By James G. Quaderer

                    Conventional RFID tags are typically manufactured individually which allows for ample physical separation between tags during the test phase; inter-tag interference and cross-talk are not significant issues.  On the contrary, because BiStatix^® tag antennas are printed with conductive ink as opposed to being assembled, one of the last steps in BiStatix^® tag manufacture almost always involves the tags being suspended on a web suitable for printing processes.  The efficiencies of the web-based process are many and these efficiencies become even greater when the tags are placed in very close proximity to one another on the web.  The close proximities of BiStatix^® tags result in possible interference and cross-talk problems.

                    One method of dealing with the interference and cross-talk problems of BiStatix^® tags on a web is to build small, custom, preferentially coupled antennas near each of the tag electrodes.  While doing this, it is necessary to be wary of cable shielding, grounding, cross-talk, and impedance issues.  One of the methods that has been used successfully to reduce cabling problems is to use a remote transformer which is placed physically close to the test electrodes but could be far from the active drive electronics.  The remote transformer steps-up the voltage from an excitation circuit very near the tag under test so that a low voltage, low impedance driver may be used at the excitation source to mitigate most of the cabling problems mentioned above.  However, design of a remote transformer system requires specific knowledge of step-up transformer voltage ratios, impedance ratios, shielding, and grounding not to mention electronic components and field wiring.  In addition, the use of a remote transformer requires a reader which allows access to the low voltage portion of it’s excitation circuit.

                    There is another method of coupling to only one BiStatix^® tag which uses more readily–available materials and readers.  Figure 1 illustrates the front view of a shielding method that uses passive materials for allowing communication to only one tag.

                    Figure 2 is a cross-sectional drawing along section 7 of Figure 1.

                    The reader 2 emits an electric field 3 toward the tags 1.  The tag under test is the one shown at the center of Figure 1, with the cross section 7 bisecting it.  It is desired that only the tag under test communicate with the reader 2 and that all other tags shown in Figure 1 be inhibited from communicating with the reader 2.

                    The exposed tag under test upper antenna electrode 101 is capacitively coupled to the reader 2 via the electric field 3.  The return path for the signal from the tag under test is from the tag’s lower antenna electrode 102 via capa...