Abstract:
In a radio transceiver operating in a time division duplex system, a cancellation use transmission signal which is reverse in phase and equal in amplitude to a leakage transmission signal from an antenna in the signal transmission is supplied to an input port of the receiving amplifier of a receiver connected to an antenna system applied in common to the signal transmission. The transmission signal is fed via a circulator, which separates a transmission and reception path, to the antenna. The leakage transmission signal from the antenna is inputted via the circulator to the receiving amplifier. The cancellation use transmission signal is fed to the receiving amplifier via a second circulator which distributes the transmission signal and cancellation use signal disposed at an output side of the transmitting amplifier.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of preventing transmission spurious response in a radio transceiver, and in particular, to a method of preventing transmission spurious response in a receiver section and a radio transceiver. 
     2. Description of the Related Art 
     In a conventional transceiver using a time division duplexing (TDD) system, a transmission frequency is quite similar to or equal to a reception frequency. 
     Consequently, it is pretty difficult to attenuate sufficiently a component of the transmission signal which leaks into the reception signal by using only a band-pass filter. 
     Japanese Patent Laid-Open application No. 9-116459 describes a technology to remove such interference in a radio transceiver having an antenna used commonly for transmitting and receiving signals. According to the laid-open application, an apparatus to remove interference between transmission and reception signals operates as follows. In a transceiver in which, as shown in FIG. 1, a transmission signal fed from a transmitting section  31  via band-pass filter (BPF)  32  which limits bandwidth of transmitting signal is applied via first metallic path  41  to transmission and reception circulator  33  and guided into antenna  34  by circulator  33 , and reception signal fed from antenna  34  is delivered by circulator  33  to band-pass filter  35  which passes the reception signal via second metallic path  43  and transferred to receiver section  36  after filtering. 
     The transceiver further comprises circulator  45  which distributes transmission signal between transmitting section  31  and circulator  33 , receiver side directional coupler  44  between receiving section  36  and circulator  33  and third metallic path  42  including amplifier  51  and phase shifter  52  between circulator  45  and coupler  44 . The difference between a length of the path from circulator  45  via first and second paths  41  and  42  to coupler  44  and an associated length therefrom via third path  42  to coupler  44  is set to λ/2 (λ=wavelength) or an odd-number multiple thereof so as to cancel a leakage component of the transmission signal. 
     The leakage component of the transmission signal flows into an input port of receiving section  36  due to, for example, a reverse directional or directive distribution of circulator  33 . Using coupler  44 , a leak signal from circulator  45  is regulated in amplitude and phase by amplifier  51  and phase shifter  52  to be fed via third path  42  as a transmission signal component. 
     The signal component cancels interference caused by the leakage transmission signal via metallic path  43  through circulator  33 . This cancellation secures removal of interference between the transmission and reception signals even when these signals are similar in frequency to each other. Therefore, the radio frequency bandwidth can be efficiently used by minimizing the difference between the frequencies. 
     The technology of the laid-open application is effective in that the leakage transmission wave from circulator  45  is canceled through the matching operation with the path length, the amplitude, and the phase shift to resultantly suppress interference between transmission and reception. However, the laid-open application describes no measure to remove a transmission spurious response component having a frequency similar to the transmission frequency. 
     In the transceiver using the TDD system, when the transmission signal is transmitted, a high powered transmission signal is inputted into the receiving amplifier even if it is not supplied power yet, causing to generate a distortion due to non-linear characteristic of input stage of the receiving amplifier and generates not only expected receiving signal frequency but unexpected harmful frequencies so called as the spurious response. 
     Particularly, in a transceiver using two transmission frequencies at the same time, components of the two transmitting signals having different frequencies enter into a receiver thereof via unexpected routes. This causes intermodulation distortion and hence transmission spurious response. 
     SUMMARY OF THE INVENTION 
     An object of the present invention to provide a transmission spurious response preventing method in which a leakage signal due to a transmission signal sent from a transmitter with a high power is prevented by canceling the leakage signal with a signal having opposed phase and the same amplitude to the leakage transmission signal. 
     Another object of the invention is to provide a transceiver which operates in a time division duplex (TDD) system and has a receiving amplifier including an input port connected to an antenna system selected for a signal transmission. 
     The transceiver is also provided means for supplying a signal divided from the transmission signal and having the same amplitude and opposite phase to the amplitude and phase of the leakage transmission signal into the input port of the receiving amplifier which is connected to an antenna selected for the transmission, for canceling the leakage transmission signal. 
     The radio transceiver further includes a first circulator for separating a transmission and reception path, a transmitting amplifier, and a second circulator disposed in an outlet side of the transmitting amplifier. The transmission signal is supplied via the first circulator to the antenna, and the division of the transmission signal for use of canceling is fed via the second circulator to the receiving amplifier. 
     In accordance with the present invention, a radio transceiver operating in a time division duplex (TDD) transmission including: an antenna used commonly for transmitting and receiving signals; a first circulator of rotary insulating type connected to the antenna; a receiving section for receiving a signal via a first strip line and a receiving amplifier from said first circulator; a transmitting amplifier for high-frequency amplifying a transmission signal; a second circulator connected to an output port of said transmitting amplifier; a third strip line and a fourth strip line for establishing connection between a normal directivity outlet from said second circulator to said first circulator; and a sixth strip line and a seventh strip line for establishing connection between a reverse directivity outlet from said second circulator and an input port of said receiving amplifier of the receiving section. 
     In the transceiver, a transmission signal reached to the input port of the receiving amplifier via a first path including normal directivity outlet of the second circulator, said third strip line, said fourth strip line, reversed directivity outlet of said first circulator, said first strip line, and the input port of said receiving section, and a transmission signal reached to the input port of the receiving amplifier via a second path including the reverse directivity outlet of said second circulator, said sixth strip line, said seventh strip line, are equal in amplitude and reverse in phase to each other. 
     There is provided in accordance with the present invention a transmission spurious response preventing method used for a radio transceiver operating in a time division duplex (TDD) transmission, wherein the method includes the step of inputting a high-frequency signal delivered from a transmitting amplifier to an input port of a receiving amplifier of a receiving section connected to an antenna system selected for a signal transmission to cancel a leakage transmission signal by setting the amplitude of the delivered transmission signal the same to the leakage transmission signal and setting the phase of the delivered transmission signal to reversed relativity with the leakage transmission signal. 
     Conceptually, the transmission spurious response preventing method of the present invention is implemented in a radio transceiver operating in a TDD including a unit which supplies an input port of a receiving amplifier connected to an antenna system selected for transmission with a transmission signal having a phase inverted with respect to a phase of a leak transmission signal from the antenna so as to suppress the spurious emission component taking place in the receiving amplifier. 
     More specifically, as can be seen from FIG. 2, a signal from transmitting amplifier  11  is fed via a second circulator  13  and a third strip line  14  to a first switch  15 . The signal is then delivered via a fourth strip line  16  and a first circulator  2  to a first antenna  1  to be sent therefrom. Or, the signal is fed via a fifth strip line  17  and a third circulator  7  to a second antenna  2  to be transmitted therefrom. In this situation, most power of the transmission signal inputted to first or third circulator  2  or  7  is supplied to the associated antenna. However, the power is partly delivered via a first strip line  3  to a first receiving amplifier  4  or via a second strip line  8  to a second receiving amplifier  9 . Also in second circulator  13 , the transmission signal inputted thereto is partly fed via a sixth strip line  18  to a second switch  19 . Second switch  19 , a third switch  21 , and a fourth switch  23  operate under control of a signal from a controller  24  to couple sixth strip line  18  with a system associated with an antenna system to transmit signals. The signal inputted to second switch  19  is fed via a seventh strip line  20  to an input port of first receiving amplifier  4  or via an eighth strip line  22  to an input port of second receiving amplifier  9 . 
     Lengths of the strip lines are determined such that a phase difference of 180° appears between the transmission signal delivered via third, fourth, and first strip lines  14 ,  16 , and  3  to first receiving amplifier  4  and that supplied via sixth and seventh strip lines  18 ,  20  thereto as well as between the transmission signal fed via third, fifth, and second strip lines  14 ,  17 , and  8  to second receiving amplifier  9  and that transferred via sixth and eighth strip lines  18  and  23  thereto. Resultantly, two transmission signals having a phase difference of 180° therebetween are inputted to an input port of each of the receiving amplifiers connected respectively to the antenna systems selected for the transmission. This resultantly attenuates power supplied to amplifiers  4  and  9  and hence prevents transmission spurious response due to signal distortion caused by amplifiers  4  and  9 . 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a schematic block diagram showing constitution of a general radio transceiver of the prior art: 
     FIG. 2 is a block diagram showing a first embodiment of a radio transceiver to which a spurious response preventing method in accordance with the present invention is applied; 
     FIG. 3 is a diagram showing an embodiment including two transceivers in accordance with the present invention; 
     FIG. 4 is a graph for explaining signal distortion due to intermodulation in a first receiving amplifier of the transceiver before the spurious preventing method in accordance with the present invention is applied; 
     FIG. 5 is a graph for explaining signal distortion due to intermodulation in a first receiving amplifier of the transceiver after the spurious preventing method in accordance with the present invention is applied; and 
     FIG. 6 is a block diagram showing structure of a second embodiment of the radio transceiver in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the accompanying drawings, description will be given of embodiments in accordance with the present invention. 
     FIG. 2 shows in a block diagram a first embodiment of a radio transceiver to which a transmission spurious response preventing method in accordance with the present invention is applied. 
     FIG. 2 includes a first antenna  1  and a second antenna  6  which are used to transmit and to receive signals. The first embodiment includes two similar system blocks and hence description will be given primarily of one system thereof. Namely, the remaining system will be indicated in parentheses in the following description. First antenna  1  (second antenna  6 ) is connected to a first circulator  2  (a third circulator  7 . First circulator  2  (third circulator  7  has, owing to its directionality or directivity, a function to transmit a received signal via a first strip line  3  (a second strip line  8 ) to a first amplifier  4  (a second amplifier  9 ) and a function to deliver a transmission signal received via a first switch  15  and a fourth strip line  16  (a fifth strip line  17 ) to first antenna  1  (second antenna  6 ). 
     First amplifier  4  (second amplifier  9 ) coupled with first strip line  3  (second strip line  8 ) produces an output signal, which is then fed via a first receiving section  5  (a second receiving section  10 ) to a control section  24  to be demodulated therein. First amplifier  4  (second amplifier  9 ) and first receiving section  5  (second receiving section  10 ) are ordinarily powered only in a signal receiving operation, namely, these units are not powered in a signal transmitting operation. 
     A transmitting section  11  receives a signal from controller  24  and produces a transmission signal in response to the signal and then supplies the transmission signal to transmitting amplifier  12 . Amplifier  12  produces an output signal to be fed to a second circulator  13 . Circulator  13  is desired to be similar to first and third circulators  2  and  7 . The transmission signal inputted to second circulator  13  is fed, depending on directionality thereof, via third strip line  14  to a first switch  15 , which selects a signal route or path determined by controller  24 . The transmission signal is accordingly transmitted from antenna  1  or  6  as described above. Similarly, quite a little part of power of the transmission signal fed to second circulator  13  is supplied to second switch  19  via sixth strip line  18 , regardless of directionality of second circulator  13 . 
     Second switch  19  is connected, under supervision of controller  24 , to a circuit system associated with an antenna to transmit signals. The transmission signal received by second switch  19  is fed via seventh strip line  20  (eighth strip line  22 ) to third switch  21  (fourth switch  23 ). In a transmission phase, third switch  21  (fourth switch  23 ) is in a closed state such that the transmission signal is delivered to first receiving amplifier  4  (second receiving amplifier  9 ). In a receiving phase, third and fourth switches  21  and  23  are not powered and hence are in released state. 
     In the configuration, a total length obtained by adding lengths respectively of third strip line  14 , fourth strip line  16  (fifth strip line  17 ), and first strip line  3  (second strip line  8 ) to each other and a total length attained by adding lengths respectively of sixth strip line  18  to seventh strip line  20  (eighth strip line  22 ) are selected such that when a signal passes through these paths, signals outputted therefrom have a phase difference of 180°. 
     Operation of the first embodiment will be described hereunder. Table 1 shows states of switches in signal transmission and reception phases of the first embodiment. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Switch control method of first embodiment of the 
               
               
                 present invention 
               
             
          
           
               
                   
                 Reference 
                 Transmission 
                 Transmission 
                   
               
               
                 Switch 
                 numeral 
                 from 1st 
                 from second 
               
               
                 name 
                 in Fig. 1 
                 antenna 
                 antenna 
                 Reception 
               
               
                   
               
               
                 1st 
                 15 
                 1st antenna 
                 2nd antenna 
                 Not specified 
               
               
                 switch 
                   
                 system 
                 system 
               
               
                   
                   
                 selected 
                 selected 
               
               
                 2nd 
                 19 
                 1st antenna 
                 2nd antenna 
                 Not specified 
               
               
                 switch 
                   
                 system 
                 system 
               
               
                   
                   
                 selected 
                 selected 
               
               
                 3rd 
                 21 
                 Not specified 
                 Open 
                 Open 
               
               
                 switch 
               
               
                 4th 
                 23 
                 Open 
                 Not 
                 Open 
               
               
                 switch 
                   
                   
                 specified 
               
               
                   
               
             
          
         
       
     
     Description will now be given of a signal transmitting operation from first antenna  1 . 
     A transmission signal from transmitting section  11  is fed via transmitting amplifier  12  to second circulator  13 . Circulator  13  includes, due to directivity thereof, two output ports for one input port, namely, an output port according to circulating directivity and an output port reverse thereto. The former causes almost no discrepancy between the input and output signals (no insertion loss) and corresponds in this system to an output port to third strip line  14 . The latter causes a considerable difference between the input and output signals (i.e., isolation) and corresponds to strip line  18  in this circuit configuration. 
     Assuming, for example, that the output power from the transmitting amplifier is 33 dBm and the insertion loss and isolation of second circulator  13  are respectively −0.3 dB and −20 dB, then power outputted from the directivity port of second circulator  13  to third strip line  14  can be conceptually expressed as 
     
       
         33 dBm−0.3 dB×32.7 dBm 
       
     
     and is fed via third strip line  14  to first switch  15 . 
     First switch  15  is coupled with the first antenna system and hence supplies the transmission signal with a fixed insertion loss to fourth strip line  16 . Assuming that first switch  15  has an insertion loss of −0.5 dB, the power of transmission signal in an input section of fourth strip line  16  is also conceptually expressed as 
     
       
         32.7 dBm−0.5 dB=32.2 dBm. 
       
     
     The transmission signal fed via fourth strip line  16  to first circulator  2  is delivered via a port associated with directivity of first circulator  2  to first antenna  1  and then is transmitted therefrom. 
     The signal is also delivered via the port, reverse to directivity, of first circulator  2  to first strip line  3 . When first and second circulator  2  and  13  are equal in configuration to each other, the power supplied to first antenna  1  is conceptually expressed (hereinafter expressed just as same as an equation) as 
     
       
         32.2 dBm−0.3 dB=31.9 dBm 
       
     
     and power to strip line  3  is expressed as 
     
       
         32.2 dBm−20 dB=12.2 dBm, 
       
     
     which is supplied via first strip line  3  to an input port of first receiving amplifier  4 . 
     On the other hand, the power from the port reverse to directivity of second circulator  13  is fed to second switch  19  as follows. 
     
       
         33 dBm−20 dB=13 dBm. 
       
     
     As can be understood from Table 1, second switch  19  is controlled in any situation to be connected to a circuit system associated with the antenna selected for the transmission. The transmission signal having passed second switch  19  becomes 
     
       
         13 dBm−0.5 dB=12.5 dBm 
       
     
     to be supplied via seventh strip line to third switch  21 . This switch is open in the reception phase and is closed in the transmission phase. Therefore, the signal outputted from third switch  21  has a power of 
     
       
         12.5 dBm−0.5 dB=12 dBm 
       
     
     and is fed to an input port of first receiving amplifier  4 . 
     Table 2 shows paths of transmission signals from an output port of transmitting amplifier  12  to first receiving amplifier  4 . 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Paths of transmission signals from an output port of 
               
               
                 transmitting amplifier 12 to an input port of first 
               
               
                 receiving amplifier 4 in transmission from first 
               
               
                 antenna 1 
               
             
          
           
               
                   
                 Forward 
                   
                   
                   
                   
                   
               
               
                   
                 direction 
                   
                   
                   
                 Reverse 
               
               
                   
                 of 2ndd 
                 3rd 
                 1st 
                   
                 direction of 
                 1st 
               
               
                   
                 circulator 
                 strip 
                 switch 
                 4th strip 
                 1st circulator 
                 strip 
               
               
                 Path 1 
                 13 
                 line 14 
                 15 
                 line 16 
                 2 
                 line 3 
               
               
                   
               
               
                 Variation 
                 λcl 
                 λ3 
                 λs 
                 λ4 
                 λc2 
                 λ1 
               
               
                 in phase 
               
               
                 Path 2 
                 Reverse 
                 6th 
                 2nd 
                 7th strip 
                 3rd switch 
               
               
                   
                 direction 
                 strip 
                 switch 
                 line 20 
                 21 
               
               
                   
                 of 2nd 
                 line 18 
                 19 
               
               
                   
                 circulator 
               
               
                   
                 13 
               
               
                 Variation 
                 λc2 
                 λ6 
                 λs 
                 λ7 
                 λs 
               
               
                 in phase 
               
               
                   
               
             
          
         
       
     
     Description will now given of variation in phase of the transmission signal through two paths. Table 2 shows a quantity of phase variation λ taking place in each associated section. A phase difference λ diff 1  which appears when the transmission signal passes two paths is accordingly calculated as                     λ                   diff      1       =                  (       λ                 c1     +   λ3   +     λ                 s     +   λ4   +     λ                 c2     +     λ                 1       )     -     (       λ                 c2     +     λ                 6     +     λ                 s     +     λ                 7     +     λ                 s       )                   =                  (       λ                 1     +     λ                 3     +     λ                 4       )     -     (       λ                 6     +   λ7     )     +       (       λ                 c2     +   λs     )     .                                      
     The third term, (λc 2 +λs), is a fixed value to be uniquely determined, so is assumed to be zero for simplifying explanation. 
     Then, the phase difference is reduced to 
     
       
         λdiff 1 =(λl+λ 3 +λ 4 )−(λ 16 +λ 7 ). 
       
     
     When the lengths of the strip lines are selected to set the value of λdiff 1  to 180°+360°×n (n=0, ±1, ±2, ±3, . . . ), two signals having a phase difference of 180° are supplied to the input port of receiving amplifier  4 . 
     Specifically, for example, when the transmission signal has a frequency of 2.0 GHz and the substrate has a specific inductive capacity ∈ s  of 4.8, the wavelength has a period λ f  as follows.                     λ   f     =     Speed                 of                 light        /        frequency        /                   =     30   ×     10   9          /        2   ×     10   9          /        4.8                 ≈     =     3.12                     (   cm   )     .                                        
     Consequently, the 180° phase difference can be obtained between two signals by changing the length of the two strip lines about 1.56 cm each other. 
     These signals respectively have power of 12.5 dBm and 12.2 dBm and hence appropriately cancel each other due to the phase difference. In receiving amplifier  4  not powered, the resultant power is considerably less than the power which possibly causes the signal distortion. Specifically, when a high-frequency signal with high power is applied to an input stage of amplifier  4  or  9  in an inoperative state, there may appear, in addition to the fundamental transmission carrier frequency, other frequencies such as second and third harmonic components in association with cross modulation because of, for example, nonlinearity of semiconductors including p-n junctions in the input stage. Namely, the fundamental transmission carrier frequency is modulated by the transmission signal to thereby have a predetermined bandwidth, generating to a large number of spurious response components due to cross modulation. 
     However, when a low-power high-frequency signal is supplied to the input stage, such spurious response components are almost suppressed. But when high-frequency signals having two or more high-voltage waves are applied to the input stage of receiving amplifier  4  or  9 , spurious response components are apt to appear due to intermodulation of the waves. 
     Table 3 shows signal paths of transmission signals from an output port of transmitting amplifier  12  to second receiving amplifier  9 . 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Signal paths of transmission signals from an 
               
               
                 output port of transmitting amplifier 13 to an input 
               
               
                 port of second receiving amplifier 9 in transmission 
               
               
                 from second antenna 6 
               
             
          
           
               
                   
                 Forward 
                   
                   
                   
                 Reverse 
                   
               
               
                   
                 direction 
                   
                   
                   
                 direction 
               
               
                   
                 of 2nd 
                   
                 1st 
                   
                 of 3rd 
                 2nd 
               
               
                   
                 circulator 
                 3rd strip 
                 switch 
                 5th strip 
                 circulator 
                 strip 
               
               
                 Path 1 
                 13 
                 line 14 
                 15 
                 line 17 
                 7 
                 line 8 
               
               
                   
               
               
                 Variation 
                 λcl 
                 λ3 
                 λs 
                 λs 
                 λc2 
                 λ2 
               
               
                 in phase 
               
               
                 Path 2 
                 Reverse 
                 6th strip 
                 2nd 
                 8th strip 
                 4th switch 
               
               
                   
                 direction 
                 line 18 
                 switch 
                 line 22 
                 23 
               
               
                   
                 of 2nd 
                   
                 19 
               
               
                   
                 circulator 
               
               
                   
                 13 
               
               
                 Variation 
                 λc2 
                 λ6 
                 λs 
                 λ8 
                 λs 
               
               
                 in phase 
               
               
                   
               
             
          
         
       
     
     In the signal transmission from second antenna  6 , a phase difference λdiff 1  occurs when the transmission signal passes two paths as shown in Table 3. The value of phase difference is attained as 
     
       
         λ diff   2 =(λ 2 +λ 3 +λ 5 )−(λ 6 +λ 8 ). 
       
     
     When the lengths of the strip lines are selected to set the value of λdiff 2  to 180°+360°×n (n=0, ±1, ±2, ±3, . . . ), the power of transmission signals in receiving amplifier  4  can be similarly set to a value considerably lower than the power which causes the signal distortion. 
     FIG. 3 shows an apparatus including two units of transceiver shown in FIG.  2 . When an apparatus includes two or more transceivers as in this embodiment, a transmission signal of the second transceiver is supplied via antennas to the first transceiver  1 . In an input section of the receiving amplifier, there exist, for example, signals respectively having frequencies f 1  and f 2  as shown in FIG.  3 . When such signals of two frequencies are fed to receiving amplifiers  4  or  9 , a distortion components of 2×f 1 −f 2  and 2×f 2 −f 1  take place due to intermodulation and are disadvantageously transmitted as spurious response components from the antenna. 
     However, the transmission spurious response preventing method in accordance with the present invention attenuates frequency component f 1  at the first transceiver as shown in FIG.  5  and suppresses the signal distortion due to intermodulation. 
     The advantage can be similarly attained even when a transmitting device for high-frequency signals, e.g., a coaxial cable or a micro-strip line is employed in place of the strip line in the embodiment above. In such a case, the transmission spurious response can be suppressed if there appears a phase shift of 180° between the pertinent transmission signals by appropriately setting the path lengths of coaxial cables and/or micro-strip lines. 
     In the embodiments above, the amplitude and phase components are adjusted by setting the path lengths of strip lines by way of example. However, a spurious preventing circuit may be arranged to suppress occurrence of spurious response with higher precision. Namely, an amplifier having a variable amplification factor and a phase shifter capable of shifting a signal phase are disposed, for example, in the section of sixth strip line  18 . 
     Although the embodiment includes two receiving systems, the technological idea of the present invention is applicable even when the apparatus includes one transmitting system and one receiving system. Even in a transceiver including one transmission system and a plurality of receiving systems, when a leakage transmission signal sent from an antenna system to each receiving amplifier and an input signal obtained from the transmission signal via a reverse directivity output port of a circulator at an output side of a transmitting amplifier are equal in an amplitude level to each other and have a phase difference of 180° (i.e., reverse in phase to each other), it is possible to desirably suppress spurious response in the signal transmission. 
     FIG. 6 shows a configuration including additional constituent components to cope with a situation in which the phase adjustment cannot be achieved only by the strip lines. 
     The second embodiment shown in FIG. 6, differs from the first embodiment shown in FIG. 2, includes a memory  27  to store data from controller  24 , a digital-to-analog (D/A) converter  26  to convert digital data of memory  27  into analog data, and a phase shifter  25 . 
     The other components are the same as those shown in FIG.  2 . 
     In operation of the second embodiment shown in FIG. 6, phase shifter  25  changes a phase of received signals according to the converted analog voltage outputted from D/A converter  26 . D/A converter  26  produces an analog voltage in accordance with a digital signal from memory  27 . 
     Memory  27  contains data for phase correction values necessary to produce at an input port of first receiving amplifier  4  a phase difference of 180° between two transmission signals via respective paths (i.e., reverse in phase to each other) for the case of signal transmission using first antenna  1  and data for phase correction values necessary to produce at an input port of second receiving amplifier  9  a phase difference of 180° between two transmission signals via respective paths (i.e., reverse in phase to each other) for the case of signal transmission using second antenna  6 . 
     In response to a signal from controller  24 , memory  27  outputs phase correction values corresponding to antenna  1  or  6  selected. 
     As described above, in an input port of a receiving amplifier of an antenna system selected to transmit signals in accordance with the present invention, for a leakage transmission signal from a point near the antenna, it is possible to simultaneously apply a transmission signal equal in power and reverse in phase to the leakage transmission signal. 
     These signals cancel each other and the transmission signal inputted to the receiving amplifier becomes very small in the signal transmission. This therefore suppresses an event of a signal distortion which cannot be conventionally prevented only by interrupting power to the receiving amplifier. 
     While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by those embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.