Abstract:
A method of offsetting a mismatch due to user interaction when handling a portable wireless terminal in which antenna matching is changed from inductive matching to capacitive matching in response to a reactance change exceeding a threshold level and vice versa when an opposite change is detected. An antenna interface module ( 44 ) is coupled between a RF output or input stage ( 25  or  33 ) and an antenna ( 48  or  50 ). The antenna interface module includes first and second switches (SW 1/1 , SW 1/2  or SW 2/1 , SW 2/2 ), a first matching circuit including an inductive reactance ( 68  or  96 ) coupled between the power amplifier and the first switch and a second matching circuit including a capacitive reactance ( 68  or  92 ) is coupled between the RF output or input stage and the second switch (SW 1/1  or SW 2/1 ). A reactance threshold detector ( 54  or  56 ) determines if the reactance change traverses a predetermined threshold value and causes the first and second switches to be actuated so that the matching changes from inductive to capacitive or vice versa.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of International Application No. PCT/EP2009/000147, filed Jan. 13, 2009, entitled “Improvements in or Relating to Portable Wireless Devices” which claims the benefit of European Patent Application No. 08100446.7, filed on Jan. 14, 2008, entitled “Improvements in or Relating to Portable Wireless Devices” both of which are incorporated herein by reference in their entireties. 
     COPYRIGHT 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
     FIELD OF THE INVENTION 
     The present invention relates to improvements in or relating to portable wireless devices. The present invention has particular, but not exclusive, application to matching of antenna structures used in mobile phones and other portable wireless devices. 
     BACKGROUND 
     A problem with operating hand portable wireless devices having small planar antennas, such as planar inverted-F antennas (PIFAs), is that when a user holds a device, the antenna&#39;s impedance changes predominantly reactively. As a result the matching of the antenna to radio frequency circuitry is affected adversely by these reactive changes. 
     EP 1 564 896 A1 discloses altering the value of an impedance connected between a power amplifier and an antenna to achieve power control in the output stage of a power amplifier. In operation the actual load impedance at the antenna is measured and the value of the impedance is adjusted so that only a purely resistive load is experienced by the power amplifier. 
     WO 2006/054246 discloses a controlled matching stage connected between the output of a power amplifier and an antenna stage. The controlled matching stage comprises a phase detector for detecting the phase difference between a first signal derived from the power amplifier and a second signal derived from an input to a switching stage coupled to the antenna stage. The difference in phase between the first and second signals is used to adjust the impedance of the switching stage. Typically the switching stage comprises a series LC circuit comprising a fixed inductance and an adjustable capacitance. 
     GB 0 804,103A discloses an automatic tuning system using servo motors driving respectively an adjustable antenna input coupling and a sliding short circuit. The resistance and reactance of a transmission line connecting a transmitter to an antenna are sensed and the results are used in driving servo amplifiers controlling the servo motors. Measures are disclosed enabling the servo motors initially to be driven rapidly and then to move more slowly. 
     GB1 362 154 A discloses an automatic tuner for transforming the impedance of an antenna to a load resistance required for the power amplifier output stage of a transmitter. The automatic tuner uses a method of control of the tuning circuit element requiring phase and impedance inputs indicative of the reactive condition of the selected antenna. The phase input is used to control the switching of capacitors in an antenna impedance matching network and the impedance input is used to control the switching of impedances in the antenna impedance matching network. 
     WO 2006/038167 A1 discloses coupling a RF power amplifier to an antenna by way of a circuit for detecting the impedance of the antenna. The circuit detects a signal travelling from the RF power amplifier to the antenna and measures the peak current of the signal. More particularly the circuit comprises first means for sensing the peak value of the output voltage of the RF power amplifier, second means for sensing the peak of the output current of the RF power amplifier, and third means for deriving the phase between the output voltage and output current. 
     WO 2004/010595 A1 discloses a device for dynamic impedance matching between a power amplifier and an antenna. The device includes a circulator which routes a signal received from the power amplifier at a first port via a second port to the antenna. Additionally the circulator diverts a signal reflected at the antenna and received at the second port through a third port. A matching network is provided. In operation a directional coupler diverts a proportion of the signal travelling from the power amplifier to the antenna, from which the magnitude and phase of the signal may be derived, to a signal detector. The circulator routes the entire signal reflected at the antenna into the signal detector. The signal detector passes the magnitude and phase of both the signal travelling to the antenna and the signal reflected at the antenna to a controller, which evaluates the information received from the signal detector in order to determine the present impedance value of the antenna and to correct the controllable matching network containing active and passive components in accordance with the determined impedance value of the antenna. 
     BRIEF SUMMARY 
     An object of the present invention is to be able to provide an acceptable match of an RF stage to an antenna structure both in free space and when user interaction occurs. 
     According to an aspect of the present invention there is provided a portable wireless terminal comprising an antenna interface module having a first port for connection to a RF output or input stage and a second port, a threshold detector including a reactance threshold detector coupled between the second port and an antenna terminal for connection to an antenna, the antenna interface module including first and second switches, a first matching circuit including an inductive reactance coupled between the first port and a first pole of the first switch, a second matching circuit including a capacitive reactance coupled between the first port and a first pole of the second switch, second poles of the first and second switches being coupled to the second port, the threshold detector providing an output for changing the state of the first switch from a first condition to a second condition and the state of the second switch from a second condition to a first condition, or vice versa, in response to the reactance threshold detector traversing a predetermined threshold value. 
     According to a second aspect of the present invention there is provided a method of operating a portable wireless terminal comprising an antenna interface module having a first port for connection to a RF output or input stage and a second port, a threshold detector including a reactance threshold detector coupled between the second port and an antenna terminal for connection to an antenna, the antenna interface module including first and second switches, a first matching circuit including an inductive reactance coupled between the first port and a first pole of the first switch, a second matching circuit including a capacitive reactance coupled between the first port and a first pole of the second switch, and second poles of the first and second switches being coupled to the second port, the method comprising monitoring the reactance of the signal at the second port and, in response to the reactance threshold detector traversing a predetermined threshold value, changing the state of the first switch from a first condition to a second condition and the state of the second switch from a second condition to a first condition, or vice versa. 
     Aspects of the present invention are based on the realisation that in free space a transmitter RF stage can be matched to an antenna using a series inductance but user interaction, that is, the wireless terminal being held by a user, causes an inductive shift that in many cases is counterproductive. In such cases matching can be achieved by capacitive matching. As the user interaction varies from person to person it is desirable that the change from inductive matching to capacitive matching and vice versa is effected dynamically. In implementing a portable wireless terminal made in accordance with aspects of the present invention the reactance of an antenna is monitored and if a reactance change is detected that traverses a threshold value in either direction, the reactance threshold detector causes the matching to switch from inductive to capacitive or vice versa. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: 
         FIG. 1  is a block schematic diagram of a portable wireless terminal made in accordance with an aspect of the present invention, 
         FIG. 2  is a partially block schematic and partially schematic circuit diagram of an RF stage and an antenna interface module as used in the portable wireless terminal shown in  FIG. 1 , 
         FIG. 3  is a schematic circuit diagram of a reactance threshold detector, 
         FIG. 4  is a Smith chart showing contours of constant K 1  with antenna impedance, Z A , 
         FIG. 5  is a Smith chart showing contours of constant K 2  with antenna impedance, Z A , for x S =1, 
         FIG. 6  is a Smith chart showing contours of constant K 2  with antenna impedance, Z A , for x S =0.5, 
         FIG. 7  shows in diagram a) a Smith chart and in diagram b) a graph of Voltage Standing Wave Ratio as a function of frequency in MHz in the GSM850 TX band (824-849 MHz band), for Antenna Interface Module (AIM) impedances in free space and user interaction without adaptive switching, 
         FIG. 8  shows in diagram a) a Smith chart and in diagram b) a graph of Voltage Standing Wave Ratio as a function of frequency in MHz in the GSM850 TX band, for Antenna Interface Module (AIM) impedances in free space and with user interaction, 
         FIG. 9  shows in diagram a) a Smith chart and in diagram b) a graph of Voltage Standing Wave Ratio as a function of frequency in MHz in the GSM850 TX band, for AIM impedances in free space and with user interaction with adaptive switching, 
         FIG. 10  shows in diagram a) a Smith chart and in diagram b) a graph of Voltage Standing Wave Ratio as a function of frequency in MHz in the GSM 1800 TX band (1710-1785 MHz band), for AIM impedances in free space and with user interaction with adaptive switching, 
         FIG. 11  shows in diagram a) a Smith chart and in diagram b) a graph of Voltage Standing Wave Ratio as a function of frequency in MHz in the GSM 1800 TX band, for AIM impedances in free space and with user interaction, and 
         FIG. 12  shows in diagram a) a Smith chart and in diagram b) a graph of Voltage Standing Wave Ratio as a function of frequency in MHz in the GSM 1800 TX band, for AIM impedances in free space and with user interaction with adaptive switching. 
     
    
    
     In the drawings the same reference numerals have been used to indicate corresponding features. 
     DETAILED DESCRIPTION 
     For convenience of description aspects of the present invention will be described with reference to a portable wireless terminal capable of operating in accordance with various radio communication standards operable in a relatively low frequency band between 824 and 960 MHz and in a relatively high frequency band between 1710 and 2170 MHz. 
     Referring to  FIG. 1  the portable wireless terminal  10  comprises a radio transmitting and receiving stage  12  formed by an audio frequency (AF) stage  14  and a radio frequency (RF) stage  24 . The AF stage  14  has an input coupled to a microphone  16  and an output coupled to a loudspeaker  18 . The RF stage  24  has terminals coupled to respective low and high frequency RF transceiver stages  20 ,  22  forming the RF stage  24 . The transceiver stages  20 ,  22  have input/output ports coupled respectively to ports  26  to  32  and  34  to  42  of an antenna interface module (AIM)  44  to be described in greater detail with reference to  FIG. 2 . The AIM  44  has a first low frequency band antenna coupling  46  connected to a low frequency band antenna  48  and a second high frequency band antenna coupling  50  connected to a high frequency band antenna  52 . The antennas  48 ,  52  comprise any suitable antennas such as Planar Inverted-F Antennas (PIFAs). 
     The portable wireless terminal  10  further includes a microcontroller  55  for controlling the operation of the terminal  10  using control software stored in a Read Only Memory (ROM)  57 . The microcontroller  55  is coupled to the radio transmitting and receiving stage  12  to configure that stage to operate in accordance with a desired radio standard. A Random Access Memory (RAM)  59  is coupled to the microcontroller  55  and serves to store data such as data messages. A man/machine interface represented by a keypad  61  is also coupled to the microcontroller  55 . The basic operation of the portable wireless terminal  10  will be understood by persons skilled in the art without requiring additional explanation. 
     Referring to  FIG. 2 , the low and high frequency RF stages  20 ,  22 , respectively comprise a plurality of output/input stages  25  to  31  and  33  to  41 . The stages  25  to  31  respectively represent GSM850 TX 824-849/GSM900 TX 880-915; GSM900 RX 925-960; GSM850 RX 869-894/UTRA V RX 869-894 and UTRA V TX 824-849, and the stages  33  to  41  respectively represent GSM1800 TX 1710-1785/GSM1900 TX 1850-1910; GSM1900 RX 1930 1990/UTRA II RX 1930-1990; UTRA II TX 1850-1910; UTRA I RX 2110-2170 AND UTRA I TX 1920-1980. TX refers to the transmitter frequency band and RX refers to the receiver frequency band. GSM refers to Global System for Mobile Communications, and UTRA is the abbreviation used for UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access and has the following bands: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Band 
                 TX (MHz) 
                 RX (MHz) 
               
               
                   
                   
               
             
             
               
                   
                 I 
                 1920-1980 
                 2110-2170 
               
               
                   
                 II 
                 1850-1910 
                 1930-1990 
               
               
                   
                 III 
                 1710-1785 
                 1805-1880 
               
               
                   
                 IV 
                 1710-1755 
                 2110-2155 
               
               
                   
                 V 
                 824-849 
                 869-894 
               
               
                   
                 VI 
                 830-840 
                 875-885 
               
               
                   
                 VII 
                 2500-2570 
                 2620-2690 
               
               
                   
                 VIII 
                 880-915 
                 925-960 
               
               
                   
                 IX 
                 1749.9-1784.9 
                 1844.9-1879.9 
               
               
                   
                   
               
             
          
         
       
     
     The transmitter stages  25 ,  31 ,  33 ,  37  and  41  can typically comprise power amplifier stages and the receiving stages  27 ,  29 ,  35  and  39  can typically comprise a low noise amplifier and RF filtering stages. The ports  26  to  32  and  34  to  42  of the AIM  44  are coupled respectively to the stages  25  to  31  and  33  to  41 . 
     The AIM  44  comprises first and second banks  54 ,  56  of switches SW 1 / 1  to SW 1 / 4  and SW 2 / 1  to SW 2 / 4 . The switches may comprise any suitable switching means such as pHEMTs (pseudomorphic High Electron Mobility Transistors), MEMS (Micro Electro-Mechanical Systems) devices or PIN diodes. Each of the switches SW 1 / 1  to SW 1 / 4  and SW 2 / 1  to SW 2 / 4  has first and second poles. The second poles of the bank  54  are coupled to a common junction or port  58 , and the second poles of the bank  56  are coupled to a common junction or port  60 . Each of the common junctions  58 ,  60  is coupled respectively to the low frequency band antenna  48  and to the high frequency band antenna  50  by way of a respective series connection of a dc blocking capacitor  62 A,  62 B and an inductive reactance  64 A,  64 B of a threshold detector  66 A,  66 B. In the embodiment shown in  FIG. 2 , the first and second banks  54 ,  56  are controlled by the microcontroller  55  ( FIG. 1 ) to select a particular one of the ports  26  to  42  to be connected to a respective antenna  48  or  52 . Additionally for the ports  26 ,  34  having inductive/capacitive antenna matching, the switches SW 1 / 1  and SW 1 / 2  and the switches SW 2 / 1  and SW 2 / 2  are additionally controlled by dc control signals produced by the threshold detector  66 A,  66 B. 
     The port  26  is coupled to a junction  67 . The first pole of the switch SW 1 / 1  is coupled by way of an antenna matching capacitance  68  to the junction  67 . An antenna matching inductance  72  on the one hand is coupled to the junction  67  and on the other hand is coupled by way of a dc blocking capacitor  70  to the first pole of the switch SW 1 / 2 . The port  28  is coupled by way of an antenna matching capacitance  74  to the first pole of the switch SW 1 / 3 . The ports  30  and  32  are coupled by respective inductances  76 ,  78  to respective bandpass filters  82 ,  84  of a duplexer filter  80 . An output of the duplexer filter  80  is coupled by way of an antenna matching arrangement to the first pole of the switch SW  1 / 4 . The antenna matching arrangement comprises an inductance  86  and a capacitance  88  together with a shunt inductance  90  connected to ground. 
     The port  34  is coupled to a junction  91 . The first pole of the switch SW 2 / 1  is coupled by way of an antenna matching capacitance  92  to the junction  91 . An antenna matching inductance  96  on the one hand is coupled to the junction  91  and on the other hand is coupled by way of a dc blocking capacitor  94  to the first pole of the switch SW 2 / 2 . The ports  36  and  38  are coupled to respective bandpass filters  100 ,  102  of a duplexer filter  98 . An output of the duplexer filter  98  is coupled by way of an antenna matching capacitance  104  to the first pole of the switch SW 2 / 3 . The ports  40  and  42  are coupled to respective bandpass filters  108 ,  110  of a duplexer filter  106 . An output of the duplexer filter  106  is coupled by way of an antenna matching capacitance  112  to the first pole of the switch SW 2 / 4 . 
     With respect to an understanding of the present invention it will be noted from the preceding description that GSM850 TX 824-849 and GSM900 TX 880-915 share a power amplifier port in the stage  25 . Hence, the operation of the switches SW 1 / 1  and SW  1 / 2  can be chosen dynamically, in response to a dc control voltage of the reactance threshold detector  66 A. The reactance threshold detector  66 A is responsive to the change of reactance of the antenna when the wireless terminal is held by a user as opposed to being in free space, and vice versa. The same is true at the GSM1800 TX 1710-1785 and GSM1900 TX 1850-1910 power amplifier port in the stage  33  (where switches SW 2 / 1  and SW  2 / 2  can be dynamically set). The teachings of the present invention are not limited to the GSM transmit channels but can be applied to matching any or all of the other stages  27  to  31  and  35  to  41  to their respective antennas. 
       FIG. 3  illustrates a reactance threshold detector  120  that can be used for determining whether the antenna reactance has exceeded a threshold. This can be achieved using a reactance threshold detector with an inductor or capacitor of a certain value. 
     The reactance threshold detector  120  comprises a reactance X S  which can be an inductor or capacitor. A signal from a RF front end is applied to a terminal  122  and a current i 1  flows to the antenna impedance Z A  which is represented by a series arranged antenna resistance R A  and reactance X A . The voltage v 1  at the antenna side of the reactance X S  is supplied to one input of a first high impedance buffer amplifier  124 . A voltage v 2  at the other side of the reactance X S  is applied to a second input of the amplifier  124  and to one input of a second high impedance buffer amplifier  126 , a second input of which is connected to ground. The outputs of the amplifiers  124 ,  126  are limited in respective limiters  128 ,  130 , the outputs from which are multiplied in a multiplier  132 . A dc control voltage is available on a terminal  136  coupled to the output of the multiplier  132 . A filter consisting of a large value shunt capacitor  134  is also coupled to the output of the multiplier  132 . 
     The operation of the phase detector will now be described. 
     The reactance, X S  is used as a sensing element, about which the two voltages, v 1  and v 2  are monitored. The first amplifier  124  processes the difference voltage dv=v 2 −v 1 , while the second amplifier  126  operates on v 2  as drawn. This amplifier may also be configured to amplify v i . The amplifiers also serve as high impedance buffers. 
     The voltages v 1  and v 2  are functions of the antenna impedance, Z A =R A +jX A  and are given by:
 
 v   1   =i   1   |Z   A |cos(ω t±φ   1 )  1)
 
 v   2   =i   1   |Z   A   +X   S |cos(ω t+φ   2 )  2)
 
     where the phases, φ 1  and φ 2  are related to the impedances by 
     
       
         
           
             
               
                 
                   
                     φ 
                     1 
                   
                   = 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       ( 
                       
                         
                           X 
                           A 
                         
                         
                           R 
                           A 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     φ 
                     2 
                   
                   = 
                   
                     
                       tan 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       ( 
                       
                         
                           
                             X 
                             A 
                           
                           + 
                           
                             X 
                             s 
                           
                         
                         
                           R 
                           A 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   4 
                   ) 
                 
               
             
           
         
       
     
     φ 1  is the phase of the antenna impedance. φ 2  is used for reactance measurement. 
     The difference voltage, dv is given by
 
dV= i   1   |x   S |cos(cos(ω t ±90)  5)
 
     where the sign within the parentheses is positive for an inductor and negative for a capacitor. 
     Amplifying and limiting these voltages using the amplifiers  124 ,  126  and limiters  128 ,  130  removes amplitude information. Multiplying the amplified and limited versions of v 1  and dv yields:
 
 A  cos(ω t+φ   1 )cos(ω t± 90)= B  cos(2ω t+φ   1 ±90)+cos(φ 1 ∓90)  6)
 
     where A and B are constants of proportionality. 
     Filtering this with a large valued shunt capacitor  134 , as shown in  FIG. 3 , leaves only the DC part, which can be written as follows:
 
 V   DC   =∓B  sin(φ 1 )  7)
 
     Here the negative and positive signs apply to capacitive and inductive sensing respectively. 
     Similarly, v 2  and dv can be processed, which yields:
 
 V   DC   =∓B  sin(φ 2 )  8)
 
     The previous sub-section shows that a given V DC  corresponds to a particular phase. From equations (3) and (4), V DC  also corresponds to a range of antenna resistance and reactance values that may be plotted as contours on a Smith Chart. The simplest way to do this is to first express resistances and reactances in terms of real and imaginary components of reflection coefficient: the x and y axes of the chart respectively. 
     As derived in Appendix A included at the end of the description, the normalised antenna resistance, r A  and reactance, x A  are related to the real and imaginary components of reflection coefficient by 
     
       
         
           
             
               
                 
                   
                     r 
                     A 
                   
                   = 
                   
                     
                       1 
                       - 
                       
                         ρ 
                         Ar 
                         2 
                       
                       - 
                       
                         ρ 
                         Ai 
                         2 
                       
                     
                     
                       
                         
                           ( 
                           
                             1 
                             - 
                             
                               ρ 
                               Ar 
                             
                           
                           ) 
                         
                         2 
                       
                       + 
                       
                         ρ 
                         Ai 
                         2 
                       
                     
                   
                 
               
               
                 
                   9 
                   ) 
                 
               
             
             
               
                 
                   
                     x 
                     A 
                   
                   = 
                   
                     
                       2 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ρ 
                         Ai 
                       
                     
                     
                       
                         
                           ( 
                           
                             1 
                             - 
                             
                               ρ 
                               Ar 
                             
                           
                           ) 
                         
                         2 
                       
                       + 
                       
                         ρ 
                         Ai 
                         2 
                       
                     
                   
                 
               
               
                 
                   10 
                   ) 
                 
               
             
           
         
       
     
     where 
     p Ar  real part of antenna reflection coefficient 
     p Ai  imaginary part of antenna reflection coefficient 
     If v 1  and dv are processed by the phase detector, from equation (3) the phase—and, therefore V DC —is constant when x A /r A  is constant. Hence, 
                       x   A       r   A       =         2   ⁢           ⁢     ρ   Ai         1   -     ρ   Ar   2     -     ρ   Ai   2         =     1     K   1                 11   )               
where, from equation (3), K 1  is given by
 
 K   1 =cot(φ 1 )  12)
 
     Simplifying equation (11) gives
 
ρ 2   Ar +ρ 2   Ai +2 K   1 ρ Ai =1=  13)
 
     This can be rearranged to give
 
ρ Ar   2 +(ρ Ai   +K   1 ) 2 =(√{square root over (1 +K   1   2 )}) 2   14)
 
     This is the equation of a circle in the (ρ Ar , ρ Ai ) plane, centered at (0, −K 1 ) and with a radius equal to √{square root over (1+K 1   2 )}. Because K 1 =tan −1 (φ 1 ), this can also be written 
     
       
         
           
             
               
                 
                   
                     
                       ρ 
                       Ar 
                       2 
                     
                     + 
                     
                       
                         [ 
                         
                           
                             ρ 
                             Ai 
                           
                           + 
                           
                             cot 
                             ⁡ 
                             
                               ( 
                               
                                 ϕ 
                                 1 
                               
                               ) 
                             
                           
                         
                         ] 
                       
                       2 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         1 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             
                               ϕ 
                               1 
                             
                             ) 
                           
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   15 
                   ) 
                 
               
             
           
         
       
     
     Equation (14) can be used to draw contours of constant K i  on a Smith Chart that is used to represent all possible antenna impedances. From equations (7) and (12), when K 1 =∞, V DC  is zero and the contour is a line of zero reactance (a horizontal line through the centre of the chart). All other lines begin and end at the points representing short and open circuits, as shown in  FIG. 4 . 
     The contours of the previous sub-section show when the phase of the antenna impedance is constant. However contours of constant phase are sub-optimal in so far as the present invention is concerned. This is because the phase of an antenna runs approximately parallel to the contours of constant phase and only small voltage changes can be detected. The present invention is concerned with using contours of constant reactance and changes in impedance cross the contours of constant reactance substantially orthogonally. 
     Means by which a constant reactance can be measured will now be derived. 
     For constant reactance, v 2  and dv are processed by the reactance threshold detector. From equations (4) and (8) V DC  is constant when (x A +x S )/r A  is constant. Hence, 
                         x   A     +     x   s         r   A       =           2   ⁢           ⁢     ρ   Ai         1   -     ρ   Ar   2     -     ρ   Ai   2         +       x   s     ⁢           (     1   -     ρ   Ar       )     2     +     ρ   Ai   2         1   -     ρ   Ar   2     -     ρ   Ai   2             =     1     K   2                 16   )               
where, from equation (4), K 2  is given by
 
 K   2 =cot(φ 2 )  17)
 
     Simplifying equation (16) gives
 
ρ Ar   2 (1 +K   2   x   S )−2 K   2   x   S ρ Ar +ρ Ai   2 (1 +K   2   x   S )+2 K   2 ρ Ai   +K   2   x   S −1=0  18)
 
     Hence, 
                       ρ   Ar   2     -       2   ⁢     K   2     ⁢     x   s     ⁢     ρ   Ar         1   +       K   2     ⁢     x   s           +     ρ   Ai   2     +       2   ⁢     K   2     ⁢     ρ   Ai         1   +       K   2     ⁢     x   s           +           K   2     ⁢     x   s       -   1       1   +       K   2     ⁢     x   s             =   0           19   )               
Substituting
 
                         (       ρ   Ar     -         K   2     ⁢     x   S         (     1   +       K   2     ⁢     x   S         )         )     2     =       ρ   Ar   2     -       2   ⁢     K   2     ⁢     x   S     ⁢     ρ   Ar         (     1   +       K   2     ⁢     x   2         )       +       (         K   2     ⁢     x   S         1   +       K   2     ⁢     x   S           )     2         ⁢                   20   )             and                             (       ρ   Ai     +       K   2       (     1   +       K   2     ⁢     x   A         )         )     2     =       ρ   Ai   2     +       2   ⁢     K   2     ⁢     ρ   Ai         (     1   +       K   2     ⁢     x   S         )       +       (       K   2       1   +       K   2     ⁢     x   S           )     2               21   )               
in equation (19) gives
 
     
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             ρ 
                             Ar 
                           
                           - 
                           
                             
                               
                                 K 
                                 2 
                               
                               ⁢ 
                               
                                 X 
                                 S 
                               
                             
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   
                                     K 
                                     2 
                                   
                                   ⁢ 
                                   
                                     x 
                                     S 
                                   
                                 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                       2 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             ρ 
                             Ai 
                           
                           + 
                           
                             
                               K 
                               2 
                             
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   
                                     K 
                                     2 
                                   
                                   ⁢ 
                                   
                                     x 
                                     S 
                                   
                                 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             
                               K 
                               2 
                             
                             ⁢ 
                             
                               x 
                               S 
                             
                           
                           
                             1 
                             + 
                             
                               
                                 K 
                                 2 
                               
                               ⁢ 
                               
                                 x 
                                 S 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             K 
                             2 
                           
                           
                             1 
                             + 
                             
                               
                                 K 
                                 2 
                               
                               ⁢ 
                               
                                 x 
                                 S 
                               
                             
                           
                         
                         ) 
                       
                       2 
                     
                     + 
                     
                       
                         1 
                         - 
                         
                           
                             K 
                             2 
                           
                           ⁢ 
                           
                             x 
                             S 
                           
                         
                       
                       
                         1 
                         + 
                         
                           
                             K 
                             2 
                           
                           ⁢ 
                           
                             x 
                             S 
                           
                         
                       
                     
                   
                 
               
               
                 
                   22 
                   ) 
                 
               
             
           
         
       
     
     The terms on the right of this equation can be written 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           ( 
                           
                             
                               K 
                               2 
                             
                             ⁢ 
                             
                               x 
                               S 
                             
                           
                           ) 
                         
                         2 
                       
                       + 
                       
                         K 
                         2 
                         2 
                       
                       + 
                       
                         
                           ( 
                           
                             1 
                             - 
                             
                               
                                 K 
                                 2 
                               
                               ⁢ 
                               
                                 x 
                                 S 
                               
                             
                           
                           ) 
                         
                         ⁢ 
                         
                           ( 
                           
                             1 
                             + 
                             
                               
                                 K 
                                 2 
                               
                               ⁢ 
                               
                                 x 
                                 S 
                               
                             
                           
                           ) 
                         
                       
                     
                     
                       
                         ( 
                         
                           1 
                           + 
                           
                             
                               K 
                               2 
                             
                             ⁢ 
                             
                               x 
                               S 
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             1 
                             + 
                             
                               K 
                               2 
                             
                           
                         
                         
                           1 
                           + 
                           
                             
                               K 
                               2 
                             
                             ⁢ 
                             
                               x 
                               S 
                             
                           
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   23 
                   ) 
                 
               
             
           
         
       
     
     Hence, 
     
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             ρ 
                             Ar 
                           
                           - 
                           
                             
                               
                                 K 
                                 2 
                               
                               ⁢ 
                               
                                 x 
                                 S 
                               
                             
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   
                                     K 
                                     2 
                                   
                                   ⁢ 
                                   
                                     x 
                                     S 
                                   
                                 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                       2 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             ρ 
                             Ai 
                           
                           + 
                           
                             
                               K 
                               2 
                             
                             
                               ( 
                               
                                 1 
                                 + 
                                 
                                   
                                     K 
                                     2 
                                   
                                   ⁢ 
                                   
                                     x 
                                     S 
                                   
                                 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             1 
                             + 
                             
                               K 
                               2 
                             
                           
                         
                         
                           1 
                           + 
                           
                             
                               K 
                               2 
                             
                             ⁢ 
                             
                               x 
                               S 
                             
                           
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   24 
                   ) 
                 
               
             
           
         
       
     
     Once again, this is the equation of a circle in the (ρ Ar , ρ Ai ) plane. The circle is centred at (K 2 x S , 1+K 2 x S ), −K 2 /(1+K 2 x S )) and the radius is given by √{square root over (1+K 2 )}/1+K 2 x S . Again this can be used to draw contours of constant K 2 . 
     From equations (8) and (17) V DC  is zero for K 2 =∞, when the centre coordinates and radius are as follows; 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           lim 
                         
                       
                       
                         
                           
                             K 
                             2 
                           
                         
                       
                     
                     → 
                     
                       ∞ 
                       ⁢ 
                       
                         { 
                         
                           
                             
                               K 
                               2 
                             
                             ⁢ 
                             
                               x 
                               S 
                             
                           
                           
                             ( 
                             
                               1 
                               + 
                               
                                 
                                   K 
                                   2 
                                 
                                 ⁢ 
                                 
                                   x 
                                   S 
                                 
                               
                             
                             ) 
                           
                         
                         } 
                       
                     
                   
                   = 
                   1 
                 
               
               
                 
                   25 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           lim 
                         
                       
                       
                         
                           
                             K 
                             2 
                           
                         
                       
                     
                     → 
                     
                       ∞ 
                       ⁢ 
                       
                         { 
                         
                           
                             K 
                             2 
                           
                           
                             ( 
                             
                               1 
                               + 
                               
                                 
                                   K 
                                   2 
                                 
                                 ⁢ 
                                 
                                   x 
                                   S 
                                 
                               
                             
                             ) 
                           
                         
                         } 
                       
                     
                   
                   = 
                   
                     1 
                     
                       x 
                       S 
                     
                   
                 
               
               
                 
                   26 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           lim 
                         
                       
                       
                         
                           
                             K 
                             2 
                           
                         
                       
                     
                     → 
                     
                       ∞ 
                       ⁢ 
                       
                         { 
                         
                           
                             
                               1 
                               + 
                               
                                 K 
                                 2 
                               
                             
                           
                           
                             1 
                             + 
                             
                               
                                 K 
                                 2 
                               
                               ⁢ 
                               
                                 x 
                                 S 
                               
                             
                           
                         
                         } 
                       
                     
                   
                   = 
                   
                     1 
                     
                       x 
                       S 
                     
                   
                 
               
               
                 
                   27 
                   ) 
                 
               
             
           
         
       
     
     Hence, when Voc is zero, eauation (24) simplifies to 
     
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             ρ 
                             Ar 
                           
                           - 
                           1 
                         
                         ) 
                       
                       2 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             ρ 
                             Ai 
                           
                           + 
                           
                             1 
                             
                               x 
                               S 
                             
                           
                         
                         ) 
                       
                       2 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         1 
                         
                           x 
                           S 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   28 
                   ) 
                 
               
             
           
         
       
     
     This is directly equivalent to a line of constant normalised reactance, −x S  on a Smith Chart (see Appendix A). As such, this can be used to set a reactance detection threshold: a reactance below −x S  will give a negative V DC , whereas a reactance above −x S  will give a positive V DC . 
     Contours of constant K 2  are plotted in  FIG. 5  for x S =1. Clearly the contour for K 2 =∞ coincides with the x A =−1 constant reactance circle. 
     Similarly,  FIG. 6  shows contours of constant K 2  for x S =0.5. 
       FIGS. 4 and 5  indicate that a reactance threshold can be chosen by choosing an appropriate inductor, X S . If necessary the AIMS may include several different valued inductors X S  together with selection means for selecting an inductor to suit a particular application. 
     The following description describes how a reactance threshold detection circuit can be used in an antenna interface module (AIM) having an architecture shown in  FIG. 2 . In the Smith Charts shown in  FIGS. 7(   a ) to  12 ( a ), the line referenced  118  relates to the free space condition and the other lines relate to different user interactions. In  FIGS. 7(   b ) to  12 ( b ), the bold black line  120  relates to the free space condition and the other lines relate to conditions noted when different volunteers held the portable wireless device. 
       FIG. 7  shows the low-band antenna impedance, line  120 , in free space and in an experiment in which a portable wireless terminal made in accordance with the present invention was held by 63 volunteers for the GSM850 TX band. 
       FIG. 8  shows the results in which in free space the GSM850 TX band is matched with the series inductor  72  ( FIG. 2 ). However, user interaction causes an inductive shift, such that for many of the users—51 out of 63 —the inductance is counterproductive. Those user interactive results furthest clockwise from the free space condition line  118  indicate that inductance matching is ineffective and in fact many of the results are worse than without any matching. In such circumstances, it is better to switch in the capacitor  68  ( FIG. 2 ) that matches the GSM850 TX band (in free space). 
     The reactance at which this threshold is reached is used to determine the sensing inductance, x S  and is given by 
     
       
         
           
             
               
                 
                   
                     x 
                     S 
                   
                   = 
                   
                     
                       
                         x 
                         L 
                       
                       + 
                       
                         x 
                         C 
                       
                     
                     2 
                   
                 
               
               
                 
                   29 
                   ) 
                 
               
             
           
         
       
     
     where 
     x L , reactance of matching inductance (positive) 
     x C , reactance of matching capacitance (negative) 
     For the case above, the matching inductor  72  is 6.2nH (36.2Ω at 837 MHz) and the capacitor  68  is 8 pF (−23.8Ω at 837 MHz). This gives a sensing inductor  68  value of 0.83nH (4.4 S 2  at 837 MHz). If necessary the value of the capacitance  62 A may be varied to take into account changes in the series inductance of the inductor  64 A. 
     Simulating with this value, and adjusting the dc blocking capacitor  70  to tune out the inductance, gives the results shown in  FIG. 9 . Clearly the VSWR is significantly improved—38 of the 63 results where the phone is held use capacitive rather than inductive matching. 
       FIG. 10  shows the high-band antenna impedance in free space and when held by 63 volunteers for the GSM1800 TX band. 
     With inductive matching based on the free space impedance, the impedance at the input of the AIM becomes as shown in  FIG. 11 . 
     The matching inductor  96  is 3.6nH (39.5Ω at 1747 MHz), while the capacitor  92  used to match the GSM1900 TX band is 11 pF (−8.3Ω at 1747 MHz). This gives a sensing inductor  64 B value of 1.42nH (15.6Ω at 1747 MHz). 
     Simulating with this value, and adjusting the dc blocking capacitor  94  to tune out the inductance, gives the results shown in  FIG. 12 . As for the low frequency band, the VSWR is significantly improved.  26  of the 63 results where the phone is held use capacitive rather than inductive matching. 
     In both the low and high frequency bands a slightly larger valued sensing inductor would be more optimum, since the theory above does not take account of circuit losses, parasitics etc. 
     The reactance measurements can be applied to a receive channel but it is preferred for reactance measurements to be made on the transmit channels because power is supplied by a power amplifier. 
     APPENDIX A 
     The Smith Chart 
     A.1-Impedance circles 
     A reflection coefficient, p can be directly plotted onto a Smith Chart, since the coordinate system used is Cartesian for the real and imaginary components of ρ. To plot lines of constant resistance and reactance, however, a relationship with the components of ρ is determined. 
     The normalised impedance is related to reflection coefficient as follows: 
     
       
         
           
             
               
                 
                   z 
                   = 
                   
                     
                       r 
                       + 
                       
                         j 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         x 
                       
                     
                     = 
                     
                       
                         
                           1 
                           + 
                           ρ 
                         
                         
                           1 
                           - 
                           ρ 
                         
                       
                       = 
                       
                         
                           1 
                           + 
                           
                             ρ 
                             r 
                           
                           + 
                           
                             jρ 
                             i 
                           
                         
                         
                           1 
                           - 
                           
                             ρ 
                             r 
                           
                           - 
                           
                             j 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               ρ 
                               i 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   
                     A 
                     ⁢ 
                     .1 
                   
                   ) 
                 
               
             
           
         
       
     
     where 
     ρ r  real part of reflection coefficient 
     ρ i  imaginary part of reflection coefficient
         r normalised resistance determined by the ratio of antenna resistance R A  divided by the input impedance, for example 50Ω.   x normalised reactance determined by the ratio of the antenna reactance R A  divided by the input impedance, for example 50Ω.       

     This can be simplified to give 
     
       
         
           
             
               
                 
                   z 
                   = 
                   
                     
                       r 
                       + 
                       
                         j 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         x 
                       
                     
                     = 
                     
                       
                         1 
                         - 
                         
                           ρ 
                           r 
                           2 
                         
                         + 
                         
                           ρ 
                           i 
                           2 
                         
                         + 
                         
                           j2ρ 
                           i 
                           2 
                         
                       
                       
                         
                           
                             ( 
                             
                               1 
                               - 
                               
                                 ρ 
                                 r 
                               
                             
                             ) 
                           
                           2 
                         
                         + 
                         
                           ρ 
                           i 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   
                     A 
                     ⁢ 
                     .2 
                   
                   ) 
                 
               
             
           
         
       
     
     The real part gives the resistance 
                   r   =       1   -     ρ   r   2     +     ρ   i   2             (     1   -     ρ   r       )     2     +     ρ   i   2                   A   ⁢   .3     )               
and the imaginary part gives the reactance
 
                   x   =       2   ⁢     ρ   i             (     1   -     ρ   r       )     2     +     ρ   i   2                   A   ⁢   .4     )               
(A.3) can be simplified to give
 
(1 +r )ρ r   2   +r (1−2 r )+(1 +r )ρ i   2 =1  A.5)
 
     This can then be further simplified to give 
     
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             ρ 
                             
                               r 
                               - 
                             
                           
                           ⁢ 
                           
                             r 
                             
                               1 
                               + 
                               r 
                             
                           
                         
                         ) 
                       
                       2 
                     
                     + 
                     
                       ρ 
                       i 
                       2 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         1 
                         
                           1 
                           + 
                           r 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   
                     A 
                     ⁢ 
                     .6 
                   
                   ) 
                 
               
             
           
         
       
     
     This is the equation of a circle in the (ρ r , ρ i ) plane, centred at (r/(1+r),0) and with a radius equal to 1/(1+r). 
     (A.4) can be simplified to give 
     
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             ρ 
                             r 
                           
                           - 
                           1 
                         
                         ) 
                       
                       2 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             ρ 
                             i 
                           
                           - 
                           
                             1 
                             x 
                           
                         
                         ) 
                       
                       2 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         1 
                         x 
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   
                     A 
                     ⁢ 
                     .7 
                   
                   ) 
                 
               
             
           
         
       
     
     This is also the equation of a circle in the (ρ r , ρ i ) plane, but centred at (1,1/x) and with a radius equal to 1/x. 
     A.2-Admittance circles 
     It can be shown in a similar fashion that lines of constant normalised  15  conductance, g are given by 
     
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             ρ 
                             r 
                           
                           + 
                           
                             g 
                             
                               1 
                               + 
                               g 
                             
                           
                         
                         ) 
                       
                       2 
                     
                     + 
                     
                       ρ 
                       i 
                       2 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         1 
                         
                           1 
                           + 
                           g 
                         
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   
                     A 
                     ⁢ 
                     .8 
                   
                   ) 
                 
               
             
           
         
       
     
     and the lines of constant normalised admittance, b are given by 
     
       
         
           
             
               
                 
                   
                     
                       
                         ( 
                         
                           
                             ρ 
                             r 
                           
                           + 
                           1 
                         
                         ) 
                       
                       2 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             ρ 
                             i 
                           
                           + 
                           
                             1 
                             b 
                           
                         
                         ) 
                       
                       2 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         1 
                         b 
                       
                       ) 
                     
                     2 
                   
                 
               
               
                 
                   
                     A 
                     ⁢ 
                     .9 
                   
                   ) 
                 
               
             
           
         
       
     
     In the present specification and claims the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. Further, the word “comprising” does not exclude the presence of other elements or steps than those listed. 
     The use of any reference signs placed between parentheses in the claims shall not be construed as limiting the scope of claims. 
     From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the design, manufacture and use of portable wireless terminals and component parts therefor and which may be used instead of or in addition to features already described herein.