Patent Publication Number: US-2022230004-A1

Title: Daisy chain antenna

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority of AU Application No. 2019902158, filed on 20 Jun. 2019 which is incorporated by reference herein, in its entirety, and made a part of this specification. 
     TECHNICAL FIELD 
     The present disclosure broadly relates to radio frequency antennas and, more particularly, a non-limiting embodiment relates to an antenna configuration for a radio frequency identification system. 
     BACKGROUND 
     Radio frequency identification (RFID) systems typically include one or more antennas that can communicate with RFID transponders (or “tags”), as well as an RFID reader (or “interrogator”) in communication with the one or more antennas. The antennas send radio frequency (RF) signals to the RFID tags and any response received from an RFID tag by the antenna(s) is relayed to the reader for further processing. 
     In RFID systems where multiple antennas are used, for example for inventory tracking in a large area or volume such as in warehouse shelving and the like, operation of the antennas is typically controlled by one or more readers that are in communication with the antennas.  FIG. 1A  of the drawing illustrates a prior art system where a large shelf  100  has four overlapping antennas  104 , each connected via a cable  103  to a port  102  of an RFID reader  101 . The reader  101  is in communication with each of the antennas  104  that transmit RF signals so as to identify RFID tags that may be present on the shelf  100 . 
     A disadvantage of the prior art system illustrated in  FIG. 1A , is that a system with several antennas will result in a lot of cabling being used as each separate antenna coil  104  requires its own length of cable  103  to connect to the reader  101 . This can be quite cumbersome and can take up a lot of space. 
     Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application. 
     SUMMARY 
     In one aspect there is provided an RFID system comprising: an RFID reader configured to interrogate RFID antennas; an antenna array comprising two or more RFID antennas connectable to the RFID reader via a series of cable links, each RFID antenna associated with a respective cable link, each cable link having a cable length; a length compensation unit associated with each RFID antenna, the length compensation unit configured to adjust a total cable length between the RFID reader and respective RFID antenna to be an effective cable length. 
     Each RFID antenna has an antenna impedance, and each antenna&#39;s respective cable link has a cable impedance, and the antenna impedance may be different to the cable impedance so that the RFID antenna and its respective cable link are impedance mismatched. The length compensation unit associated with an RFID antenna may be configured to adjust for the total cable length between the RFID reader and the respective RFID antenna so that reflection resulting from said impedance mismatch has a predefined phase. 
     The antenna impedance of an RFID antenna may be transformed along the effective cable length to a final impedance having a resistance and substantially no reactance. 
     The antenna impedance may equal an antenna resistance without reactance, and the antenna impedance may be transformed along the effective cable length to have intermediate impedance values including reactance. The final impedance may be substantially equal to the antenna resistance without reactance. 
     The system may further comprise a controller configured to activate one RFID antenna at a time by communicating a first control signal to the antenna array. The system may further comprise a bypass switch associated with each RFID antenna, the bypass switch being responsive to the first control signal so as to either bypass the respective RFID antenna or connect the respective RFID antenna to the RFID reader. The total cable length may be a variable length depending on which one of the two or more RFID antennas is an active antenna, and the total cable length comprises a sum of cable lengths for each cable link connecting the RFID reader and the active antenna. The length compensation unit associated with the active antenna may adjust the total cable length between the RFID reader and the active antenna to be the effective cable length. 
     The length compensation unit may comprise a configuration of reactive electronic components that emulate a lengthening or shortening of the total cable length. The effective cable length may be the sum of the total cable length and a compensation length provided by the length compensation unit of an active antenna. The effective cable length may be substantially equal to a defined length. 
     Each length compensation unit may be configured to have a different compensation length, each unit&#39;s compensation length being a function of a number of cable links between said compensation unit and the RFID reader. 
     The compensation length of each length compensation unit may be adjustable. 
     The two or more RFID antennas of the antenna array may be connected in a daisy chain configuration via the series of cable links. 
     The controller may further be configured to communicate a second control signal to at least one length compensation unit for setting an adjustable compensation length of the at least one unit. 
     Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the disclosure are now described by way of example with reference to the accompanying drawings in which: 
         FIG. 1A  is a schematic representation of a prior art RFID system; 
         FIG. 1B  is a schematic representation of another prior art RFID system; 
         FIG. 2A  is a schematic representation of an RFID system with serially connected antennas; 
         FIG. 2B  is a schematic representation of a bypass switch used in the RFID system of  FIG. 2A ; 
         FIG. 2C  is another schematic representation of the bypass switch used in the RFID system of  FIG. 2A ; 
         FIG. 3A  is a schematic representation of a circuit with a mismatched antenna and cable configuration; 
         FIG. 3B  is a Smith Chart illustrating the impedance of the circuit of  FIG. 3A ; 
         FIG. 4A  is a schematic representation of the impedance in a circuit with a reduced length cable; 
         FIG. 4B  is a schematic representation of the impedance in a circuit with an increased length cable; 
         FIG. 5A  is a schematic representation of a circuit with length compensation for an increased length cable; 
         FIG. 5B  is the Smith Chart illustrating the impedances along the circuit of  FIG. 5A ; 
         FIG. 6A  is a schematic representation of a circuit with length compensation for a reduced length cable; 
         FIG. 6B  is the Smith Chart illustrating the impedances along the circuit of  FIG. 6A ; 
         FIG. 7  is a schematic representation of an embodiment of an embodiment of a length compensation unit; 
         FIG. 8  is a schematic representation of an embodiment of a length compensation unit; 
         FIG. 9  is a schematic representation of another embodiment of a length compensation unit; 
         FIG. 10  is a schematic representation of yet another embodiment of a length compensation unit; 
         FIG. 11  is a schematic representation of an embodiment of a length compensation unit having a local unit controller; 
         FIG. 12  is a schematic representation of another embodiment of a length compensation unit having a local unit controller; 
         FIG. 13  is a schematic representation of an embodiment of an RFID subsystem; 
         FIG. 14  is another schematic representation of the RFID subsystem of  FIG. 13 ; 
         FIG. 15  is a schematic representation of an embodiment of a calibration circuit; 
       and 
         FIG. 16  is a schematic representation of another embodiment of a calibration circuit. 
     
    
    
     In the drawings, like reference numerals designate similar parts. 
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1B  illustrates another prior art antenna configuration  120  that attempts to reduce the amount of cabling being used by using a serial configuration. The example illustrated in  FIG. 1B  is of a shelf  122  with three antennas  124  that are connected to a reader  126 . A length of cable  128  and a bypass switch  130  connects each consecutive antenna  124  and connects to the reader  126 . Typically, this type of configuration keeps all the antenna units powered by tapping off power at each shelf via the respective bypass switch  130 . An individual antenna  124  is activated via a control mechanism that addresses the particular antenna  124  via a unique address. This approach can be inefficient not only in terms of power usage, but also due to the complexity of implementing an addressing system. 
     Typically, conventional RFID systems (such as those illustrated in  FIGS. 1A and 1B ) are configured to operate as impedance matched systems. The input impedance of the antennas  104 ,  124  would be matched to the cable  103 ,  128  which would be matched to the output impedance of the reader  101 ,  126 . Matching is done to simplify design and to maximise power transfer in RF systems. However, in RFID systems, impedance matched systems suffer from narrow bandwidth operation and consequently low data rates. For RFID systems that use antenna coils the interrogation signal is an oscillating magnetic field. The interrogation field is reactive and maximum power transfer is not a useful measure of performance since the reactive interrogation field is lossless. Any circuit losses are due to resistive losses and do not constitute a useful part of the interrogation signal. Maximising the power transfer is equivalent to maximising the losses which is not useful for a reactive system. Because of this, the solution proposed herein is an RFID system that is not impedance matched. 
     Impedance Mismatched Operation 
     In an impedance mismatched RFID system the output impedance of the RFID reader is not matched to the impedance of the connecting cable, and the impedance of the connecting cable is not matched to the impedance of the RFID antenna. The benefits of this type of mismatching include wide bandwidth and high data rate operation. 
     Typically the connecting cable will be a coaxial cable with an impedance of Zo=50 ohms, however other types of cables may also be used. The antenna is typically a series tuned coil with a low impedance of a few ohms, for example 2 ohms to 5 ohms. When connected to the cable, the antenna  206  and cable are impedance mismatched. Likewise the reader output impedance will be low, typically 10 ohms, which is also mismatched to the cable impedance. The antenna impedance is transformed along the cable, and if the cable is a specific fixed and correct length, then the transformed impedance will be a predictable value. A drawback of a typical mismatched system is the requirement for the cable to have a specific fixed correct length. One option is to include additional lengths of cable per cable link to ensure that the specific length is provided, but this would result in bulky cabling. Accordingly it would be useful to find a way to provide reliable mismatched operation where the cable length is not the required defined length. 
     System Overview 
       FIG. 2A  of the drawings illustrates an RFID system  200  that has multiple antennas, for example as implemented in a cabinet  210  having several shelves  212 . The RFID system  200  has an RFID reader  202  and a controller  222 . In some embodiments the RFID reader  202  powers the controller  222 , and in some embodiments the controller  222  forms part of the reader  202 . 
     In some embodiments, the RFID system  200  has an RFID reader  202  configured to interrogate RFID antennas, and the system  200  has an antenna array  204  that has two or more RFID antennas  206  connectable to the RFID reader  202  via a series of cable links  208 , each RFID antenna  206  associated with a respective cable link  208 . Each cable link  208  has a certain cable length. The system  200  also has a length compensation unit  215  associated with each RFID antenna  206 , and the length compensation unit  215  is configured to adjust a total cable length between the RFID reader  202  and its respective RFID antenna  206  to be an effective cable length. The system  200  has a bypass switch  214  associated with each RFID antenna  206 , each bypass switch  214  operable to either bypass the respective RFID antenna or connect the respective RFID antenna to the RFID reader  202 . In some embodiments the bypass switch  214  and the length compensation unit  215  form a combined unit with shared functionality, for example having a shared local controller. 
     In this way, the antennas  206  are connected in series, in a daisy chain arrangement, each RFID antenna  206  connected to the RFID reader  202  via a respective cable link  208  and via a respective length compensation unit  215 . In the system  200  the cable length to each antenna  206  becomes increasingly longer and does not have a single fixed length as required for typical impedance mismatched operation. This variable total cable length is accommodated by including the length compensation units  215 . By effectively adding or subtracting cable length, a length compensation unit  215  is able to ensure that the effective cable length is substantially equal to the defined cable length required for impedance mismatched operation. 
     Each RFID antenna  206  has an antenna impedance ZA, and each antenna&#39;s respective cable link  208  has a cable impedance Zo. The antenna impedance ZA is different to the cable impedance Zo so that the RFID antenna and its respective cable link are impedance mismatched. The length compensation unit  215  associated with an RFID antenna  206  is configured to adjust for the total cable length between the RFID reader and the respective RFID antenna so that reflection resulting from the impedance mismatch between the RFID antenna and the cable link is controlled to have a predefined phase. The phase of the reflection affects the impact that the reflection has on the operation of the reader and antenna, and the correct value of the phase when it arrives at the reader end will ensure that the impedance transformation is correct. 
     In this way, the system of  FIG. 2A  is configured for mismatched operation and the length compensation unit  215  makes the necessary electrical length adjustments required to provide reliable mismatched operation where the cable length is not of a fixed correct length. 
     Length Compensation 
     In order for the reader  202  to see the appropriate defined cable length, the length compensation unit  215  is configured so that the necessary electrical length adjustments are made to provide reliable mismatched operation where the cable length is not of a fixed correct length. The length compensation device is chosen to appear electrically as an extra length of cable where the cable is too short, or as a ‘negative’ length of cable where the cable is too long. 
       FIG. 3A  of the drawings illustrates an embodiment where the impedance of an antenna  501  Z1 is given by R1. The impedance of the antenna  501  is not matched to the impedance Zo of the cable  502 . The impedance Z1 is transformed down the length of the cable to a value of Z2, which is given by R2+jX at the far end of the cable. The total impedance as seen by a reader is adjusted by compensation element  503  with impedance −jX so that the total impedance Z3, as seen by the reader, is equal to R2. 
       FIG. 3B  shows a Smith Chart  300  that illustrates the changes in impedance from the antenna (point A), through the cable (point B) and including the compensation element  503  (point C). 
     A: Z1=R1 
     B: Z2=R2+jX 
     C: Z3=R2 
     The embodiment illustrated in  FIGS. 3A and 3B  is dependent on a defined cable length l, which is typically a relatively short length of 1.5 meters which is less than ⅛ of a wavelength. Where the cable length deviates, the impedance seen by the reader is not optimal for reader performance. For example, as illustrated in  FIG. 4A , a shorter cable  402  would result in a load impedance Z3 of R2−jX1, and as illustrated in  FIG. 4B , a longer cable  404  would result in a load impedance Z3 of R2+jX2. 
     As different antennas  206  in the antenna array  204  are selected, the characteristics of the connecting cabling change because a varying cable length is included from the reader  202  to the antenna selected to be the active antenna. For example, the total cable length will be different when a first antenna  206 . 1  is connected to the reader  202  via the first bypass switch  214 . 1 , when compared to the total cable length when a third antenna  206 . 3  is connected to the reader  202  via the third bypass switch  214 . 3 . 
     The RFID reader  202  is configured to operate with a predefined load impedance ZL. Therefore, as different antennas  206  in the antenna array  204  are selected, the total impedance seen by the reader  202  will change due to the changing total cable length. In order to adjust the total impedance seen by the reader  202  to be substantially equal or close to the predefined load impedance ZL, the length compensation unit  215  included in the connection between the reader  202  and the active antenna  220  provides a length compensation that causes the total impedance as seen by the reader  202  to be substantially equal to the predefined load impedance ZL. In this way, the same effective cable length is seen from the reader  202  irrespective of which antenna is activated. 
     Ideally, the RFID antenna has an antenna impedance that has a resistance but no reactance. For mismatched operation, it is also preferable that this antenna resistance is transformed along the connecting cables to a transformed impedance that has a resistance and not a reactance, in other words with zero phase. However, as shown in  FIG. 4B , when the connecting cable is too long or too short, the transformed impedance includes a reactance. 
     In some embodiments, the compensation units compensate for an increase in total cable length using a passive element configuration  510  as illustrated in  FIG. 5A . The impedance of the circuit shown in  FIG. 5A  is illustrated in the Smith Chart  512  of  FIG. 5B  where the impedance changes are: 
     At A, including −jΔX compensation impedance: Z1=R1−jΔX;
 
At B where the additional Δl has been compensated for: Z2=R2;
 
At C following the defined cable length l: Z3=R3+jX; and
 
At D including a default pre-compensation −jX: Z4=R3.
 
     In some embodiments, the compensation units compensate for a decrease in total cable length using a passive element configuration  610  as illustrated in  FIG. 6A . The impedance of the circuit shown in  FIG. 6A  is illustrated in the Smith Chart  612  of  FIG. 6B  where the impedance changes are: 
     At A: Z1=R1; 
     At B, including jΔX compensation impedance: Z2=R1+jΔX;
 
At C following a reduced cable length: Z3=R4+jX; and
 
At D including a default pre-compensation −jX: Z4=R4.
 
     It will be understood that various different configurations of capacitive and/or inductive elements may be used for a set of compensation units  215  associated with an array  204  of antennas  206  as shown in  FIG. 2A . In one embodiment  700 , illustrated in  FIG. 7  of the drawings, five antennas  206  are connected in series via cable links  208 . The total cable length l from the reader to a selected active antenna is equal to the sum of an initial cable length l1 and the cable lengths up to the active antenna  220 , n×Δl n . 
     In this embodiment, the default compensation length is considered to be: 
         l=l 1+Δ l 1+Δ l 2,
 
     with the shorter connecting cables of the first two antennas requiring additional compensation lengths of Δl1+Δl2 and Δl2 respectively, and the longer connecting cables of the last two antennas requiring a reduction in length with compensation lengths of −Δl3 and −Δl3−Δl4 respectively. 
     To do this, a configuration of compensation units is used that provides a default compensation  702  of −jX associated with a middle antenna  704 , and capacitive  706  and inductive  708  compensation units positioned further and closer than the middle antenna  704 , respectively, to the reader with reactance as follows: 
       − jX≡     =     +Δ     1 +Δ   2  
 
       + jX   1 ≡Δ   2  
 
       + jX   2 ≡Δ   1 +Δ   2  
 
       − jX   2 ≡−Δ   3  
 
       − jX   4 ≡−Δ   3 −Δ   4  
 
     In this way, the length compensation units include a configuration of reactive electronic components that emulate a lengthening or shortening of the total cable length, as required. 
     Bypass Switching 
     The controller  222  activates one RFID antenna  206  at a time by communicating a bypass control signal to the antenna array  204 . The controller  222  controls switching between the antennas  206  by controlling the bypass switches  214 . The bypass switches are responsive to the bypass control signal so as to either bypass the respective RFID antenna or connect the respective RFID antenna to the RFID reader. 
     In some embodiments a 3-bit control line may be provided, for example, enabling switching between the antennas at each daisy chain position. In other embodiments a combined RF, DC and control signal is transmitted from the controller  222  along the daisy chain cable towards the antenna array  204 , and this bypass control signal causes the selected bypass switch  214  to switch in the selected length compensation unit  215  together with a selected antenna  220 . In some embodiments the control signal is carried on the RF signal and/or the DC power signal according to the methods described in the International Patent Application published as WO2009/149506 A1, the contents of which are incorporated herein by reference. In some embodiments the control signal is imposed on a DC power signal that also provides power to a local unit controller  1102  that controls the bypass switch  214 . This is described in more detail elsewhere herein with reference to  FIG. 12 . In some embodiments, the control signal from the controller  222  may direct the operation of a local antenna controller  1310  as described elsewhere herein with reference to  FIG. 13 . It will be understood that communication between the RFID reader  202  and the antennas  206  are bidirectional, for example via one or more of the controllers  222 ,  1102 ,  1310 . 
     The connection from the RFID reader  202  to an antenna  206 , or alternatively past an antenna  206  and to the following cable  208  is made via a bypass switch  214 .  FIGS. 2B and 2C  of the drawings show, as an example, the first bypass switch  214 . 1  of the system  200  illustrated in  FIG. 2A . Switch  214 . 1  can either connect cable  208 . 1  to the first antenna  206 . 1  via length compensation unit  215 . 1 , or bypass antenna  206 . 1  and connect cable  208 . 1  to cable  208 . 2  that leads to the second antenna  206 . 2 . In the illustrated example, the second antenna  206 . 2  has been selected to be the active antenna  220 . Each bypass switch  214  operates to connect a selected active antenna  220  to the cable  208  from the reader  202  while at the same time disconnecting the next piece of cable  208  following the switch  214 . So in this example, switch  214 . 2  disconnects cable  208 . 3 . Alternatively, when the bypass switch  214  is set to bypass an antenna  206 , then the switch  214  disconnects its respective antenna and connects the preceding and subsequence cable links  208  so that the reader  202  can connect to the next antenna along the daisy chain. In this example, switch  214 . 1  will switch to bypass antenna  206 . 1  and to connect cables  208 . 1  and  208 . 2 . 
     In some embodiments the bypass switches may be implemented using pin diodes. In other embodiments the bypass switches may be implemented using relays. 
     Adjustable Compensation 
     In some embodiments the controller  222  controls the compensation units  215  where the compensation length of each length compensation unit is adjustable. 
       FIG. 8 - FIG. 12  illustrate embodiments of adjustable length compensation units. 
     The length compensation can be made adjustable by making the impedance value selectable using switches, as shown in  FIG. 8  which illustrates a first embodiment of an adjustable length compensation configuration  800 . The length compensation illustrated has binary weight inductive impedance each selectable using shunt switches  802 . The inductive impedance can be adjusted from zero to +j7X in steps of jX. 
       FIG. 9  shows a second embodiment of an adjustable length compensation configuration  900  where the adjustment can be made in a positive and a negative direction with inductance  902  and capacitance  904  respectively. The binary weight inductive impedances are each selectable using shunt switches  802  and the total series impedance can be adjusted from −j3X to +j4X in steps of jX. 
       FIG. 10  shows a third embodiment of an adjustable length compensation configuration  1000  where the adjustment can be made in a positive and a negative direction, with an alternative combination of inductance and capacitance. The binary weight capacitive impedances are each selectable using a shunt switch  802  and the total series impedance can be adjusted from −j4X to +j3X in steps of jX. 
     In some embodiments the shunt switches  802  may be implemented using pin diodes. In other embodiments the shunt switches  802  may be implemented using mechanical switches that are manually set. In other embodiments the shunt switches  802  may be implemented using relays. It is possible to use mechanical latched relays in applications where switching is infrequent so that relays can be latched and stay set. This is the case in applications where adjustments are made at power up, and thereafter only infrequently as particular antennas in the array are selected, and where the configurations are preferably saved on power down. 
       FIG. 11  shows another embodiment of a length compensation unit  1100  where a combination RF, DC and control signal is transmitted from the reader  202  along the daisy chain cable towards the antenna array  204 , and the control signal causes the selected bypass switch  214  to switch in the selected length compensation unit  215 . The control signals are imposed on a DC power signal. The DC power signal provides power to a local unit controller  1102  that operates the bypass switch  214 . The control signal  1104  also directs the operation of the unit controller  1102  to control the shunt switches  802  of the length compensation unit  1100 . In this way the unit controller  1102  can adjust the length compensation (under the direction of the reader&#39;s controller  222 ) to ensure that the correct length compensation is selected for the active antenna. 
       FIG. 12  illustrates yet another embodiment of a length compensation unit  1200  using the same combination RF, DC and control signal  1104  as described with reference to  FIG. 11 . As illustrated here, when the reader  202  deselects an antenna  206  the bypass switch  214  disconnects the length compensation unit  1200 , however the unit controller  1102  remains active and is able to hold the switch  214  in the ‘bypass’ state so that the reader  202  can communicate with the next antenna&#39;s unit controller. The process is repeated for the following antenna and so on. In this way the reader  202  is able to communicate with every antenna  206  in the daisy chain in a sequential and repeatable manner. If power is disconnected than all bypass switches  214  revert to the open state which automatically places the reader  202  in connection with the first antenna  206 . 1  in the daisy chain due to the default setup of the first switch  214 . 1 . The process of sequential control and operation can then be repeated. 
     While  FIGS. 8-12  show 3 shunt switches per configuration, requiring 3 bits of control, a greater or lesser number of shunt switches may be used depending upon the length compensation resolution required. 
     Local Controller 
     In  FIGS. 13 and 14  of the drawings, an embodiment of an antenna subsystem  1300  is illustrated. The subsystem may, for example, be associated with one of the shelves  212  in the cabinet  210  illustrated in  FIG. 2A . 
     The subsystem  1300  has a plurality of antennas  1302  in communication with a tuner  1304  via a first multiplexer  1306 , and in communication with the length compensation unit  215  and bypass switch  214  via a second multiplexer  1308 . The subsystem  1300  has a local antenna controller  1310  that controls the operation of the tuner  1304 , the multiplexers  1306 ,  1308 , the antennas  1302  via the multiplexers  1306 ,  1308 , the length compensation unit  215 , and the bypass switch  214  (for example, via the length compensation unit  215 ). 
     As in the embodiments illustrated in  FIGS. 11 and 12 , the subsystem may receive a combination RF, DC and control signal  1104  and when the reader  202  deselects an antenna  206  the bypass switch  214  disconnects the length compensation unit  215  while the antenna controller  1310  remains active and is able to hold the switch  214  in the ‘bypass’ state so that the reader  202  can communicate with the next antenna&#39;s controller. 
     The tuner  1304  adjusts the resonant frequency of the antenna coils by adjusting the tuning capacitance such that the antenna coil is tuned to be resonant. When tuned, the antenna input impedance has a real value that is low, since the preferred antenna is a series resonant coil with low resistance and no reactance. 
     In some embodiments, the subsystem  1300  may be operatively connected to one or more additional devices, displays, sensors, indicators, etc., for example to provide a user interface for the shelf  212  and/or cabinet  210 . Indicator lights may show where a tag is located, a display may show picking information relevant to the shelf  212 , etc. 
     Calibration 
     Prior to operation, for example at installation of the RFID system  200  or when the system is powered up, an initial effective cable length must be measured to ascertain any adjustment to the cable length compensation required. 
       FIG. 15  shows a first embodiment of a calibration circuit  1500 , and  FIG. 16  shows a second embodiment of a calibration circuit  1600 . Placing a shorting calibration switch  1502  at the antenna end  1504  of a length compensator  215  allows the reader  202  to set or calibrate the compensation length. The calibration switch  1502  is closed under the direction of the reader&#39;s controller  222  via a local controller, e.g. an antenna controller  1310 . Once closed, the reader  202  can monitor the phase and amplitude of the RF signal it delivers to the active antenna. The switch  1502  behaves as a low impedance load and the phase (of the phase and amplitude) should be consistent with this value. In other words, the RF current should be relatively large, the voltage relatively low, and the phase between the current and voltage should be zero degrees (in phase). If the compensation length is too short or too long, the current will be low, the voltage high, and the phase between the current and voltage will either be positive above zero or negative below zero. The reader&#39;s controller  222  adjusts the shunt switches  802  of the length compensation unit  215  to get the phase angle as close to zero as possible, at which point the length compensation will be at the best achievable setting. 
     Since the length compensation will remain fixed unless antennas or cables are physically moved or altered, the compensation settings will mostly not change once set. Mechanical latched relays can be used in this application so that relays can be latched and stay set. In this case the adjustments can be made at power up, and thereafter only infrequently if required. The configuration is advantageously saved on power down by the latching relays. 
     Unbalanced to balanced operation of the length compensation can be achieved by placing a balun between the compensation unit  215  and the antenna  206 . The cable  208 , bypass switch  214  and compensation unit  215  operate unbalanced whilst the antenna  206  can operate in a balanced state. Balanced operation has been found to be beneficial for reducing interference at, and stray coupling from, the antenna  206 . The circuit of  FIG. 15  provides for unbalanced operation, and the circuit of  FIG. 16  provides for balanced operation. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.