Abstract:
A control system for the transmitting power of a multiband antenna of especially a mobile station. The antenna ( 300 ) has at least two radiating elements (B 31 , B 32 ) corresponding to different operating bands. A separate part is arranged for each element in the antenna switch of a radio device using TDD technology. When one radiating element (B 31 ) is connected to a transmitter (TX 1 ), the other radiating element (B 32 ) is connected to a control circuit (DET, PCU) for a radio-frequency power amplifier (PA 1 ) of the transmitter. By an electromagnetic coupling (CP) between the radiating elements is then produced to the control circuit a signal (M 1 ) indicating the transmitting power, and the transmitting power can be kept as desired by controlling the power amplifier. The electromagnetic coupling between the elements is arranged suitable in view of the power control. The arrangement achieves space savings on the circuit board of the radio device as the relatively large directional couplers can be left out, and the attenuation in the transmission paths from the power amplifiers to the antenna will be lower.

Description:
[0001]    The invention relates to an arrangement for controlling the transmitting power of a multiband antenna of especially a mobile station. The invention further relates to a method for controlling the transmitting power of a multiband antenna of especially a mobile station.  
         BACKGROUND OF THE INVENTION  
         [0002]    It is common to set limits on the transmitting power of radio devices in order to keep the interference level in receivers low enough. Transmitting power restrictions are particularly well-grounded in mobile terminals because of the great number and density of the devices. On the other hand, the transmitting power naturally has to be high enough for getting the transmitted signal through. If the impedance of the transmitting antenna is known and stays constant, the transmitting power of a radio transmitter can be set with good accuracy by choosing suitable component values for the radio-frequency power amplifier. However, in mobile stations, for example, the immediate surroundings have an effect on the transmitting power. Dielectric material and conductive material in particular change the impedance matching of the antenna and, hence, the transmitting power. Therefore it is advantageous to measure the real transmitting power, and control the radio-frequency power amplifier so that the transmitting power stays at the set value.  
           [0003]    The transmitting power measurement is usually realized using a directional coupler, which is part of the antenna feedline. FIGS. 1 and 2 show an example of such a known arrangement. FIG. 1 shows a known antenna, applicable in mobile stations, and FIG. 2 shows schematically the antenna end of a radio device utilizing that antenna. The antenna in this example is a dual-band antenna since the invention also applies to an arrangement in a multi-band device. The antenna  100  in FIG. 1 is a planar inverted F antenna (PIFA). It comprises a ground plane  110  on a surface of a circuit board  105  in a radio device, and a radiating plane  120  elevated from the ground plane. The radiating plane is supported by a dielectric frame  150  on the circuit board  105 . The radiating plane is galvanically connected to the ground plane by a short-circuit conductor  111  at a short-circuit point S. An antenna feed conductor  112  is galvanically connected to the radiating plane through a via hole in the circuit board  105  at a feed point F. Furthermore, the radiating plane  120  has a non-conductive slot  125  starting from the edge thereof such that, viewed from the short-circuit point S, the plane is divided into two branches of different lengths: a first branch B 1  goes round along the edges of the plane and a second, shorter branch B 2  lies in the middle section of the plane. The branches also have different electrical lengths so that the antenna has a lower operating band corresponding to the first branch, and an upper operating band corresponding to the second branch.  
           [0004]    In FIG. 2 the antenna  100  can be connected by means of the antenna switch ASW to a transmitting or receiving part in the radio device. The radio device at issue thus utilizes time division duplex (TDD) technology. In this example the antenna switch has five positions. At position 1 the antenna is connected to a first receiver RX 1 , at position 4 to a second receiver RX 2 , and at position 5 to a third receiver RX 3 . The first receiver can be e.g. in accordance with the GSM900 system (Global System for Mobile telecommunications), the second one with the GSM1800 system, and the third one with the GSM1900 system. In this case the aforementioned upper operating band of the antenna  100  is wide enough to cover the frequency range of both the GSM1800 and GSM 1900 system. The radio device includes respective three transmitters. The first transmitter TX 1  comprises, along the direction of signal propagation, a first power amplifier PA 1 , first antenna filter TF 1 , and a first directional coupler DC 1 , connected in series. The first directional coupler is connected to the antenna when the antenna switch is at position 2. The second TX 2  and third TX 3  transmitters share, along the direction of signal propagation, a second power amplifier PA 2 , second antenna filter TF 2 , and a second directional coupler DC 2 , connected in series. The second directional coupler is connected to the antenna when the antenna switch is at position 3.  
           [0005]    When the first transmitter TX 1  is active, power amplifier PA 1  receives a radio-frequence signal TS 1  which is amplified and fed to the antenna. From pole p 1  of the first directional coupler DC 1  it is obtained a first radio-frequency measurement signal M 1  proportional to the strength of the field propagating towards the antenna. When, for example, the matching of the antenna becomes worse for external reasons, the strength of the field reflected from the antenna increases and the strength of the propagating field decreases. The real transmitting power is proportional to the square of the strength of the propagating field, so the measurement signal M 1  serves as an indicator of the transmitting power. The measurement signal is brought to a detector DET which outputs a signal ML proportional to changes in the level of said measurement signal. In a power control unit PCU, signal ML is compared with a reference level corresponding to a certain transmitting power, and, on the result basis, the first power amplifier PA 1  is controlled by a control signal C 1 . If an external factor causes the transmitting power for example to drop, control C 1  changes such that it increases the amplification in the power amplifier until the level of signal ML again equals said reference level. The reference level is set by software through a bus in the radio device. Similarly, when the second or third transmitter is active, it is obtained from the second directional coupler DC 2  a second measurement signal M 2  proportional to the strength of the field propagating towards the antenna, and on grounds of detection result of the signal M 2  the second power amplifier PA 2  is controlled by a control signal C 2 .  
           [0006]    A disadvantage of the arrangement according to FIG. 2 is that the directional couplers, being relatively large components, take up an impractically large space on the circuit board. Moreover, they cause extra attenuation in the signal to be transmitted, which is particularly disadvantageous in that part of the transmitter which follows the radio-frequence power amplifier.  
         SUMMARY OF THE INVENTION  
         [0007]    An object of the invention is to alleviate said disadvantages associated with the prior art. An arrangement according to the invention is characterized in that which is specified in the independent claim 1. A method according to the invention is characterized in that which is specified in the independent claim 6. Preferred embodiments of the invention are presented in the other claims.  
           [0008]    The idea of the invention is basically as follows: An antenna has at least two radiating elements corresponding to different operating bands. For each element is given a separate part of its own in the antenna switch of a radio device using TDD technology. When a radiating element is connected to a transmitter, an other element is connected to a control circuit of the radio-frequency power amplifier of the transmitter in question. The control circuit thus gets a signal indicative of transmitting power through an electromagnetic coupling between the radiating elements, and the transmitting power can be kept at a desired level by controlling the power amplifier. The electromagnetic coupling between the elements is arranged suitable in view of the power control.  
           [0009]    An advantage of the invention is that space is saved on the circuit board of a radio device as the relatively large directional couplers can be left out. Another advantage of the invention is that, for the reason mentioned above, attenuation decreases on the transmission paths leading from the power amplifiers to the antenna, reducing energy consumption and warming of the power amplifiers. A further advantage of the invention is that as the number of components drops, the production costs of the radio device drop, too. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The invention is now described more closely. The description to follow refers to the accompanying drawings where  
         [0011]    [0011]FIGS. 1,2 illustrate an arrangement according to the prior art for controlling the transmitting power,  
         [0012]    [0012]FIG. 3 illustrates an example of the antenna according to the invention,  
         [0013]    [0013]FIG. 4 is a block diagram illustrating an arrangement according to the invention for controlling the transmitting power,  
         [0014]    [0014]FIG. 5 illustrates a second example of the antenna according to the invention,  
         [0015]    [0015]FIG. 6 illustrates examples of antenna tuning ways in an arrangement according to the invention, and  
         [0016]    [0016]FIG. 7 is a flow diagram illustrating the method according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    [0017]FIGS. 1 and 2 were already discussed in conjunction with the description of the prior art.  
         [0018]    [0018]FIG. 3 shows an example of a dual-band planar antenna according to the invention corresponding to the antenna shown in FIG. 1. A substantial difference to FIG. 1 is that now the slot  325  in the radiating plane  320  of the antenna  300  both starts from an edge of the plane and ends up at an edge of the plane whereby the first B 31  and second B 32  radiating elements are galvanically isolated from one another. Between them there is only an electromagnetic coupling CP. Consequently, each radiating element needs a feed conductor of its own. The first radiating element B 31 , or the first element in short, has a first feed conductor  312  connected thereto at a feed point F 1 , and the second radiating element B 32 , or the second element in short, has a second feed conductor  314  connected thereto at a feed point F 2 . Similarly, each radiating element has a short-circuit conductor of its own: a first short-circuit conductor  311  connected at point S 1 , and a second short-circuit conductor  313  connected at point S 2 . In this example the feed and short-circuit conductors are of the spring contact type and they are a part of the same metal sheet as the radiating element at issue. When installed, a spring force presses them against the circuit board  305  of the radio device. The first element B 31  is physically and electrically longer than the second element wherefore it is used to produce the lower operating band of the antenna.  
         [0019]    [0019]FIG. 4 shows an example of an arrangement according to the invention for controlling transmitting power, corresponding to the arrangement of FIG. 2. In FIG. 4, the radiating elements of the antenna  300  are represented by arrow-shaped symbols pointing upwards. Between them there exists said electromagnetic coupling CP. The antenna switch ASW now comprises two separate parts: a first part having three positions, and a second part having four positions. When the first part of the antenna switch is at position 1, the first element B 31  is connected to the first receiver RX 1 . When the second part of the antenna switch is at position 3, the second element B 32  is connected to the second receiver RX 2 , and when it is at position 4, the second element is connected to the third receiver RX 3 . The radio device has three transmitters respectively. The first transmitter TX 1  comprises, connected in series along the direction of signal propagation, a first power amplifier PA 1  and a first antenna filter TF 1 . At position 2 of the first part of the antenna switch the first antenna filter is connected to the first element B 31  of the antenna. This situation is depicted in FIG. 4. The second TX 2  and third TX 3  transmitters share, along the direction of signal propagation, a second power amplifier PA 2  and a second antenna filter TF 2 , connected in series. At position 2 of the second part of the antenna switch the second antenna filter is connected to the second element B 32  of the antenna. At position 3 of the first part of the antenna switch ASW the first element B 31  is connected to the detector DET. At position 1 of the second part of the antenna switch the second element B 32  is connected to the detector DET, which situation is depicted in FIG. 4.  
         [0020]    When the first transmitter TX 1  is active, power amplifier PA 1  receives a radio-frequency signal TS 1  which is amplified and fed in accordance with FIG. 4 to the part of antenna corresponding to the first element B 31 . Due to said electromagnetic coupling CP, part of the energy fed is transferred into the circuit of the second element B 32 . The electromagnetic coupling is arranged to be so weak that the portion of the energy transferred is relatively small; the isolation attenuation of the elements is 15 to 20 dB, for instance. The idea is to obtain a signal of high enough level from the second element for the purpose to measure the field fed. Thus, instead of a separate directional coupler, the second antenna element B 32  is used as a measurement element. The second element produces a first radio-frequency measurement signal M 1  proportional to the strength of the field propagating towards the first element of the antenna. This is suitable, like measurement signal M 1  in FIG. 2, for indicating the transmitting power. The measurement signal is brought to detector DET which outputs a signal ML proportional to the change of level thereof. The level of signal ML is compared in a power control unit PCU with a reference level corresponding to a certain transmitting power, and, on the result basis, the first power amplifier PA 1  is controlled by a control C 1 . The feedback described keeps in this case, too, the level of the measurement signal ML equal to the reference level, i.e. keeps the transmitting power nominal. The reference level is set by software through a bus in the radio device. Similarly, when the second or third transmitter is active, instead of a separate directional coupler, the first antenna element B 31  is used as a measurement element. In that case the second part of the antenna switch is at position 3 and the first part is also at position 3. The first element gives a second radio-frequency measurement signal proportional to the strength of the field propagating towards the second element of the antenna, and on grounds of this detection result the second power amplifier PA 2  is controlled by a control signal C 2 .  
         [0021]    [0021]FIG. 5 shows a second example of an antenna according to the invention. The difference to FIG. 3 is that now the slot  525  in the radiating plane  520  of the antenna  500  ends up in the inner region of the plane, instead of an edge of the plane. The end point of the slot  525 , or the closed end, is relatively close to that end of the radiating plane where the antenna feed arrangement is located. In the area between the closed end of the slot and said end of the plane there are the short-circuit points of the antenna, which there are two in the example of FIG. 5: a first short-circuit point S 1  to which a first short-circuit conductor  511  is connected, and a second short-circuit point S 2  to which a second short-circuit conductor  513  is connected. There may also be just one short-circuit conductor. This would be e.g. a relatively wide spring contact, like the ones depicted in FIG. 3, and it would be located at the closed end of the slot  525  at the gable of the antenna. Viewed from the short-circuit area, the radiating plane is divided by the slot  525  into two branches, or radiating elements, of different lengths: The first element B 51  is limited by a first long side of the plane, the end opposite to the short-circuit area, and by a portion of a second long side. The second, shorter element B 52  is limited by a second portion of the second long side of the radiating plane. The first element B 51  has a first feed conductor  512  connected thereto at a feed point F 1 , and the second element B 52  has a second feed conductor  514  connected thereto at a feed point F 2 . Between the first and second elements there is a certain electromagnetic coupling CP. It is utilized for controlling the transmitting power of the antenna, as in the arrangement according to FIGS. 3 and 4.  
         [0022]    Compared to the prior art, antennas in the arrangements according to FIGS. 3, 4 and  5  need more feed and short-circuit conductors and the antenna switch is larger. However, the exclusion of two directional couplers means that as a whole, the components of the radio device require less space and the overall manufacturing costs are smaller. The appropriate isolation attenuation between the radiating antenna elements is set by determining the width of the slot between the elements and by element design. Arrangements for the isolation attenuation naturally have an effect on the resonance frequencies of the antenna and thus on the locations of the operating bands. Therefore, the resonance frequencies need to be tuned separately after the tuning of the isolation attenuation. FIG. 6 shows examples of ways to tune antenna resonance frequencies in an arrangement according to the invention. FIG. 6 shows a radiating plane  620  divided into a first element B 61  and second element B 62 . These have feed points F 1 , F 2  and short-circuit points S 1 , S 2 , respectively, of their own. The ground plane is not drawn. The electrical length and, thus, the fundamental resonance frequency of the first element B 61  are determined by a first tuning slot  626  directed from an edge of the element towards the center region of the element, and by a first extension  621  located at the farther end of the element relative to the short-circuit point S 1  and directed towards the ground plane. At the ground-plane-side end of the extension  621  there is further a fold parallel to the ground plane, directed into the interior of the antenna. The electrical length and, thus, the fundamental resonance frequency of the second element B 62  are determined by a second tuning slot  627  directed from an edge of the element towards the center region of the element, and by a second extension  622  directed from a side of the element towards the ground plane. These arrangements make it possible to tune the resonance frequencies at gigahertz frequencies within a range of about one hundred megahertz.  
         [0023]    [0023]FIG. 7 illustrates in a flow diagram the method according to the invention. The flow diagram starts with the beginning of transmitting period. At step  701  it is defined which of the two operating bands of the antenna will be used for the transmitting. This depends on which of the transmitters is active, which is known by the power control unit PCU shown in FIG. 4, for example. If the first transmitter, which uses the lower operating band, is activated, the first power amplifier is connected in accordance with step  702  to the first element radiating in the lower operating band of the antenna. In addition, the second radiating element of the antenna, which is used as a measurement element in this situation, is connected to the detector used in the transmitting power control. These connections may be controlled by the aforementioned power control unit, for example. At step  703  it is detected the radio-frequency measurement signal brought to the detector, which signal depends on the transmitting power. At step  704  it is compared the level of the detected measurement signal against a reference level corresponding to nominal power. If the measured level is below the reference level, the amplification of the power amplifier currently in use is increased in accordance with step  705 . If the measured level is above the reference level, the amplification of the power amplifier currently in use is decreased in accordance with step  706 . The control may also have hysteresis such that when the measured level equals the reference level within a certain tolerance, the amplification will not be changed. If at step  701  it appears that the upper operating band is needed, the second power amplifier is connected in accordance with step  707  to the second element radiating in the upper operating band of the antenna. In addition, the first radiating element of the antenna, which is used as a measurement element in this situation, is connected to the detector used in the transmitting power control. The operation then continues in accordance with steps  703  to  706 .  
         [0024]    Solutions according to the invention were described above. The invention is not limited to those. The antenna elements may be other than planar elements, and the number of operating bands in the antenna may be greater than two. The structure of the antenna end of the transmitters may differ from those described. The invention does not restrict the implementation of the antenna switch, detector and power control unit. For example, control operation in the power control unit may be analog or digital, being program-based in the latter case. The inventional idea can be applied in various ways within the scope defined by the independent claims 1 and 6.