Patent Publication Number: US-6987486-B2

Title: Ground arrangement for a device using wireless data transfer

Description:
TECHNICAL FIELD OF THE INVENTION 
   The invention relates to the ground arrangement of a device using wireless data transfer, such as a mobile phone. By means of ground arrangement one tries to improve the electric properties of the device antenna as well as the performance of the device. 
   BACKGROUND OF THE INVENTION 
   Wireless terminal devices such as mobile phones must meet various requirements. As the physical size of the devices gets smaller, their size imposes their own requirements e.g. on the device structure, and particularly on the ground structure of the device. The antenna is a very important part of a terminal device operating at high frequencies, since it converts the signal received from the transmitter into electromagnetic waves and the electromagnetic waves into the signal to be received. 
   The antenna has to be capable of receiving and sending signal as well as possible under all circumstances. The frequency and output of a signal passing through the antenna may vary even to a great extent depending on the device, system and conditions, so as concerns the functioning of the device, it is substantial that the antenna operates under all circumstances as well as possible. In practice all antennas need a properly operating ground arrangement in order to efficiently function as emitters. The ground arrangement is herein also referred to as the “antenna ground”, which is generally used to refer to the earth current arrangement that is used to achieve a low-impedance coupling to the ground potential or to the common reference point. 
   In practice, interfering signals are received into the antenna, which render difficult the normal functioning of the device, at least to some extent. The devices in use themselves cause interfering signals that may e.g. harm the operation of the components in the device. 
   Devices operating at a radio frequency cause electric and magnetic fields while in use. Such a place or point at which there is, or at which there each time is created, a concentration of electric or magnetic field is called a so-called hot spot. From the distribution of currents induced by the fields it is possible to approximately calculate the specific absorption rate (SAR), which indicates the output absorbed (e.g. by the bodily tissue) per mass unit. By means of the ground arrangement of the antenna it is possible to affect the magnitude of the SAR value. 
   In addition to the output, the SAR value depends e.g. on the frequency, the distance of the antenna in relation to the user, the service position of the phone and the antenna type. While the SAR values of the devices on the market are very small, there is an attempt to design the new devices in such a manner that one could get the SAR values smaller than before. 
   It is known to attach to the terminal devices various additional components that help extend the antenna ground of the terminal device when necessary, which means that the antenna amplification is bigger. The user has had e.g. to detach an additional component from the terminal device in order to introduce the ground extension. The aforementioned solution is therefore cumbersome in use. 
   The objective of the invention is to improve the features of the device as concerns the above-mentioned facts and to achieve a method more workable than before for controlling and changing the ground configuration of the device. In addition, the objective is to achieve a solution by means of which it is possible to direct the earth currents to a place as optimal as possible in such a manner that the created field concentrations are as far as possible from the user, in which case it is possible to obtain a small SAR value. 
   SUMMARY OF THE PRESENT INVENTION 
   The objective of the invention is to achieve a solution for a wireless terminal device by means of which it is possible to simply and flexibly optimize the electric properties of the device antenna, the performance of the device as well as to minimize the SAR value to be measured from the device. 
   This goal is achieved by a solution that has been described in the independent claims. 
   The idea of the invention is to form in the device, in addition to its actual ground lead i.e. ground route, one or more additional ground leads, and to change by means of it/them the ground configuration of the antenna, such as the effective length and/or area of the ground leads by automatically forming a galvanic coupling between the ground lead and one or more additional ground leads based on predetermined coupling criteria and by automatically releasing the coupling in question based on predetermined release criteria. In this manner it is possible to affect not only the SAR value but also the electric performance of the antenna. The ground lead and additional ground leads as well as the coupling criteria are designed for the device beforehand in such a manner that the desired features are achieved under varying transmission or reception circumstances. 
   The solution in accordance with the invention is used to achieve not only a small SAR value but also a low-loss antenna structure that has a VSWR value (VSWR=Voltage Standing Wave Ratio) as small as possible. By means of the invention it is also possible to achieve an antenna structure that operates in each situation with an optimal amplification. 
   In one advantageous implementation mode of the invention, the additional ground lead/leads are at least partly disposed in the cover part of the device, which enables one to implement various additional ground leads, and the place of the hot spots can be changed when necessary. The additional ground leads may be disposed on the surface of the cover material or inside it. 
   In another advantageous implementation mode of the invention, the additional ground leads are disposed in different layers of a multi-layer PC board. In this manner it is possible easily to implement additional ground leads that possess even very different sizes and shapes. The additional ground leads are easy to implement in respect of manufacturing technique on a multi-layer PC board, which makes the solution also cost-effective. 
   Since the effective length of the ground leads can be increased when necessary, it enables one to easily implement various ground configurations, from which the one each time most suitable is chosen. In practice, especially the frequency band to be used has an effect on the selection of the ground configuration (i.e. the additional ground lead each time most suitable). 
   The ground configuration in accordance with the invention can be easily implemented, and the user does not have to do anything in order to introduce a new ground configuration, but the introduction is performed automatically based on predetermined criteria. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention and in order to show how the same may be carried into effect reference will now be made to the accompanying drawings, in which: 
       FIGS. 1 and 2  represent one terminal device according to the invention. 
       FIG. 3  represents one implementation mode of a ground arrangement. 
       FIG. 4  represents another implementation mode of a ground arrangement. 
       FIG. 5  represents the ground arrangement of  FIG. 4 , when the additional ground lead is in use. 
       FIG. 6  represents a third implementation mode of the ground arrangement. 
       FIG. 7  represents a terminal device according to the invention. 
       FIG. 8  represents one implementation mode of the location of the additional ground lead. 
       FIG. 9  represents another implementation mode of the location of the additional ground lead. 
   

   DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS 
     FIG. 1  shows a front view of a typical (subscriber) terminal device. The terminal device may be e.g. a mobile phone  40 , which comprises, among other things, keys  10  and a display  20 . The terminal device may also be some other device than a mobile phone, since from the standpoint of the invention substantial is only the fact that the device is capable of sending and receiving via an air interface. 
   The terminal device of  FIG. 1  comprises a cover or enclosure structure, of which in the figure can be seen the front cover, i.e. the so-called A cover  30 . The cover structure can be made e.g. of plastic, glass fiber or metal or a combination thereof. 
     FIG. 2  shows a side view of the terminal device, which allows the cover structure of the terminal device to be better seen. In addition to the front cover, the terminal device comprises a rear cover, i.e. a so-called B cover, and an accumulator space  61  for the accumulator. The proposed terminal device comprises, in addition, an accumulator cover  60  for closing the accumulator space. The accumulator cover is thus in this implementation mode part of the cover structure of the terminal device. The A and B covers of the device as well as the accumulator cover can be detached from the rest of the device structure. The terminal device further comprises a circuit board  70 , which is disposed inside the enclosure structure. 
   The ground arrangement according to the invention comprises an actual ground lead, one or more separate additional ground leads and coupling means, which enable one to automatically achieve a galvanic coupling between the ground lead and one or more additional ground leads in order to change the ground lead configuration to correspond to the changed transmission and reception circumstances, if desired. The number of cover parts of the terminal device is thus not so important as concerns the operability of the invention, instead substantial is that the device cover offers a space in which it is possible, e.g. in the manufacture phase of the device cover, to form one or more additional ground leads for the ground solution in accordance with the invention. The possible locations of the actual ground leads and additional ground leads are handled in more detail hereinafter. 
   The actual ground lead and additional ground leads are designed, in terms of qualities, (such as length, area and shape) beforehand in such a manner that the coupling and release of coupling of the additional ground leads enable one to achieve the desired effect on the functioning of the device. The physical measures of the additional ground leads are typically determined based on the frequency and output used. The seeking for the desired effect may happen also by way of experiment, as will be described hereinafter. It is possible to form even more additional ground leads, from which the one each time most suitable is chosen in order to provide the desired grounding network. It is also possible that from the additional ground leads, one chooses each time more than one to be connected to the actual ground lead. 
   In the selection of the additional ground lead to be used each time it is possible to take into account e.g. the transmission frequency and bandwidth of the antenna. This enables one to get the SAR and VSWR values of the device as small as possible. Furthermore, this enables one to optimize the antenna loss, and the antenna can be made to operate each time with optimal amplification. 
     FIG. 3  is a skeleton diagram which shows the device components substantial from the standpoint of the invention that are closely related to the operation of the antenna. The terminal device thus comprises an antenna  80 , which may basically be of any known type, e.g. a so-called internal or a so-called external antenna. There are practically various antenna types, but the type to be used is of no great importance from the standpoint of the invention. At its simplest, the antenna is a so-called rod antenna, which is made of an electrically conductive material, and the resonance frequency of which is determined by the so-called electrical length of the antenna. Therefore, the effective length of a rod antenna e.g. in a mobile phone is typically e.g. λ/4, 3λ/8 or 5λ/8, wherein the wavelength λ is determined based on the frequency band used by the device. 
   The antenna may also be e.g. a so-called helix antenna, in which case the cylindrical coil acts as an antenna. The electrical properties of the helix antennas to be used in mobile phones are typically λ/4, 3λ/4 or 5λ/4. In mobile phones, e.g. so-called micro strip antennas and so-called PIFA antennas (PIFA=Planar Inverted F Antenna) are used in addition. The PIFA antennas enable one to achieve a particularly good radiation pattern and low VSWR value. In addition, the PIFA antennas are suitable for use at a wide frequency band. 
   The terminal device may comprise even several separate antennas, e.g. if the antenna utilizes several different frequency bands. The same mobile station may be used to establish connections e.g. in the frequency ranges of 900 MHz, 1800 MHz and 1900 MHz. 
   For the sake of simplicity, let it be stated that the device comprises, as shown in  FIG. 3 , only one antenna  80 , which in this case is a rod antenna. The antenna is connected at its one end to the antenna feed point  81 , which is disposed on the device&#39;s circuit board  70 , which comprises various components  71 . In this case, the circuit board is a so-called single-layer PC board. In addition to these, on the circuit board there has been formed a ground lead  72  made of an electrically conductive material, which acts as an antenna ground lead. The ground lead thus represents the ground plane in which there is the ground potential acting, and it acts not only as an antenna ground lead but also as the ground of the components on the circuit board. 
   The length, area and shape of the ground lead have an effect on the functioning and properties of the antenna and device, which is why the physical measures and shape of the ground lead may vary even to a great extent for each case specifically. 
   The physical properties of the ground lead have thus an effect e.g. on the SAR and VSWR values of the device as well as on the antenna amplification and antenna losses. The solution in accordance with the invention renders possible the functioning of the antenna at frequencies more several than before and in a frequency band wider than before. In practice it has been found that the solution in accordance with the invention enables one to increase the antenna amplification by over 0.5 dB and the band by over 3% as compared to the corresponding known methods. 
   The device further comprises a switch  90  disposed on the circuit board, which switch  90  is connected to the lead  72 . More specifically, the switch  90  is connected to point A of the lead, which is the remotest point in the lead with respect to the antenna feed point  81 . As discussed above, the length, shape and area of the ground lead affect the functioning of the antenna. There has been marked in the figure point B from which there is the shortest distance to the antenna feed point  81 . As concerns the functioning of the antenna important is the shortest total length of the lead  72  from the antenna to the remotest point. In other words, as concerns the antenna, the effective length of the ground lead corresponds to the aforementioned shortest route along the lead  72  from point B to point A. Therefore, the length of the lead  72  “visible” to the antenna may be changed by means of an additional ground lead, if necessary. 
   In the example of  FIG. 3 , the device comprises three electrically conductive additional ground leads  73 ,  74  and  75  that are of unequal length and are connected to the switch  90 . The one end of each of the additional ground leads is free. While the additional ground leads are in this example like a long lead, their physical measures may vary in various ways, and they may be even very different in respect of their width, length and shape. If necessary, at least one additional ground lead is connected by means of a switch to the actual ground lead  72 , thereby increasing the effective length of the ground lead  72 . Each additional ground lead may, in addition, be located in a different place with respect to the antenna, in which case the additional lead connected to the actual ground lead may affect even to a great extent the SAR and VSWR values of the antenna and device, although the lengths of the additional leads would correspond to one another a lot. 
     FIG. 4  shows another implementation mode of the terminal device. The device comprises on a circuit board  70  three switches  91 ,  92  and  93 , which are connected to the ground lead  72 , which also in this case consists of a conductive pattern on a circuit board. In this case, there is in the device only one additional ground lead  75 . The switches  91 - 93  are so disposed that each of them connects the point of the ground lead corresponding to the switch to a certain point of the additional ground lead. In the case as shown in  FIG. 4 , all the switches are open, which means that the additional ground lead is not connected to the actual ground lead  72 . 
   The galvanic coupling between a ground lead and one or more additional ground leads may be implemented e.g. using components based on the so-called MEMS technology (MEMS=Micro-Electro Mechanical Systems), which components are very small as concerns their physical size in comparison to the corresponding macroscopic counterparts, such as the semiconductor switches. The switches can be e.g. micro relays that have been manufactured using the MEMS technology. Thanks to its process engineering and structure, a micro relay is easy to encapsulate in ordinary SMD enclosures (SMD=Surface Mount Devices). Furthermore, the switches may be implemented e.g. using a PIN diode that is suitable for use in switching applications of various high-frequency signals. 
     FIG. 5  shows the terminal device of  FIG. 4  in a situation in which the additional ground lead  75  is connected to the actual ground lead  72  by the switch  91 . In this situation, the switch  91  is thus closed, thereby galvanically connecting the additional ground lead to the actual ground lead. In the situation as shown in  FIG. 5 , the effective total length of the antenna ground consists of the route B-C-D-E-F. In addition, due to the coupling of the additional ground lead, one has managed to change the shape of the ground. 
   The device comprises a control means  95 , which is practically a microprocessor that controls the closing and opening of the switches. The microprocessor controls the switches based on predetermined switching and release criteria. Typically these criteria include at least the piece of information on the frequency band and/or transmission output used by the device. In a simple embodiment, for each frequency range, there may be an additional ground lead of its own, which is connected to the ground lead  72 , when the device uses the frequency range in question. It is also possible to determine for the device with what length of ground lead and ground configuration one may obtain the most preferred SAR and VSWR values in relation to the frequency and frequency band used. When the most preferred ground lead configurations have once been determined, the lead configurations corresponding to them as well as the switching means with their corresponding switching and release criteria may be introduced in the manufacture phase of the devices. 
   When the device is in use, it is possible, e.g. in conjunction with the change of the frequency band, to change the ground routes (based on a definition made beforehand) in such a manner that the desired features are achieved, such as the maximal antenna amplification or the moving of the hot spot to a preferable location as concerns the SAR value. 
   When an additional ground lead is connected to serve as an extension of the actual ground lead, not only the length of the ground lead will change but also the additional ground lead is disposed in such place or position in relation to the actual ground lead that also the shape of the ground lead has changed to correspond to the changed transmission or reception circumstances. By changing the shape of the ground lead it is possible to further improve the features of the antenna, and it enables one to avoid the problems caused by various interference fields. 
   When the shape of the ground is changed, at the same change also the routes via which the high-frequency ground currents pass that have an effect e.g. on the magnitude of the SAR value. By automatically changing the shape of the ground routes it is possible to control the location of the hot spots in the device. The electric and magnetic fields created are directed in the device structure to such a place in which the fields are more far off from the user of the device. 
     FIG. 6  illustrates one advantageous implementation mode of a terminal device that comprises a multi-layer PC board  70 .  FIG. 6  shows of the circuit board a layer that has been totally coated with an electrically conductive material. In other words, the presented layer  72 ′ of the circuit board functions as the actual ground lead. 
   In  FIG. 6 , the terminal device comprises an antenna  80 , which in this case is a PIFA antenna. The terminal device comprises an antenna feed line  82 , which at the first end is connected to the feed point  81  disposed on the circuit board, and at the second end to the antenna  80 , which is a conductive material. The feed line is, however, not connected to the ground lead at the end where the feed point  81  is, instead the terminal device comprises, in addition, a short circuit, i.e. a short circuit wire  84 , whose first end is connected to the ground lead  72 , and the second end to the antenna. The point at which the first end of the short circuit wire  84  is connected to the ground lead  72 ′ is called a ground contact, and it is marked with reference numeral  83 . The one end of the short circuit wire  84  of the antenna and the one end of the feed line  82  are galvanically connected to one another through the antenna. 
   In the structure as shown in  FIG. 6 , the switch  90  has been placed on the circuit board as far as possible from the ground contact  83  of the antenna in order that the actual ground would be as long as possible from an antenna standpoint. As can be further seen from the figure, the ground contact and switch  90  have been placed in the opposite corners of the circuit board with respect to one another in order to get the length of the actual ground lead  72 ′ as big as possible from an antenna standpoint. The effective length corresponds in this case to the distance between the ground contact  83  and point G marked in the figure. When an additional ground lead is connected to serve as an extension of the actual ground lead, the total length of the ground lead of the device can be increased even more from an antenna standpoint, which enables one to move the hot spots as far as possible from the user of the device. The additional ground leads may be disposed on the circuit board or in the cover structure of the device. 
     FIG. 7  shows a front view of one device in accordance with the invention. The device as shown in the figure comprises additional ground leads  73 ,  74  and  75  on the inner surface of the A cover  30 . In the solution as shown in the figure, the additional ground leads are straight wires, but the shape of the wires may vary. The additional ground lead may be disposed also in the B cover, accumulator cover or some other structural element. The additional ground lead is invisible to the user, unlike in the above-mentioned prior-art embodiments, since in the solution in accordance with the invention, the additional ground lead is automatically introduced. 
     FIGS. 8 and 9  show in more detail the possible locations of the additional ground in the device structure. In the example of  FIG. 8 , the additional ground lead  73  is inside the cover structure. The additional ground lead may be placed in between the inner and outer surface of the cover structure most easily in the manufacture phase of the device cover. If the cover structure is e.g. plastic, it is quite easy to implement a conductive additional ground in the cover structure in the manufacture phase. In principle the cover can even wholly be made of an electrically conductive material, but in that case the cover has to be isolated from the additional ground lead. 
   In the implementation mode as shown in  FIG. 9 , the additional ground  73  is disposed on the surface of the device cover. The wire film which forms the additional ground lead may e.g. be glued or attached in some other suitable manner e.g. to the inner surface of the device cover. The additional ground lead may thus be disposed e.g. in the A cover, B cover, accumulator cover, or on the inner surface of some other device component. It may be disposed e.g. in the accumulator space of the device, in which case it would be on the outer surface of the B cover. The additional ground lead may be disposed e.g. in the rear cover of the mobile station, which enables one to move the hot spot as far as possible from the user of the device. 
   The disposition of the additional grounds in the cover is in that sense preferable that the cover structure provides a possibility of implementing additional ground leads that are physically even very different and big in respect of their area, and at the same time the hot spots may be directed to the desired place in the device. It is, however, possible to implement the additional ground leads in such a manner that they are wholly disposed on the circuit board. Since there normally is only a little space on the circuit board, they may be disposed in one or more (additional) layers of a multi-layer PC board (FIG.  6 ). For example, the additional ground lead for each frequency range may be disposed in its own layer. It is advantageous to dispose the actual ground lead wholly on the circuit board, regardless of whether one uses a one-layer or a multi-layer PC board in the device. 
   The ground plane (ground lead plus additional ground leads) has at each frequency an optimal minimum length, which enables one to achieve sufficient antenna amplification. The decreasing of the SAR value is achieved e.g. in such a manner that the hot spot is moved in the device to such a place in which it is possible to measure for the device a small SAR value. The SAR and VSWR values are not directly dependent on each other. When a good VSWR value is obtained, it is likely that also the antenna amplification increases. In order to achieve good antenna amplification, the antenna coupling has to be good (a small VSWR value), the length of the ground lead has to be optimal and the antenna emitter has to be disposed in a free place as concerns the RF features. If one generalizes a little, it can be said that the length of the necessary ground plane is the bigger the smaller is the frequency. The length of the necessary ground plane is, however, dependent on many factors, e.g. on the location of the ground, its shape and the rest of the device structure. 
   While the invention has been described above with reference to the examples according to the attached drawings, it should be understood that the invention is not limited to them, instead a person skilled in the art can vary the proposed solutions without departing from the inventive idea. If there are in the device e.g. several antennas, each of them may have at least partly their own ground arrangements, as described above, or there may be common additional ground leads for all antennas.