Abstract:
An array antenna calibration apparatus simple in configuration and inexpensive while ensuring an accurate calibration of an array antenna is provided. This array antenna calibration apparatus includes supply means supplying original calibration signals to a plurality of antenna elements constituting an array antenna, the original calibration signals being orthogonal to one another among the antenna elements; a phase and amplitude characteristic calculation means calculating correlations between calibration signals, which are emitted from the antenna elements and received by the adjacent antenna elements, and the original calibration signals related to the received calibration signals; a relative calibration factor calculation means obtaining a relative calibration factor among all the antenna elements constituting the array antenna based on phase and amplitude characteristics of the respective antenna elements; and calibration means calibrating transmission signals to be supplied to the respective antenna elements based on the relative calibration factor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an array antenna calibration apparatus for use in a radio base station and the like. 
     2. Description of the Related Art 
     In order that a digital beam-forming apparatus forms an accurate transmission beam, it is necessary to make the phase characteristics and amplitude characteristics of signals emitted from respective antenna elements uniform. 
     FIG. 5 is a block diagram of a conventional array antenna calibration apparatus. 
     The array antenna calibration apparatus according to the conventional art includes beam formers  13  for users  1  to N, respectively, a user signal multiplexing section  12 , multipliers  10 , adders  5 , transmitters  3 , couplers  17 , antennas  1 , a power synthesizer  18 , a receiver  7 , a calibration factor calculation section  9  and a calibration signal generator  4 . 
     Each beam former  13  forms a beam having a directivity for each user. The user signal multiplexing section  12  multiplexes the beams for the respective users 1 to N and outputs user multiplex signals for six transmitting systems, respectively. Each multiplier  10  multiplies the user multiplexed signal by a corresponding calibration factor. The calibration signal generator  4  generates a calibration signal corresponding to each user multiplexed signal. Each adder  5  adds the corresponding calibration signal to the corresponding user multiplexed signal multiplied by the calibration factor. Each transmitter  3  transmits the corresponding user multiplexed signal which is multiplied by the corresponding calibration factor and to which the corresponding calibration signal is added. The coupler  17  branches a part of each transmission signal and supplies the branched signal to the power synthesizer  18  and the remaining signal to the antenna  1 . Each antenna  1  transmits the signal supplied from the coupler  17 . 
     The power synthesizer  18  synthesizes the powers of the signals supplied from the six couplers  17 . The receiver  7  receives the power-synthesized signals. The calibration factor calculation section  9  calculates a calibration factor for each user multiplexed signal based on the signal received by the receiver  7 , and supplies the calculated calibration factor to the corresponding multiplier  10 . 
     The calibration signals have such signal patterns to be orthogonal to one another among the transmitting systems. Due to this, the calibration factor calculation section  9  performs a correlation processing for the signals synthesized and received by the power synthesizer  18 , whereby the phases and amplitudes of the calibration signals for the respective antenna elements can be measured. The calibration factor calculation section  9  also calculates the calibration factors of the respective transmitting systems based on the measured phases and amplitudes. 
     The above-stated conventional antenna array calibration apparatus has a disadvantage in that fluctuations in the characteristics of the couplers  17  and the antenna elements  1 — 1  to  1 - 6  cannot be corrected. Further, although the conventional array antenna calibration apparatus can measure the characteristics of the couplers  17  and the antenna elements  1 — 1  to  1 - 6  in advance and correct the fluctuations using a table, the apparatus disadvantageously requires high accuracy in measurement and stability in characteristics. In addition, to suppress a fluctuation in the characteristics of cables which connect the couplers  17  to the antenna elements  1 — 1  to  1 - 6 , it is necessary to arrange the couplers  17  in the vicinity of the corresponding antenna elements  1 — 1  to  1 - 6 . To do so, each coupler  17  needs a waterproof structure, with the result that the coupler becomes disadvantageously expensive. 
     To solve these disadvantages, a method adapted for an apparatus constituted as shown in FIG. 6 has been conventionally proposed. Namely, a calibration signal-receiving station  19  which includes a receiver  7  and a calibration factor calculation section  9  is disposed within a sight range. The receiver  7  receives calibration signals transmitted from base station array antennas  1 — 1  to  1 - 6  and having signal patterns orthogonal to one another. The calibration factor calculation section  9  calculates calibration factors by measuring the phases and amplitudes of the respective signals. With this configuration, however, it is necessary to notify the obtained calibration factor to the correction factor receiving section  20  of each base station by a cable or radio communication means. As a result, the system becomes disadvantageously complicated and expensive. Further, it is disadvantageously necessary to dispose the calibration signal receiving station  19  within the sight range of the base station. Besides, it is disadvantageously necessary to grasp the accurate positional relationships between the base stations and the signal generating station. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved to solve the above-stated disadvantages. It is an object of the present invention to provide an array antenna calibration apparatus which is simple in configuration and inexpensive while ensuring an accurate calibration of an array antenna, and an array antenna calibration method therefor. 
     According to the present invention, there is provided an array antenna calibration apparatus comprising: supply means for supplying original calibration signals to a plurality of antenna elements constituting an array antenna, the original calibration signals being orthogonal to one another among the antenna elements; phase and amplitude characteristic calculation means for calculating correlations between calibration signals, emitted from the antenna elements and received by the adjacent antenna elements, and the original calibration signals related to the received calibration signals; relative calibration factor calculation means for obtaining a relative calibration factor among all the antenna elements constituting the array antenna based on phase and amplitude characteristics of the respective antenna elements; and calibration means for calibrating transmission signals to be supplied to the respective antenna elements based on the relative calibration factor. 
     In the array antenna calibration apparatus according to the present invention, the antenna elements constituting the array antenna may be classified into a first group and a second group, and the relative calibration factor calculation means comprises: first relative calibration factor calculation means for obtaining a relative calibration factor among all of the antenna elements belonging to the first group based on the phase and amplitude characteristics of all the antenna elements of the first group; second relative calibration factor calculation means for obtaining a relative calibration factor among all of the antenna elements belonging to the second group based on the phase and amplitude characteristics of all the antenna elements of the second group; third relative calibration factor calculation means for obtaining a relative calibration factor between the first group and the second group based on the phase and amplitude characteristics of one of the antenna elements belonging to the first group and the phase and amplitude characteristics of one of the antenna elements belonging to the second group; and fourth relative calibration factor calculation means for obtaining a relative calibration factor among all the antenna elements constituting the array antenna based on the relative calibration factor among all the antenna elements belonging to the first group, the relative calibration factor among all the antenna elements belonging to the second group, and the relative calibration factor between the first group and the second group. 
     The array antenna calibration apparatus according to the present invention may comprise: synthesizing means for synthesizing the calibration signal received by one of the antenna elements belonging to the second group from one of the antenna elements belonging to the first group with the calibration signal received by the one antenna element belonging to the first group from the one antenna element belonging to the second group, and the relative calibration factor between the first group and the second group may be obtained based on the phase and amplitude characteristic obtained by the phase and amplitude characteristic calculation means based on the synthesized calibration signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the configuration of an array antenna calibration apparatus in one embodiment according to the present invention; 
     FIG. 2 is a block diagram showing the important sections of the calibration apparatus shown in FIG.  1  and the operation thereof FIG. 3 is a block diagram showing the configuration of the array antenna calibration apparatus in the other embodiment according to the present invention; 
     FIG. 4 is a block diagram showing the important sections of the calibration apparatus in the other embodiment and the operation thereof; 
     FIG. 5 is a block diagram showing the configuration of the array antenna calibration apparatus according to the first conventional art; and 
     FIG. 6 is a block diagram showing the configuration of the array antenna calibration apparatus according to the second conventional art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention will be described hereinafter in detail with reference to the accompanying drawings. 
     FIG. 1 is a block diagram showing the configuration of an array antenna calibration apparatus in one embodiment according to the present invention. 
     Referring to FIG. 1, the array antenna calibration apparatus in this embodiment comprises a calibration signal generator  4  which generates calibration signals for making uniform the phase characteristics and amplitude characteristics of signals emitted from antenna elements  1 — 1  to  1 - 6  which constitute a linearly arranged array antenna, adders  5  which add the calibration signals to respective user multiplexed signals, circulators  6  which fetch electromagnetically coupled signals from the adjacent antenna elements, a receiver  7  which receives the signals fetched by the respective circulators  6 , an RF switch  8  which switches the input signals of the receiver  7 , a calibration factor calculation section  9  which detects a calibration signal from the output of the receiver  7  and calculates a calibration factor, multipliers  10  which multiply the user multiplexed signals by the calibration factors calculated by the calibration factor calculation section  9 , and a power synthesizer  11  which synthesizes the electromagnetically coupled signals from the antenna elements adjacent to the antenna elements  1 — 1  and  1 - 6  on the both ends of the linear array antenna. Respective transmitting systems comprises employing orthogonal signal patterns which have no correlations with one another. 
     A calibration method in this embodiment will be described with reference to FIG.  2 . Calibration signals C 1  to C 6  are orthogonal to one another. The calibration signals C 1  to C 6  are superposed on the user multiplexed signals at an equal amplitude and with an equal phase, and input into the transmitters  3 , and transmitted from the antenna elements  1 — 1  to  1 - 6 . The calibration signals C 1  to C 6  can be fetched without the interference of the user multiplexed signals by subjecting the user multiplexed signals to frequency division multiplexing (FDM), time division multiplexing (TDM) or code division multiplexing (CDM). Further, by using signal patterns orthogonal to one another and having no correlation to one another, the respective calibration signals C 1  to C 6  can be fetched independently of one another. 
     Now, the calibration method will be described while paying attention only to the calibration signals. The calibration signals C 1  and C 3  which are transmitted from the antenna elements  1 — 1  and  1 - 3 , respectively are received by the antenna element  1 - 2  due to the electromagnetic coupling between the antenna elements. The received signals C 1 +C 3  are fetched by the circulator  6  and input into the P 1  port of the RF switch  8 . Likewise, the signals C 2 +C 4 , C 3 +C 5 , and C 4 +C 6  are input into the P 2  port, P 3  port and P 4  port of the RF switch  8 , respectively. The calibration signal C 2  is fetched by the circulator  6  of the antenna element  1 — 1  and the calibration signal C 5  is fetched by the circulator  6  of the antenna element  1 - 6  due to the electromagnetic coupling. These calibration signals C 2  and C 5  are synthesized with each other by the power synthesizer  11 , and input into the P 5  port of the RF switch  8 . 
     The ports of the RF switch  9  are sequentially changed over, the input signals of the P 1  to P 5  ports are demodulated and converted into baseband signals by the receiver  7 . The calibration factor calculation section  9  measures the phases and amplitudes of the respective calibration signals and calculates calibration factors. When the P 1  port is connected to the receiver  7 , calibration signals C 1 +C 3  are received by the receiver  7 . The calibration signals C 1  and C 3  have signal patterns orthogonal to each other and having no correlation to each other. Due to this, a correlation processing is performed based on the respective signal patterns, whereby the phases and amplitudes of the calibration signals C 1  and C 3  are obtained, and a factor for making the amplitudes and phases of the signals C 1  and C 3  uniform is obtained. Likewise, by changing over the port of the RF switch  8 , factors for making uniform the amplitudes and phase of the signals C 2  and C 4 , those of the signals C 3  and C 5 , those of the signals C 4  and C 6 , and those of the signals C 2  and C 5  are obtained. By employing the factors thus obtained, a calibration factor for making uniform the phases and amplitudes of all the calibration signals C 1  to C 6  is obtained. Since the calibration signals C 1  to C 6  are input into the respective transmitters  3  at equal amplitude and with equal phase, the measured amplitudes and phases of the C 1  to  6  indicate fluctuations in the amplitude and phase characteristics of the corresponding antenna elements and cables. Accordingly, by multiplying the calibration factors obtained from the measured values by the input signals, it is possible to make uniform the amplitude and phase characteristics of the respective transmitting systems. 
     The embodiment of the present invention will be described with reference to FIG.  3 . FIG. 3 shows the configuration of the base station of a CDMA communications system which employs a linear array antenna. The transmission signal of each user is subjected to complex weighting by the beam former  13  of the user, thereby generating a signal to be transmitted from the antenna element for the user. The transmission signal of the antenna element generated by the beam former  13  is spread by the spreader  15  of a code multiplexing section  14 , and the spread signals of all the users are multiplexed by a signal synthesizer  16  for each antenna element. 
     The user multiplexed spread signal of each antenna element output from the code multiplexing section  14  is multiplied by the calibration factor, which is calculated by the calibration factor calculation section  9 , by the multiplier  10 . The calibration signal, which is generated by the calibration signal generator  4 , is added to each multiplied signal by the adder  5 , the calibration signal-added signal is modulated by the transmitter  3  and emitted from each of the antenna elements  1 — 1  to  1 - 6 . Orthogonal signal patterns which have no correlation to one another are generated by the calibration signal generator  4 , and added to the respective antenna elements  1 — 1  to  1 - 6 . 
     A part of the RF signal emitted from each antenna element is electromagnetically coupled with the adjacent antenna elements and fetched by the circulators  6  of the adjacent antenna elements. By changing over the RF switch  8 , the coupled signals from the adjacent antenna elements can be sequentially received by the receiver  7 . 
     The signals received by the receiver  7  are demodulated and then converted into baseband digital signals. The calibration factor calculation section  9  calculates calibration factors for correcting the phase and amplitude characteristics of the transmitting systems of the respective antenna elements. Since the receiver  7  does not perform an inverse spread processing, the user multiplexed spread signals are suppressed and only the calibration signals can be fetched. 
     The operation of this embodiment will be described with reference to FIG.  2 . The signals emitted from the respective antenna elements  1 — 1  to  1 - 6  receive fluctuations in the characteristics of the transmitter, the antenna elements  1 — 1  to  1 - 6 , the circulators  6  and the connection cables, and these signals can be expressed as follows: 
     
       
           x   i =( C   i ( t )· U   i ( t ))· a   i ( t ) exp ( jφ   i ( t ))  (1)  
       
     
     where 
     C i (t): calibration signal of antenna element  1 - i    
     U i (t): user multiplexed spread signal 
     a i : amplitude fluctuation of transmitting system of antenna element  1 - i    
     φ i : phase fluctuation of transmitting system of antenna element  1 - i    
     The transmission signals from the adjacent antenna elements on both sides are electromagnetically coupled to the antenna element  1 - i  (i=2 to 5), whereby signals x i-1 (t)+x i+1 (t) are fetched by the circulator  6  of the antenna element  1 - i  and received by the receiver  7  through the RF switch  8 . The calibration signals C 1  to C 6  are signals which are not spread, the user multiplexed spread signals are signals which have been spread, and the receiver  7  does not perform the inverse spread processing. Therefore, the user multiplexed spread signals are suppressed and only the calibration signals can be fetched by the receiver  7  as follows: 
     
       
           y   i ( t )= C   i-1 ( t )· a   i-1 ( t ) exp ( j·φ   i-1 ( t ))+ C   i+1 ( t )· a   i+1 ( t ) exp ( j·φ   i+1 ( t ))  (2)  
       
     
     The calibration signals C 1  to C 6  employ the following orthogonal signal patterns which have no correlation to one another.                        1   T            ∑     i   =   nT         (     n   +   1     )        T                C   i          (   t   )       ·       C   j          (   t   )             =     1        (     i   =   j     )                   =     0        (     i   ≠   j     )                     (   3   )                                
     Accordingly, if the characteristic fluctuation of each antenna element is slow enough to be able to be approximated with a constant value within a calibration signal pattern cycle T, a component C i+1 (t) can be eliminated and the phase and amplitude characteristics of the transmitting system of the antenna element  1 -( i− 1), through which a calibration signal pattern C i-1 (t) passes, can be measured by obtaining the correlation between the calibration signal y i (t) and the calibration signal pattern C i-1 (t).                  h     i   -   1            (   n   )       =         ∑     t   =   nT         (     n   +   1     )        T                y   i          (   t   )       ·       C     i   -   1            (   t   )           ≅       ∑     t   =   nT         (     n   +   1     )        T                a     i   -   1            (   t   )       ·     exp        (     j                     φ     i   -   1            (   t   )         )                     (   4   )                                
     Likewise, by obtaining the correlations between the calibration signal y i (t) and the calibration signal pattern C i+1 (t), the component C i-1 (t) can be eliminated and the phase and amplitude characteristics h i+1  of the transmitting system of the antenna element  1 -( i+ 1), through which the component C i+1 (t) passes, can be measured. 
     Consequently, a calibration factor corr i  for making uniform the amplitude and phase characteristics of the antenna elements  1 -( i− 1) and  1 -( i+ 1) adjacent to the antenna element  1 - i  can be obtained as follows: 
     
       
           h   1+1 ( n )= corr   i ( n )· h   i-1 ( n )  (5)  
       
     
     The calibration factors of the six antenna elements shown in FIG. 2 are expressed as follows: 
     
       
           h   3 ( n )= corr   1 ( n )·h 1 ( n )  
       
     
     
       
           h   4 ( n )= corr   2 ( n )·h 2 ( n )  
       
     
     
       
           h   5 ( n )= corr   3 ( n )·h 3 ( n )  
       
     
     
       
           h   6 ( n )= corr   4 ( n )·h 4 ( n )  (6)  
       
     
     As shown in the configuration of FIG. 2, the circulator outputs of the antenna elements  1 — 1  and  1 - 6  are synthesized with each other by the power synthesizer  11 . The output of the power synthesizer  11  is demodulated by the receiver  7 , whereby the signals C 2 +C 5  are fetched. The calibration factor calculation section  9  performs a correlation processing based on the calibration signal patterns by the above-stated method, whereby the amplitude and phase characteristics of the calibration signals C 2  and C 5  can be measured. If the amplitudes and phases of the power synthesizer  11  and the respective circulators  6  are made uniform in advance, the calibration factor can be obtained from the measured amplitude and phase characteristics of the calibration signals C 2  and C 5  as follows: 
     
       
           h   2 ( n )= Corr   5 ( n )· h   5 ( n )  (7)  
       
     
     By employing the calibration factors obtained by the expressions (6) and (7), the respective calibration factors with the calibration factor h 1  as reference can be expressed as follows: 
     
       
           h   2 ( n )= corr   5 ( n )· h   5 ( n )= corr   5  ( n )· corr   3 ( n )· corr   1 ( n )· h   1 ( n )  
       
     
     
       
           h   3 ( n )= corr   1 ( n )· h   1 ( n )  
       
     
     
       
           h   4 ( n )= corr   2 ( n )· h   2 ( n )= corr   2 ( n )· corr   5 ( n )· corr   3 ( n )· corr   1 ( n )· h   1 ( n )  
       
     
     
       
           h   5 ( n )= corr   3 ( n )· h   3 ( n )= corr   3 ( n )· corr   1 ( n )· h   1 ( n )  
       
     
     
       
           h   6 ( n )= corr   4 ( n )· h   4 ( n )= corr   4 ( n )· corr   2 ( n )· corr   5 ( n )· corr   3 ( n )· corr   1 ( n )· h   1 ( n )  (8)  
       
     
     Hence, the calibration factors with the antenna element  1 - i  as reference, can be obtained as follows: 
     
       
         Corr 1 (n)=1  
       
     
     
       
           Corr   2 ( n )=1/( corr   5 ( n )· corr   3 ( n )· corr   1 ( n ))  
       
     
     
       
           Corr   3 ( n )=1/ corr   1 ( n )  
       
     
     
       
           Corr   4 ( n )=1/( corr   2 ( n )  corr   5 ( n )· corr   3 ( n )· corr   1 ( n ))  
       
     
     
       
           Corr   5 ( n )=1/( corr   3 ( n )· h   3 ( n )= corr   3 ( n )· corr   1 ( n ))  
       
     
     
       
           Corr   6 ( n )=1/( corr   4 ( n )· corr   2 ( n )· corr   5 ( n )· corr   3 ( n )· corr   1 ( n ))  (9)  
       
     
     FIG. 4 shows another embodiment according to the present invention. The outputs of antenna elements  1 - 7  and  1 - 8 , to which non-reflection terminating units  2  are connected in FIG. 2 but not connected in FIG. 4, are synthesized by the power synthesizer  11 . The signals due to the coupling of the antenna elements  1 - 7  and  1 - 8  with the antenna elements  1 — 1  and  1 - 6 , respectively, are received by the receiver  7 . By doing so, calibration signals C 1 +C 6  are fetched and the calibration factor calculation section  9  can obtain the calibration factor between the calibration signals C 1  and C 6 . Similarly to the preceding embodiment, the outputs of the circulators  6  of the antenna elements  1 - 2  to  1 - 5  are received by the receiver  7 , whereby the calibration signals C 1 ⇄C 3 , C 4 +C 2 , C 3 +C 5  and C 4 +C 6  are fetched and the calibration factor calculation section  9  can obtain calibration factors for the respective calibration signal pairs. As a result, as in the case of the preceding embodiment, the calibration factors with the antenna element  1 — 1  as reference, can be obtained as follows: 
     
       
         
           h 
           6 
           =corr 
           5 
           ·h 
           1  
         
       
     
     
       
         
           h 
           3 
           =corr 
           1 
           ·h 
           1  
         
       
     
     
       
         
           h 
           4 
           =corr 
           2 
           ·h 
           2  
         
       
     
     
       
         
           h 
           5 
           =corr 
           3 
           ·h 
           3 
           =corr 
           3 
           ·corr 
           1 
           ·h 
           1  
         
       
     
     
       
         
           h 
           6 
           =corr 
           4 
           ·h 
           4  
         
       
     
     
       
         
           h 
           4 
           =corr 
           5 
           /corr 
           4 
           ·h 
           1  
         
       
     
     
       
           h   2   =corr   5 /( corr   4   ·corr   2 )· h   1   (10)  
       
     
     
       
         Corr1=1  
       
     
     
       
           Corr 2= Corr 4· Corr 2/ Corr 5  
       
     
     
       
           Corr 3=1 /Corr 1  
       
     
     
       
           Corr 4= Corr 4/ Corr 5  
       
     
     
       
           Corr 5=1/( Corr 3· Corr   1)    
       
     
     
       
           Corr 6=1/ Corr 4  (11)  
       
     
     The present invention is also applicable to the base stations of a TDMA communications system and an FDMA communications system. If the present invention is applied to the TDMA communications system, a calibration signal is input by using an allocated calibration signal time slot or an empty time slot, and is measured. If the present invention is applied to the FDMA communications system, a calibration signal is input by using an allocated calibration signal frequency channel or an empty frequency channel, and is measured. 
     Furthermore, the present invention is applicable to a circular array antenna in which the antenna elements of the linear antenna shown in the embodiments are arranged on a circumference except for the non-reflection terminating antenna elements. 
     Moreover, in the embodiment shown in FIG. 1, the signals received by the two antennas  1 — 1  and  1 - 6  are synthesized with each other by the power synthesizer  11  and the synthesized signal is supplied to the RF switch  8 . Alternatively, the number of inputs of the RF switch may be increased without providing the power synthesizer  11 , and the signal received by the antenna element  1 — 1  and that received by the antenna element  1 - 6  maybe separately supplied to the RF switch  8 . In this case, it is possible to obtain the phase and amplitude characteristics of the transmitting systems of the antenna elements according to a similar expression to the expression (4). 
     In the embodiment shown in FIG. 4, the signals received by the two antenna elements  1 - 7  and  1 - 8  are synthesized with each other by the power synthesizer  11  and the synthesized signal is supplied to the RF switch  8 . Alternatively, the number of inputs of the RF switch may be increased without providing the power synthesizer  11 , and the signal received by the antenna element  1 - 7  and that received by the antenna element  1 - 8  may be separately supplied to the RF switch  8 . In this case, it is possible to obtain the phase and amplitude characteristics of the transmitting systems of the antenna elements according to a similar expression to the expression (4). 
     As described so far, according to the present invention, it is advantageously possible to correct fluctuations in amplitude and phase characteristics including even the radiation characteristics of the antenna elements without providing an external calibration signal receiving station. 
     Further, since it is possible to calibrate the characteristics including even those of the circulators for fetching the calibration signals and the connection cables from the circulators to the antenna elements, each circulator can be arranged at an arbitrary place between the transmitter and the antenna element. Therefore, differently from the conventional art, it is advantageously unnecessary to arrange the circulator in the vicinity of the corresponding antenna element so as to suppress a characteristic fluctuation in the cable between the coupler for fetching the calibration signal and the antenna element, to provide the circulator with the water proof structure, and to provide cables for feeding the calibration signals into a house. 
     Further, it is unnecessary that the circulators for fetching calibration signals except for the calibration signals synthesized by the power synthesizer have the same characteristics. Therefore, it is advantageously possible to employ inexpensive circulators. 
     Moreover, since the power synthesizer which is required to make characteristics uniform is a two-branch power synthesizer, it is advantageously easy to make characteristics uniform, compared with the conventional multiple-branch power synthesizer.