Electric Field Cancellation
Apparatus For BiStatix®
Tags On A Web
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...