RFID Paperclip Tags
Pavel V. Nikitin, K. V. S. Rao, and Sander Lam
Intermec Technologies Corporation
6001 36th Ave W, Everett, WA, 98203, USA
{pavel.nikitin , kvs.rao , sander.lam} @ intermec.com
Abstract— In this paper, we present RFID tags which double as
paperclips. These tags use standard metal paperclip bodies as
antennas. This way, the paper holding function of each paperclip
is augmented by its RFID functionality. Paperclip tags can be
designed so that as they slide on or off the stack of papers, their
antenna wires either touch or separate, changing tag sensitivity
and causing tags to be activated or deactivated. We provide a
survey of the prior work, describe the concept of a paperclip tag,
demonstrate several prototypes accompanied by experimental
results and electromagnetic simulations, and discuss possible
applications and future work.
a
b
c
I.
INTRODUCTION
All passive UHF RFID tags available on the market today
have specially designed antennas, specific for particular
applications. One of many RFID applications is document
tracking and management.
To track documents using RFID, it was earlier
proposed to attach standard flexible RFID inlays to the front
page of a document [1]. Another existing approach is to attach
rigid tags to file holders in a file cabinet [2]. Finally, both
Hitachi and Hewlett Packard have introduced chips which
operate at 2.4 GHz ISM band and can be mounted directly on
objects: Hitachi mu-chip [3] and HP Memory Spot chip [4].
Those chips use on-chip antennas and thus can only be read
with special readers from very close (sub-mm) range (an
optional external antenna can be used for Hitachi mu-chip). In
all approaches described above, labels, rigid tags, or chips must
somehow be attached to the paper document.
d
Fig. 1. Prior work on document tracking using RFID: a – inlays
attached to the paper (from [1]), b –rigid tags attached to file holder
(from [2]), c - Hitachi mu-chip (from [3]), d – HP Memory Spot chip
and its reading device (from [4]).
At the same time, there exists an object which has
remained an attribute of almost any office during for over one
hundred years. This object is a simple paperclip. Most well
known “Gem” paperclip design was introduced in late 1800’s
and is still in use today. The photograph of this paperclip is
shown in Figure 2. There exist many other paperclip designs,
some which are shown in Figure 3. Many more paperclip
designs can be found, for example in [5].
Fig. 2. Most well known “Gem” paperclip.
Fig. 3. Some other paperclip designs.
While modern paperclips can be made of various materials
such as plastic or recycled wood, most paperclips are still
made of galvanized steel wire. We propose to recognize that
the metal wire bodies of such paperclips can be reused as
RFID tag antennas.
In this way, an ordinary paperclip can also acquire RFID
functionality. Such RFID paperclip can still be used for its
primary purpose (mechanically holding together sheets of
papers) while at the same time acting as an RFID tag storing
data. This idea was briefly described by us in [6] and is
developed in details in this paper.
II.
PAPERCLIP TAG CONCEPT
We propose to reuse the metal wire body of a generic
paperclip as a tag antenna, as illustrated in Figure 4. In order
to achieve proper chip impedance matching and antenna gain
characteristics, the shape of the paperclip may need to be
accordingly modified.
Such a tag can operate in three different modes described
below.
• Mode 1 (short range): as is, when the paperclip tag is
not attached to the paper (for example, when all
paperclip tags are still in the box). In this mode,
paperclip wires touch each other, creating a short
circuit and strongly affecting tag antenna impedance
and gain. As a result, the tag range is minimal, and
the tag can be considered to be deactivated.
• Mode 2 (long range): the paperclip is attached to the
paper (or stack of papers). In this position, the
paperclip wires are separated by one or more sheets
of paper. The paperclip tag can be designed to have a
longer read range in this position, and the tag can be
considered to be activated.
• Mode 3 (adjusted range): the paperclip is attached to
the paper which, in its turn, has one or more
conducting strips attached to it. Such strips can act as
external antenna elements, allowing one to adjust the
paperclip tag range and/or its resonant frequency.
The three modes of operation are illustrated in Figure 5.
Mode 1: short range
Short circuit
Mode 2: long range
Paper
Open circuit
Mode 3: adjusted range
Paper
Conducting strip acts as an
external antenna element
Fig. 4. RFID paperclip tag concept.
Fig. 5. Three modes of operation of paperclip tag: short range mode
(top), long range mode (middle), adjusted range mode (bottom).
III.
PROTOTYPES AND MEASUREMENTS
A. Prototypes
To verify the concept, several prototypes of RFID
paperclip tags were built and tested. The prototypes are
summarized in the Table 1. Most of the prototype tags used
unfolded steel wires from the standard paperclips of various
sizes. Those wires are tin plated and can be easily soldered.
All these prototypes were initially designed and optimized
experimentally, which was easier and faster than using
modeling and simulation for these paperclips which involved
3-D structures of curved wires soldered to ICs or to PCBs with
ICs. Later, we used simulations for analysis of the tag
performance and for understanding of the physical effects
which we observed in experiments.
The read range of prototype paperclip tags (as is and
on a single piece of A4 sized office printer paper) was tested
in anechoic chamber using our wideband RFID tag testing
equipment (1 W reader and a linearly polarized 6 dBi antenna,
both covering 800-100 MHz band) described in [7]. The
results are shown in Figure 6. The testing distance (between
the reader antenna and the tag) was 2 feet, and hence the read
range below that value was not detected.
Tag
IC
Photographs
1
Monza 2
nevertheless it proved that the concept works: the paperclip
wire body can be used as a tag antenna.
The second prototype (tag 2) was made using the
unfolded wire from the large paperclip (its length was about
right for this tag antenna). The RFID IC was NXP G2iL [9] in
TSSOP package, mounted on a piece of 60 mil FR4 PCB
board to which the wire was soldered. When attached to paper,
it had maximum read range of about 18 feet at 880 MHz
(shown in Figure 6) but the protruding sharp edges of the wire
made it difficult to slide it on and off the paper easily.
The third prototype (tag 3) was made using the same
unfolded steel wire from the standard large paperclip. The
RFID IC was Monza 4 QT [8] in UDFN package, also
mounted on a piece of 60 mil FR4 PCB board to which the
wire was soldered. The design was modeled after the Niagara
paperclip [5]. When attached to the paper, it had maximum
read range of about 19 feet at 865 MHz (shown in Figure 6).
Again, the protruding sharp edges of the wire made it difficult
to slide it on and off the paper easily.
Finally, the fourth prototype (tag 4) was made using
the same wire and the same RFID IC as the previous prototype
(tag 3), but the design was modified to follow the shape of
improved Niagara paperclip with rounded edges [5]. On our
opinion, this design worked the best in terms of compromise
between its mechanical paper holding ability and its electrical
performance. We chose to tune it to the middle of US ISM
band. When attached to the paper, tag 4 had maximum read
range of about 17 feet at 915 MHz (shown in Figure 6).
24
(TSSOP
package)
Tag 2, as is
Tag 3, as is
Tag 4, as is
22
20
(TSSOP
package)
3
18
G2iL
Monza 4
(UDFN
package)
16
Range (ft)
2
Tag 2, on 1 sheet
Tag 3, on 1 sheet
Tag 4, on 1 sheet
14
12
10
8
6
4
4
Monza 4
(UDFN
package)
2
0
800
820
840
860
880
900
920
940
960
980
1000
Frequency (MHz)
Fig. 6. Read range (ft) of paperclip tag prototypes (for 4 W EIRP in
free space).
TABLE I.
PAPERCLIP TAG PROTOTYPES
The first prototype (tag 1) was used as proof of
concept verification. The RFID IC used in it was Impinj
Monza 2 [8] in TSSOP package. It was directly soldered to the
tin plated steel wire. Read range of that prototype was only
about 2 feet and the connection to the chip was not sturdy but
All tags described above were linearly polarized. The
gain of these small tags was modest, on the order of -1 dBi.
Tags 2, 3, and 4 employed a T-matching structure which is
well known in RFID tag antenna design community [10-12].
The frequency shift observed in Figure 6 when a
paperclip tag is placed on one sheet of paper is mainly due to
the fact that the paper separates the antenna wires, which are
otherwise crossed and touching each other. The presence of
paper removes the electrical short between those wires, thus
changing the tag antenna impedance and gain. For studying
the effect of the number of paper sheets on the tag
performance and the effect of the adjusted range mode (due to
conducting strip on the paper), we picked prototype tag 4,
shown on detailed photograph with dimensions in Figure 7.
C. Adjusted range mode
To prove that the tag performance can be adjusted in terms
of range and frequency (we are referring to the adjusted range
mode illustrated in Figure 5), we tested the tag 4 on 1 sheet and
20 sheets of paper and on then on the same stacks but with
conducting strip on the first sheet. The strip was made of
copper foil (10 mm wide, 135 mm long) and attached to the
paper as shown in Figure 9.
Fig. 7. Detailed view and dimensions of paperclip tag 4.
B. Effect of number of paper sheets
To see the effect of the number of pieces of paper that the
paperclip tag holds together on its performance, we tested the
tag 4 on stacks of 1, 5, 10, 20, and 30 sheets of standard A4
office printer paper. The results are shown in Figure 8. As one
can see, the strongest effect is due to inserting 1 sheet of paper.
As mentioned above, this separates the antenna wires, which
are otherwise crossed and touching each other. However, those
wires are still in the close proximity to each other and thus are
strongly coupled.
24
Tag 4, as is
Tag 4, on 5 sheets
Tag 4, on 20 sheets
22
20
Tag 4, on 1 sheet
Tag 4, on 10 sheets
Tag 4, on 30 sheets
As one can see from the data in Figure 10, the tag
performance indeed can be adjusted by the conducting strip. In
this particular example, the conducting strip increases the tag
antenna gain and significantly improves tag range (up to 36 ft
at 955 MHz).
18
16
Range (ft)
Fig. 9. Paperclip tag 4 on a plain sheet of paper (top) and on the same
sheet of paper with attached copper foil strip (bottom).
14
12
45
10
40
8
35
4
30
2
0
800
820
840
860
880 900 920
Frequency (MHz)
940
960
980
1000
Fig. 8. Read range (ft) of paperclip tag 4 on 1, 5, 10, 20, and 30
sheets of office paper (for 4 W EIRP in free space).
Adding more paper increases the spacing between the wires,
reducing the coupling. At the same time, the paper stack acts
as a sheet of dielectric which increases the equivalent
electrical length of the tag antenna and shifts the antenna
resonant frequency down. As a result, the tag resonance shifts
down in frequency as more and more sheets of paper are
added to the stack.
Range (ft)
6
Tag 4, on 1 sheet
Tag 4, on 20 sheets
Tag 4, on 1 sheet with copper strip
Tag 4, on 20 sheets with copper strip
25
20
15
10
5
0
800
820
840
860
880
900
920
940
960
980
1000
Frequency (MHz)
Fig. 10. Read range (ft) of paperclip tag 4 on plain sheet of office
paper and on the piece of paper with copper strip.
D. Simulations
We performed modeling and simulation of tag 4 using
Ansoft HFSS. Here we present three cases: tag as is, tag on 1
sheet of paper, and tag on 1 sheet of paper with conducting
strip. The paper sheet was assumed to be 0.1 mm thick and
have dielectric constant of 3. Modeling the exact geometry of
the hand bent and soldered wire was difficult. We
approximated it to the best extent with straight cylinders. The
wire diameter was 1 mm. The snapshots of HFSS geometry of
these three cases are shown in Fig. 11-13.
50
Tag 4, as is (HFSS)
45
40
Range (ft)
35
30
Tag 4, on 1 sheet (HFSS)
Tag 4, on 1 sheet with strip (HFSS)
Tag 4, as is (data)
Tag 4, on 1 sheet (data)
Tag 4, on 1 sheet with strip (data)
25
20
15
10
5
0
800
820
840
860
880
900
920
940
960
980
1000
Frequency (MHz)
Fig 14. Read range for three cases (data and HFSS)
3
2
1
Gain (dBi)
Fig. 11. HFSS geometry for simulation case 1: tag 4 as is.
0
-1
Tag as is
Tag on 1 sheet
Tag on 1 sheet with strip
-2
-3
-4
800 820 840 860 880 900 920 940 960 980 1000
Frequency (MHz)
Fig. 15. Maximum tag antenna gain (HFSS simulation).
0
Tau (dB)
-3
Fig. 12. HFSS geometry for case 2: tag 4 on 1 sheet of paper.
-6
-9
-12
-15
-18
Tag as is
Tag on 1 sheet
Tag on 1 sheet with strip
800 820 840 860 880 900 920 940 960 980 1000
Frequency (MHz)
Fig. 16. Tag impedance matching coefficient (HFSS simulation).
Fig. 13. HFSS geometry for case 3: tag 4 on 1 sheet of paper with
conducting strip.
The range was calculated from simulations as shown in [7].
The complex impedance of Monza 4 chip mounted on FR4
board was measured experimentally and approximated by the
following formula:
⎛ f − f low ⎞
⎟ , (1)
Z ic ( f )[ Ohm ] = 18 − j180 − (7 − j 40 )⎜
⎜f −f ⎟
high
low
⎠
⎝
where f low =800 MHz and f high =1000 MHz.
Simulation results are presented in Fig. 14-16. One
can see that the agreement between data and simulations is
reasonable, given the complexity of the bent wire geometry.
Adding a sheet of paper and then a conducting strip improves
both antenna gain and impedance matching (although one can
see from Fig. 16 that there is still room for improvement). We
also simulated tag performance on multiple stacks of multiple
sheets of paper (modeled as thick paper slabs) and observed
that adding more paper resulted in lower tag resonant
frequency although the exact amount of frequency shift was
not as measured (most likely due to the fact that the stack of
loose papers cannot be modeled as a solid dielectric slab).
IV.
APPLICATIONS
A. Document tracking
One main envisioned application of RFID paperclip tags is
document tracking and management. Imagine an office where
information about each paper or stack of papers is contained in
the memory of its RFID paperclip. Furthermore, assume that
the current localization technology for UHF RFID tags [13-16]
will eventually mature and reach the point where one would be
able to tell the tag location with affordable hardware. Then
one could potentially locate a misplaced document in a papercluttered office by simply “searching” for the tag with
appropriate document record using the localization-enabled
RFID reader.
B. Direct data exchange between paperclip tags
The concept of smart document tracking can be taken one
step further. Imagine paper document stacks whose paperclips
“know” what other documents are located nearby. This can
potentially be done with direct tag-to-tag communication
between paperclip tags.
Direct tag-to-tag communication is currently possible only
between specially designed battery-powered active tags (see
for example, [17]). However, passive UHF RFID tags in close
proximity of each other (such as paperclip tags in stack of
papers) can potentially communicate directly even in the
absence of RFID reader, as long as some external RF source is
available to power up the tags (see [18] for example of using
external RF source). The tags can communicate by modulating
and backscattering to each other the RF CW signal from the
source as illustrated in Figure 17. Of course, the
communication protocol would need to be modified
accordingly to allow tags discover each other (for example,
tags could periodically backscatter queries to each other). This
idea was previously briefly described by us in a patent
application [19]. Note that the concept of direct tag-to-tag
communication applies not only to paperclip tags but to any
tags in close proximity of each other.
Fig 17. Concept of direct communication between paperclip tags. Tag
A transmits data to tag B by backscattering RF CW signal from the
external source. Tag B demodulates this signal and thus receives data
directly from tag A. Tag B can respond to tag A in a similar manner.
Sample link budget presented in Table 2 shows that
the proposed direct communication between paperclip tags is
indeed possible for the tags which are in close proximity of
each other. If we assume that the coupling loss between the
tags is 10 dB or less, then the received power of -12 dBm is
more than sufficient for operation of even small tags,
especially if they use latest sensitive RFID ICs.
Transmitted RF CW EIRP
36 dBm
Free space path loss from CW source to the
tag A over 1 m distance at 915 MHz
32 dB
Backscatter modulation loss of the tag A
6 dB
Coupling loss between the tags A and tag B
10 dB
Backscattered signal power seen by tag B
-12 dBm
TABLE II.
LINK BUDGET FOR DIRECT TAG-TO-TAG COMMUNICATION
To experimentally verify the concept of tag-to-tag
communications,
we have performed experimental
measurements using the setup shown and photographed in
Figures 18 and 19. We used our prototype tag 4 (shown in
Figure 6) as a paperclip tag. The 1 W RFID reader connected
to 6 dBi linearly polarized antenna was constantly
interrogating paperclip tag from the distance of approximately
1 m in an anechoic chamber. A half-wavelength broadband
dipole antenna was placed 1 cm away from the paperclip tag.
The dipole was tuned to be well matched to 50 Ohms in 915
MHz ISM band and was connected via coaxial cable directly
to the real time spectrum analyzer.
Fig. 18. Experimental setup for verification of possibility of direct
tag-to-tag communication between paperclip tags.
V.
Dipole antenna
Paperclip tag
Fig. 19. Antenna fixture used in experimental setup for verification of
possibility of direct tag-to-tag communication.
The spectrum analyzer performed non-coherent
demodulation by amplitude envelope detection, just like an
RFID tag IC. Thus, the signal from the dipole antenna
observed on the spectrum analyzer was very similar to
demodulated RF signal inside the IC on a tag which is
properly matched and placed in the vicinity of our paperclip
tag.
Figure 20 shows the signals observed by the dipole
antenna: the original reader signal and the backscattered
modulated signal from the paperclip tag. One can see that the
backscattered signal seen by the dipole antenna has enough
modulation depth and hence should be detectable by another
nearby tag.
DISCUSSION AND FUTURE WORK
One can imagine that the concept of a paperclip tag
(reusing paperclip body as an antenna) opens a whole new
field for RFID tag design. Now an antenna designer must
ensure that a tag not only has good read range, but also retains
its primary paperclip functionality (can be attached to papers
conveniently and can hold them together well). The same
antenna reuse concept can be applied to other document
holders (metal document binders, etc.).
The proposed paperclip tags have several attractive
features such as:
• Paperclip tags can be used anywhere where ordinary
paperclips are used (badges, money tracking [20], etc.)
• Paperclip tags can be designed to automatically
enable/disable as they are slid on/off the paper
• Paperclip tags can be made economically competitive
due to their double functionality as normal paperclips
There are still quite a few questions to be answered which
we have not covered in this work such as:
• Which out of many existing paperclip designs are best
suited to be reused (without or with minimal shape
modifications) with modern RFID ICs as tags?
• What is the best (reliable, low loss, and low cost) method
to attach RFID IC to the paperclip wire antenna
terminals?
• What should be the design rules which specify the tradeoff between mechanical and electrical functionalities of a
paperclip tag?
• How to effectively adjust the resonance of the single wire
paperclip tag (or how to make it broadband)?
• How does the coupling between the closely spaced
paperclip tags affect their performance?
Some of these questions have been partially answered in
existing literature. For example, the trade-off between the
small antenna size and its performance has been well covered
in antenna literature [21-22] whereas tag-to-tag coupling has
been studied in RFID literature [23-25]. However, we believe
that any of the questions above can become a subject for
fruitful future research. Electromagnetic modeling and
simulation tools will play the key role in answering those
questions.
VI.
Fig. 20. Demodulated voltage observed by the dipole antenna
(approximate equivalent of another tag) in the vicinity of the
paperclip tag interrogated by the RFID reader as shown in Figure 18
at 915 MHz.
CONCLUSIONS
In this paper, we described a concept of a passive
UHF RFID tag which uses paperclip body as an antenna while
also functioning as a regular paperclip. Sliding the tag on or
off the paper allows one to increase/decrease tag range
(activate/deactivate the tag), while an additional conducting
strip placed on the document can enhance tag range as was
demonstrated experimentally. We believe that as the world
moves forward to smarter document management systems, the
RFID paperclip tags described in this paper can potentially
find their use in many modern office applications.
ACKNOWLEDGEMENTS
We would like to thank three anonymous reviewers
who read our manuscript and made many valuable comments
and suggestions on both the form and the content of the
presented research. Their comments and suggestions, which
we tried to address to the best of our extent, helped us to
significantly improve this paper.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
“RFID activated paperclip tag”, US Patent 7280044
“Object locator and methods therefore”, US patent 5689238
M. Usami, “An ultra-small RFID chip: mu-chip”, IEEE Asia-Pacific
Conference on Advanced System Integrated Circuits, 2004
J. McDonnell et al., “Memory spot: a labeling technology”, IEEE
Pervasive Computing, vol. 9 , no. 2 , 2010, pp. 11 - 17
“History
of
the
paperclip”
[Online].
Available:
http://www.officemuseum.com/paper_clips.htm
“Systems and methods for wirelessly marking media”, US patent
7724142
K. V. S. Rao, P. V. Nikitin and S. Lam, “Antenna design for UHF RFID
tags: a review and a practical application”, IEEE Transactions on
Antennas and Propagation, vol. 53, no. 12, pp. 3870-3876, Dec. 2005
Impinj
Monza
RFID
ICs
[Online].
Available:
http://www.impinj.com/products/
NXP UCODE ICs [Online]. Available: http://www.nxp.com
G. Marrocco, “The art of UHF RFID antenna design: impedancematching and size-reduction techniques”,
IEEE Antennas and
Propagation Magazine, vol. 50, no. 1, Feb. 2008, pp. 66 – 79
N. Mohammed, K. Demarest, D. Deavours, “Analysis and synthesis of
UHF RFID antennas using the embedded T-match”, IEEE RFID
conference, 2010, pp. 230-236
P. Lacouth et al., “New RFID's chip matching technique using Artificial
Neural Networks”, IEEE International Conference on RFID-Technology
and Applications (RFID-TA), 2010, pp. 189 - 193
M. Bouet, A. L. dos Santos, “RFID tags: positioning principles and
localization techniques”, 1st IFIP Wireless Days, 2008, pp.1 – 5
P. Nikitin et al., “Phase based spatial identification of UHF RFID tags”,
IEEE RFID conference, 2010. pp. 102-109
D. Arnitz, U. Muehlmann, K. Witrisal, „UWB ranging in passive UHF
RFID: proof of concept”, Electronics Letters, vol. 46 , no. 20, 2010 , pp.
1401 – 1402
C. Angerer, R. Langwieser, M. Rupp, “Direction of Arrival Estimation
by Phased Arrays in RFID”, EURASIP Workshop on RFID Technology,
2010, pp. 57 - 61
Axcess
active
tags
[Online].
Available:
http://www.axcessinc.com/products/tags.html
J.-S. Park et al., “Extending the Interrogation Range of a Passive UHF
RFID System With an External Continuous Wave Transmitter”, IEEE
Trans. on Instr. and Measurement, vol. 59, no. 8, 2010, pp. 2191 – 2197
“Stochastic communication protocol method and system for RFID tags
based on coalition formation, such as for tag-to-tag communication”, US
patent application 20080252424
T. Kim et al., “Design of an UHF RFID tag antenna for paper money
management system”, IEEE APMC Conference, 2009, pp. 1056-1059
R. C. Hansen, “Fundamental limitations in antennas”, Proceedings of the
IEEE, vol. 69 , no. 2, 1981 , pp. 170 – 182
S. R. Best, “The performance properties of electrically small resonant
multiple-arm folded wire antennas”, IEEE Antennas and Propagation
Magazine, vol. 47 , no. 4, 2005 , pp. 13 – 27
L. Feng, C. XiaoSheng, T. Ye, ”Performance analysis of stacked RFID
tags”, IEEE RFID conference, 2009, pp. 330-337
Y. Tanaka et al., ” Change of read range for UHF passive RFID tags in
close proximity“, IEEE RFID conference, 2009, pp. 338-345
L. Feng, C. XiaoSheng, T. Ye, ” The “weak spots” in stacked UHF
RFID tags in NFC applications”, IEEE RFID conference, 2009, pp. 181186