A Dual-Resonant Planar Microstrip Antenna Design for UHF RFID
Using Paperboard as a Substrate
Mutharasu Sivakumar, Daniel D.Deavours
Information and Telecommunication Technology Center
University of Kansas
email: {muthus, deavours}
Abstract— Passive UHF RFID tags are commonly used in the
supply chain, and generally use dipole antennas. It is well
known that the performance of these tags degrade when placed
near water or metal continents. Thus, it is desirable for tags to
have consistent performance regardless of the contents of
containers. Microstrip antennas offer a potential solution, but
variations in the dielectric properties of corrugated paperboard
with varying moisture content provide unique challenges. In
this paper, we investigate whether microstrip antennas are
appropriate for cardboard containers, and show a novel dualresonant microstrip antenna and matching circuit that
provides good performance over a wide range of dielectric
properties. The antenna design can be realized using low-cost,
commodity “inlay” and “smart label” manufacturing
techniques.
T
I. INTRODUCTION
HE majority of passive UHF RFID tags available in the
market are tuned for optimal performance on free space
or when attached to corrugated paperboard/fiberboard
(commonly called “cardboard”). That is because the great
majority of tags are destined to be integrated into “smart
labels” for tagging paperboard for use in the supply chain
(e.g., Wal-Mart “mandates”). While stripline dipoles offer
good performance and low manufacturing costs (as little as
$0.10 US in high volume as of 2006), their primary
limitation is that the performance is highly dependent on the
materials to which they are attached. In the case of
paperboard containers, the contents of the container can
have a dramatic affect on tag performance. Recent advances
in lower-power IC designs offer some improvements, but
those are being offset by lower interrogator usage in order to
limit the interrogation zones.
The materials of greatest significance are metal and water.
Metal in the form of canned goods are relatively easy to
manage by finding the air gaps between cans. The most
difficult products are those that contain metallic foil in their
packaging, which is common, e.g., in dishwashing
detergent. Water-based products are also difficult if they are
packaged in rectangular-shaped containers so that there is no
predictable air gap.
In principle, these limitations can be overcome by the use
of microstrip antennas [1]. There are a number of microstrip
Manuscript received November 24, 2006.
antenna solutions for UHF RFID [2] [3] [4] and [5], but the
main limitation of those approaches is the complexity in
manufacturing those tags, and thus they are not amenable to
use in the supply chain. Indeed, the cost of some microstrip
tags can exceed the cost of a cardboard container. In
contrast, [1] provides a solution that requires no connection
from the antenna and the ground plane, and thus is
potentially viable for use in the supply chain. The only
modification is that a metal foil needs to be applied to the
inside of the container, and the paperboard becomes the
substrate of the microstrip antenna. We discuss the
particular antenna design below.
The primary technical challenge to this approach is that
the dielectric properties of paperboard can change
substantially with humidity and temperature. If the relative
dielectric constant changes, for example, from 1.3 to 1.6
with increase in relative humidity, the resonant frequency of
the antenna can shift from 915 MHz to under 800 MHz, and
render the narrowband antenna practically unusable.
Furthermore, the loss tangent increases substantially with
the increased water absorption, resulting in poor impedance
match.
The purpose of this paper is to show that we can design
microstrip antennas that take into account the changing
dielectric properties of paperboard under different humidity
conditions. We do this by designing a very simple dualresonant patch antenna and matching circuit. The elements
are designed so that the first antenna and matching circuit
resonates and provides a good match with the lower
dielectric constant and loss tangent, and the second antenna
and matching circuit resonates and provides a good match at
the higher dielectric constant and loss tangent. The
increased complexity of our proposed solution is the
application of a metallic foil to a portion of the interior of
the container.
II. APPROACH
In this section, we review known properties of paperboard
and previous work on planar microstrip antennas.
A. Properties of paperboard
Corrugated cardboard or corrugated paperboard (hereafter
paperboard) can be classified based on the number of paper
walls. Cardboard commonly comes in three forms: single
faced, in which flutes have paper on one side, single walled,
in which flutes are sandwiched between paper walls, and
double walled, in which two layers of flutes are contained
by three paper walls. Single-walled paperboard is the type
most commonly used in the supply chain.
Paperboard can also be classified in the type of flutes and
number of flutes per unit length. The “C” flute is the most
common, and the flute density of 42 flutes every 12 inches,
i.e., approximately one flute per 2/7th of an inch. For this
paper, we use a sample of “C” flute, 42 flutes per 12 inches,
and 150 mils (0.150 inches, 3.81 mm) thick.
Table I: Sorption / desorption of paperboard [2].
40% RH
90% RH
8 g/100g (Sorption)
16 g/100g (Sorption)
1° C
10 g/100g (Desorption)
18 g/100g (Desorption)
6 g/100g (Sorption)
14 g/100g (Sorption)
40° C
10 g/100g (Desorption)
18 g/100g (Desorption)
Temp.
The main limitation of using paperboard as a substrate for
a microstrip “patch” antenna is the fact that dielectric
properties of the paperboard can change over time
depending on the water content in the cardboard. Water
content in cardboard is typically measured in grams of water
per 100 grams of solid substance. Water content follows a
sorption / desorption isotherm based on the relative humidity
and temperature [6]. When the relative humidity increases,
the sorption rate is used to calculate the moisture content,
and when the relative humidity decreases, the desorption
rate is used. Table 1 illustrates a typical sorption /
desorption isotherm. As Table I indicates, water content can
vary between 6 and 18 g/100g.
Equilibrium moisture content of single walled paperboard
at 20 degree Celsius and 54.4% relative humidity
(approximately room temperature conditions) is 9.34 grams
per 100 grams of solid substance [4], although we measured
a lower rate for our paperboard sample (see Section 3).
Considering temperatures between 1 and 40 degree Celsius
and RH between 40 and 90 percent, the range of water
contents we are looking for would be from 6 to 18 grams per
100 grams of solid paper substance. To be conservative, we
consider water content rates between 5 and 25 g/100g. In
Section 3, we measure the dielectric properties for
paperboard with varying moisture content.
B. Microstrip Antennas
Traditional microstrip antennas for RFID tags (e.g., [3]),
require some direct electrical connection between the
antenna or matching circuit and the ground plane. For lowcost packaging, introducing a via is simply not cost
effective. A completely planar microstrip antenna design in
which the antenna, matching circuit, and RFID IC are all on
the same plane (e.g., [1]) is ideally suited for such an
application.
Using this method, the only additional step is to include a
ground plane in the paperboard, which can be performed
relatively easily at manufacturing time, either by integrating
a metal foil into the paper wall, or by applying a metal foil
tape to the interior of the box before it is formed. The
microstrip inlay (antenna and IC on a PET substrate) can be
converted to a “smart label” (label with inlay affixed) and
applied using the usual label-application method.
To exercise this approach, what is necessary is to design a
planar microstrip tag for the paperboard substrate. The
primary difficulty of this approach is that the dielectric
properties of paperboard can change substantially with
changes in relative humidity. What we will show is that a
simple, rectangular antenna and matching circuit that is
designed for relatively low water content becomes
practically unreadable at higher moisture contents. Thus, we
need to develop a microstrip antenna and matching circuit
that is able to match to the range of different dielectric
properties of the paperboard.
III. DIELECTRIC PROPERTIES OF PAPERBOARD
If we can find the percentage of water by volume for
various values of water content then we will be able to
obtain a theoretical bound on the dielectric constants we
need to operate over, provided we know the dielectric
properties of water over the range of temperatures and water
behaves the same way electrically when bonded with the
paper molecules and at free state.
Dielectric properties of water have been evaluated
extensively and available in literature [7]. Values of relative
permittivity and loss tangent for water at different
temperatures are tabulated in Table II.
Table II: Dielectric properties of water at 915 MHz.
tan δ
T (°C)
εr
0
87.0
0.095
25
78.2
0.044
40
73.1
0.031
We found that the density of our dry cardboard sample
has a density of 0.14 g/cc. It is well known that water has a
density of 1 g/cc. A simple calculation shows that water
content between 6 and 18 g/100g yields 0.84% to 2.52%
water by volume respectively. Assuming a simplistic model
of proportionality, and assuming that the dielectric constant
of cardboard is 1 (air), we could estimate that the dielectric
constant of cardboard varies between 1.66 and 2.97,
respectively. Based on measurements we show below later,
this simplistic model is not valid, and clearly the water is not
behaving electrically the same way as it does in liquid form.
Thus, we must obtain direct measurement of the dielectric
constant of the cardboard in order to design to that material.
We found the dielectric properties of different materials
by constructing a resonant rectangular microstrip antenna
and measuring the S-parameters using a network analyzer.
We start by estimating a value for dielectric constant and
loss tangent of the paperboard. Using the assumed dielectric
properties, we constructed a microstrip patch antenna,
measured the impedance characteristics over a broad
frequency range, then used data fitting and a method of
moments simulation package to estimate the dielectric
properties of the paperboard. The antenna design was
fabricated using the paperboard as substrate as shown in
Figure 1.
Figure 1: Microstrip test apparatus for paperboard. The
reverse side is laminated with a metal foil.
Table III: Measured dielectric properties of paperboard.
Water content
εr
tan δ
(g/100g)
~0
1.2
0.01
6.5
1.24
0.014
10
1.3
0.02
25
1.45
0.05
moisture content at room temperature and approximately
55% relative humidity was 6.5 g/100g, which differs from
[8]. We can conclude that different paperboard behaves
differently, and thus results from this paper should not be
overly generalized.
IV.
SINGLE RESONANT ANTENNA DESIGN
Once we know the dielectric properties of the substrate,
we first investigated whether a single-resonant patch
antenna such as [1] can be used to give the required
performance.
A. Design
We began by attempting to design the antenna with roomtemperature conditions (i.e., εr = 1.24 and tan δ = 0.014).
We chose the patch antenna to have a width of 40 mm,
which can safely fit in a 2-inch label. As illustrated in
Figure 2, we shaped the antenna so that the matching circuit
could safely fit within the 40 mm width. We determined the
length to be 128 mm. The matching circuit was constructed
with 3 mm transmission lines with 3 mm spacing, and the
matching circuit is placed in a 32 mm by 12 mm cavity.
This yields a resonant frequency of about 932 MHz with a
resonant impedance of 89 + j111 for 6.5g/100g paperboard.
The resonant frequency was intentionally high and the
impedance intentionally too large in order to obtain good
performance over the range of dielectric constants.
5
0
Dry
6.5
10
25
For accuracy, we repeated the procedure a number of
times until we were satisfied with the accuracy. Relative
permittivity and loss tangent of paperboard at room
temperatures conditions found using the above procedure
were 1.24 and 0.014 respectively. Note that the measured
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Five samples of paperboard of same dimensions were
taken. The samples were dried completely, weighed, and
stored in an air tight bag. After about 24 hours, they were
weighed again to make sure there was no change in water
content. Then we added water to obtain a 10g, and 25g of
water per 100g of dry paperboard. The samples were kept
for about 24 hours in an air tight bag to reach uniform
moisture content throughout the paperboard. Finally, the
dielectric constants of all the samples were measured using
the microstrip method. The dielectric constant and loss
tangent that we obtained are tabulated in Table III.
Dielectric constant and loss tangent of paperboard with
25g/100g water content was 1.45 and 0.05 respectively.
Dielectric constant and loss tangent of paperboard with no
water content was 1.2 and 0.01 respectively. These specify
the upper and lower bound of the dielectric properties of the
paperboard respectively.
Gain (dBi)
-5
Frequency (MHz)
Figure 2: Simulated antenna gain of single patch for various
moisture levels.
One novel aspect of the design in Figure 5 over [1] is that
that [1] forces relatively high current densities through 1 mm
transmission lines, while Figure 2 uses 3 mm transmission
lines. Using wider lines yielded a measurable increase in
performance. The matching circuit was designed to provide
a good match to 65 –j110 Ohms. For the performance
criterion, we chose a gain of -5 dBi, which should be
readable reliably to about 16 feet (about 5 meters), and
found that this design gives about 44 MHz of bandwidth
with that criterion. The simulated gain values are shown in
Figure 3.
When we varied the dielectric properties of the material,
the simulation results showed the gain curves in Figure 2.
While 10g/100g still gives acceptable performance,
25g/100g is completely unacceptable. Experimentation with
simulation indicates that about 15g/100g is enough to reduce
the gain to unacceptable levels. Thus, the change in
dielectric constant due to 90% relative humidity is sufficient
to render this tag as below-adequate performance.
120
Figure 5: Image of single patch design for paperboard.
100
Dry
Real Impedance (Ohm)
6.5
10
80
25
60
40
20
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900
890
880
0
Frequency (MHz)
Figure 3: Simulated input resistance of single patch for
various moisture levels.
Assuming minimum transmit power obtained provides the
minimum turn on power for the chip (a known quantity), we
can find gain of the tag antenna using Friss’ transmission
equation. What we obtain is a result that includes the
product of the tag antenna gain (Gt) and the power
transmission coefficient (τ). We call this product the
realized gain. The results for the realized gain are shown in
Figure 6 for 6.5g/100g. Note that for 25g/100g, we were not
able to achieve a single read at any frequency at 1 meter.
0
-0.5
160
Dry
-1
10
25
-1.5
Gain (dBi)
Imaginary Impedance (Ohm)
6.5
80
-2
-2.5
-3
-3.5
Also, it was found that for higher values of loss tangent the
port impedance reduced drastically which lead to poor
impedance matching. Figures 3 and 4 show the simulated
real and imaginary input impedance for the tag, which we
found to have good agreement with measured impedance
values. Note that the 6.5g/100g and 10g/100g show a good
impedance match over a portion of the band, but 25g/100g
has unacceptably low real impedance over the FCC band.
B. Measurements
Next, we constructed a rectangular tag, attached the RFID
IC, and measured its performance. An image of the
constructed tag is shown in Figure 5. Note that a metal foil
was attached to the reverse side. We used an instrumented
RFID reader in which we varied the frequency and power
settings from 903 to 927 MHz in order to find the minimum
power setting required to stimulate the tag. The tag was
placed a fixed distance (1 meter) from the reader antenna.
Note that the “turn-on power” measures both gain and
impedance match.
927
924
921
918
915
912
909
Figure 4: Simulated input reactance of single patch for
various moisture levels.
-4
906
Frequency (MHz)
903
960
950
940
930
920
910
900
890
880
0
Frequency (MHz)
Figure 6: Measured antenna gain of single patch antenna at
6.5g/100g. At 25g/100g, the tag was unreadable.
C. Interpretations
The tag shows excellent performance at 6.5g/100g, and
we would expect similar performance at 10g/100g. This tag
should be readable well over 20 feet away. However, with
higher moisture contents, the tag becomes completely
unreadable. The tag experiences very poor performance at
the higher moisture levels both because the antenna is not
resonant at the higher dielectric constants and because it
experiences poor impedance matching that happens when
the antenna is non-resonant and because there is a
significant reduction in the resonant resistance due to the
higher tan δ.
From these results, we conclude that a suitable microstrip
patch antenna is not suitable to address the variance in
dielectric constant for paperboard. Traditional methods of
making the antenna more broadband, such as making the tag
wider or thicker, are not practical for this problem.
If we are to achieve good performance over a considerable
variation in the dielectric constant of the paperboard, we
must develop new techniques to address this unique
challenge.
V.
DUAL-RESONANT ANTENNA DESIGN
In Section 4, we concluded that a single-resonant patch
antenna with a single, balanced feed structure is not suitable
to work suitably over the range of dielectric properties of
paperboard. We noted two factors that caused problems:
poor antenna efficiency due to non-resonant behavior, and
poor impedance matching. Any solution will need to
adequate address both of those challenges.
Here, we propose the use of two resonant patch antennas.
Unlike previous work of [1], we use a single, edge feed off
of the non-radiating edge of each antenna. Each antenna is
designed to resonate at a significantly different frequency.
We note that the non-resonant impedance of a patch antenna
is small in magnitude, and we use that small-magnitude
impedance as an approximation for a reference to ground. A
simple transmission line length is used to increase the
reactive component of the impedance in order to provide
good impedance matching.
maintains a good match and reasonable gain for higher
values of relative permittivity and loss tangent.
A dual patch antenna was designed taking all these factors
into consideration. The design with dimensions is shown in
Figure 7.
Figure 7: Dual-resonant antenna design with measurements.
Next, we present simulation results for the antenna.
Figure 8 shows the simulated antenna gain for varying
moisture rates. We note that the -5 dBi gain bandwidth at
6.5g/100g is approximately 81 MHz, almost twice as large
as the single rectangular patch.
5
0
Dry
6.5
10
25
-5
Gain (dBi)
By using this approach, we are able to not only match to
two different frequencies, but to two different impedances as
well.
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A. The Design
For comparison purposes, we choose width of the antenna
form factor to be the same 40 mm as in section 4. The two
patches are offset in terms of resonant frequency, one has a
resonant frequency of about 925 MHz and the other patch
has a resonant frequency of about 973 MHz. The lengths of
these patches were 148 mm and 136 mm respectively. The
patches were 15mm wide and were placed 10mm apart.
Frequency (M Hz)
Figure 8: Simulated antenna gain of dual patch for various
moisture levels.
350
300
Dry
6.5
Real Impedance (Ohm)
250
10
25
200
150
100
50
1000
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880
A rectangular patch antenna fed offset from center of the
non-radiating edge has a small reactive impedance. This
reactive impedance can be increased as required for
matching by changing length and width of the transmission
line. Again, we make use of the fact that at frequencies in
which one patch is resonating; the other patch has very low
impedance and acts as a virtual ground, eliminating the need
for any via.
Frequency (MHz)
Adjusting lengths and offsets of transmission lines
feeding the two patches, we can control the impedance for
both the resonances separately. This proves to be a great
advantage because for higher values of water content, both
the relative permittivity and loss tangent goes up resulting in
the lower resonant frequencies and a decrease in Q. Hence
we can adjust the impedance of the resonances so that one
resonance maintains a good match and a good gain for lower
values of relative permittivity and the other resonance
Figure 9: Simulated input resistance of dual patch for
various moisture levels.
Next, we show the simulated real and imaginary input
impedance for varying moisture contents in Figures 9 and
10. Note how the tag is able to maintain excellent
impedance matching (to 65 –j110Ω) over the entire band for
all the tested moisture contents. The second, large peak at
988 MHz for dry paperboard is the same peak that gives an
almost ideal peak of 83Ω at 919 MHz with 25g/100g. The
reactive impedance gives excellent results except for the
higher frequencies at 25g/100g, which will begin to degrade
performance significantly.
240
Dry
Imaginary Impedance (Ohm)
160
C. Interpretations
The measured results show generally good matching with
simulated results, and validate this approach. At 6.5g/100g,
the tag showed better-than-expected effective gain. It is
possible that the moisture content was not fixed for the
duration of the experiment, for example, and the tag took on
excess moisture. It is also very possible that the paperboard
samples were not uniform.
6.5
10
25
80
0
1000
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955
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895
880
-80
At 25g/100g, the measured effective gain matches closely
with simulation results. We expected to see a peak gain of
about -8 dBi at lower frequencies with a decrease in
performance at the higher frequencies due to poor
impedance matching. The measured results show exactly
this.
Frequency (MHz)
Figure 10: Simulated input reactance of dual patch for
various moisture levels.
From the above graph we observe that up to a water
content of 25 g/100g the antenna maintains a gain over -8
dBi over all dielectric conditions.
B. Measurement
As before, the design was etched into inlays on copper
clad polyester. Figure 11 illustrates one of the tags used for
testing. The inlays were then fabricated into tags using
paperboard at room temperature and humidity conditions
(6.5g/100g) and with 25g/100g. The tag was tested using
the same process described in Section 4.2. The performance
results are shown in Figure 12.
As a basis for comparison, a tag designed for optimal
performance at 6.5g/100g with a similar form factor as the
tag in Section 4 will give a gain of about -6 dBi. This poor
performance is primarily due the large loss component of the
dielectric constant.
VI. CONCLUSION
In this paper, we considered the applicability of a
microstrip “patch” antenna and its suitability for providing a
consistently high level of performance regardless of the
contents of the container, and with minimal impact on
current processes. The proposed approach requires (1) the
addition of a foil ground plane to the inside of the
paperboard container, and replacing commodity RFID
antennas (typically dipoles) with a planar microstrip
antenna.
We first characterized the dielectric properties of
paperboard under different moisture conditions. We found
that the dielectric constant may vary between 1.2 and 1.45,
and the loss tangent between .01 and .05.
Figure 11: Image of dual-patch design for paperboard.
0
-2
-4
Gain (dBi)
-6
25
-8
6.5
-10
-12
Our first attempt of using a single microstrip patch
antenna based on [1] performed well at the lower moisture
contents, but completely fails at higher moisture contents.
We developed a novel, dual-resonant structure that
provides both frequency and impedance diversity so that we
are able to achieve both good gain and good impedance over
the range of dielectric constants. We present extensive
simulation data as well as experimental results that show
good consistency between the two.
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921
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Frequency (MHz)
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Figure 12: Measured antenna gain of dual-patch antenna.
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