  505 ). After this, the component is finished with respect to its radiation characteristics. 
     FIG. 6  shows an example of an antenna element according to the invention as installed in a radio device. The radio device is presented as a simplified cross-section in which the outer cover COV and the circuit board PCB are seen. The conductive upper surface of the circuit board is of the signal ground GND and also functions as the ground plane of the antenna. The antenna element, which comprises a dielectric substrate  610  and its conductive coating  620 , can be made of a thin circuit board, for example. The element is supported above the ground plane by support legs SUP, total amount of which is such as required for the sufficient support. In addition, the figure shows the antenna feed conductor FC and the short-circuit conductor SC. 
     FIG. 7  shows another example of an antenna element according to the invention as installed in a radio device. The radio device is also here shown as a simplified cross-section in which the outer cover and the circuit board PCB are seen. The conductive upper surface of the circuit board is of the signal ground GND and also functions as the ground plane of the antenna. In this example, the antenna element is formed of a part  710  of the outer cover of the radio device and a conductor foil  720  fastened to its inner surface by glueing, for example. Said part of the outer cover thus functions as the dielectric substrate of the element. An area between the radiating branches according to the invention is formed on the conductor foil after the foil has been fastened. The antenna feed conductor FC and the short-circuit conductor SC are also seen in  FIG. 7 . 
     FIG. 8  shows an example of band characteristics of the antennas using an element according to the invention. It presents curves of the reflection coefficient S 11  as a function of frequency. Curve  81  has been measured from a known antenna using an element according to  FIG. 1 , curve  82  from an antenna using an element according to  FIG. 2 , curve  83  of an antenna using an element according to  FIG. 3 , and curve  84  of an antenna using an element according to  FIG. 4 . The antenna is designed to operate in the systems GSM850 (Global System for Mobile telecommunications), GSM900, GSM1800 and GSM1900. The bands required by the two former are on the frequency range 824 to 960 MHz, which is the lower operating band BI of the antenna. The bands required by the two latter are on the frequency range 1710 to 1990 MHz, which is the upper operating band Bu of the antenna. The measurements have been performed on prototypes. It is seen from the curves that with a small amount of additional tuning, the reflection coefficient of all the antenna versions is better than −5 dB on the whole area of both operating bands. In addition, it can be seen that leaving conductive coating on the intermediate area between the radiating branches of the antenna does not deteriorate the band characteristics of the antenna, but on the contrary, improves them slightly. In addition, the antenna corresponding to curve  84  and  FIG. 4  has been dimensioned to operate on the band of the Bluetooth system, and therefore the reflection coefficient falls deeply above the frequency of 2.4 GHz. The width of the topmost band is almost 100 MHz. 
     FIG. 9  shows an example of the efficiency of antennas using an element according to the invention. The efficiencies have been measured from the same structures as the matching curves of  FIG. 8 : Curve  91  shows the change of the efficiency in a known antenna using an element according to  FIG. 1 , curve  92  in an antenna using an element according to  FIG. 2 , curve  93  in an antenna using an element according to  FIG. 3  and curve  94  in an antenna using an element according to  FIG. 4 . On the lower operating band the efficiencies vary in the range 0.3 to 0.7, and on the upper operating band in the range 0.3 to 0.65. With respect to efficiency, the antenna according to the invention, corresponding to  FIG. 2 , also beats the prior art antenna corresponding to  FIG. 1 . 
   The qualifiers “upper” and “lower” in this description and the claims refer to the positions of the antenna element presented in  FIGS. 5 and 7  to  9 , and they have nothing to do with the position in which the devices are used. 
   Antenna elements according to the invention have been described above. The shapes of the antenna element and its radiators can naturally differ from those presented. The inventive idea can be applied in different ways within the limits set by the independent claims  1  and  7 .