Regular paper
Linearly polarized compact extended U-shaped chipless RFID tag
F. Salemi
a, H.R. Hassani
a,⇑, S. Mohammad-Ali-Nezhad
baElectrical and Electronics Engineering Department, Shahed University, Tehran, Iran
bElectrical and Electronics Engineering Department, University of Qom, Qom, Iran
a r t i c l e i n f o
Article history:
Received 22 October 2019 Accepted 13 February 2020
Keywords:
Compact chipless RFID tag U-shaped resonator
Multi-resonance compact chipless RFID tag
a b s t r a c t
A novel compact linearly polarized extended U-shaped chipless radio frequency identification (RFID) tag is presented. To compact the size of the tag on a single dielectric substrate, several U-shaped strips are placed on the top and several strips are placed on the bottom surface of the substrate. The top and bottom strips are connected through vias. In doing so, a smaller strip length leading to a smaller tag, on the top surface is needed for the resonance required. The proposed chipless RFID tag operates over 2–8 GHz. For an 8-bit encoded structure the substrate RO4003 has size of 7.310.2 mm2equivalent to 0.09kg0.
13kg. The proposed tag is simple in design, has good angular stability up to 50°and has a surface size compaction of more than 60% in comparison to the previously reported compact chipless RFID tags over a similar frequency range. An equivalent LC circuited that describes the behavior of the proposed tag is also provided. The designed chipless RFID tag is fabricated and measured. The simulated and measured results are in good agreement.
Ó2020 Elsevier GmbH. All rights reserved.
1. Introduction
Radio frequency identification (RFID) uses radio frequency waves to encode the response data scattered from the tags[1].
RFID technology has a broad field of applications in commerce, industry, medicine, science, and other areas[1]. This system con- sists of two main elements, the RFID tag, and the RFID reader[2].
The system properties include, transmit data without physical con- tact, unique identification possibility and transmit data only if the reader requests[1].
RFID tags can be divided into two groups: chip tags [3,4]and chipless tags[2]. Due to the complexity and cost of the silicon chips the chipless tags have received much attention in recent years[5].
A chipless RFID tag is simple to fabricate, has a higher lifetime and can be fabricated in different sizes[2].
Different forms of Chipless RFID tags have been reported previ- ously. Several designed tags are based on different encoding tech- niques[6], such as frequency[7,8], time[9]and phase[10]domain.
In the frequency domain technique resonant elements with differ- ent lengths to encode information are used. In the other designs, symmetrical structures independent of polarization are used such as the square[11], circular[12]and trefoil rings[13]. Tags based on high-density data are proposed in[14–16]. Other tags reported recently are based on genetic algorithm such as those in[17,18].
Due to the importance of size compaction within different mod- ern portable devices, compact chipless RFID tag has been used in recent years. Size reduction in such tags can be achieved through the shape of the resonators[19]. Different shapes that use nested resonators have been used to reduce the size of the tags[20–22].
In[20], a compact tag based on U-shaped slot resonators in which the resonant frequency of each slot is adjusted by varying the length of the U-shaped slot is proposed. In that work, to encode 4-bit, the substrate used has a surface area of 0.21kg 0.26kg
(where kg is the guided wavelength). This tag was also printed using conductive ink but the magnitude of the RCS response is reduced. In[21], another compact chipless RFID tag that consists of eight slot ring resonators within a circular patch is proposed.
The radius of the patch is 0.25kg and this tag due to the symmetry of the structure is independent of orientation. In[22], a multi-band compact chipless RFID tag based on four C-shaped slots within a patch is reported. To reduce the size of the tag, short circuits are placed along the length of a few of the slots resulting in quarter- wavelength resonators and the size of the tag for a 4-bit structure reduces to 0.21kg 0.29kg. Other compact chipless tag cases reported are nested square loops[23], nested ring resonators inkjet printed on paper[24]and dual-polarized U-shaped tag[25].
In all of the compact chipless RFID tags reported above, to cover the lower frequency spectrum, an increase in resonant length is required that leads to an increase in the size of the substrate and the tag. To the best knowledge of the authors, no reports of any
https://doi.org/10.1016/j.aeue.2020.153129
1434-8411/Ó2020 Elsevier GmbH. All rights reserved.
⇑ Corresponding author.
E-mail address:[email protected](H.R. Hassani).
Contents lists available atScienceDirect
International Journal of Electronics and Communications (AEÜ)
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a e u e
work in which the other side of the substrate is used to further reduce the size of the chipless RFID tags are given.
In this paper, the design of a compact chipless RFID tag in the frequency-domain that consists of several strips placed on the top and bottom surface of the substrate and connected through vias is presented. In doing so, the surface area of the substrate can be reduced by more than 60% for operation over the same bandwidth if compared to the conventional case in which the strips are placed on one side of the substrate. The proposed chip- less tag can be used in most applications of RFIDs especially in cases where size is of importance. The proposed structure is simu- lated in HFSS software package and a sample is fabricated.
2. Antenna design
(A) Design of the compact chipless RFID tag
In the compact chipless RFID tags that have been reported so far, size reduction is achieved through the structure shape. In those structures, the relevant resonators are placed on the top layer of the substrate and resonant frequency is dependent on the resonant length.
In this paper, to further reduce the size of the RFID tag both the top and bottom surface of the substrate are used to provide the res- onant length at the required resonant frequency. As such, strips are placed on both the top and bottom surface of the substrate. These two top and bottom strips are connected through vias, allowing the current to flow between the two strips, thus, increasing the length of the strips.
For a constant guided wavelength (kg), unlike the previous works reported in the literature, half of the total resonant length required is placed underneath the substrate. This reduces the sub- strate surface required and makes the structure more compact. The proposed tag is made from several strips that are placed on both sides of the RO4003 substrate with size 7.310.2 mm2equivalent to 0.09kg0.13kg, with thickness of 0.8 mm and relative permit- tivity of 3.55. The radar cross section (RCS) response of the tag is encoded intoN-bit in the frequency domain. The designed tag is simulated using high frequency structure simulator (HFSS) software.
The layout of the proposed linear polarized compact chipless RFID tag based on the extended U-shaped resonators is shown in Fig. 1. In this figure, several U-shaped strips are placed on the top and a few strips are placed on the bottom surface of the single dielectric substrate with the top and bottom strips connected through vias. For all strips, the side length of the lower strip is the same as the corresponding side length of the strip placed above it.
Wsub and Lsub are the width and length of the substrate, respectively, Ss is the separation between the strips, Ws is the width of the strips,his the height of the substrate,Lsiis the strip length andWsiis the side length of the lower strip (withi = 1, 2,
. . ., N) which is equal to the side length of the upper strip on top
of it, andSeis the gap between the edge of the substrate and the closest strip to it. As shown in the enlarged figure,his the height andRvis the radius of the vias. The strips are numbered according to their lengths. Strip number 1 with the longest resonant length is denoted as U1and strip numberNwith the lowest resonant length is denoted as UN.
Initially, consider a single U-shaped strip (for example element U5) without the lower extension placed on top of the substrate, as shown inFig. 2, named Case 1. In element U5, the strip lengthLs5is 5.7 mm and any of the two side lengthsWs5is 4.8 mm making the total length equal to 15.3 mm. From this figure, the RCS response of this strip shows a single resonance at 12 GHz (minimum RCS value). Next, the lower strips and vias are added to the end of the U-shaped strip increasing the overall length of the strip, shown inFig. 2 as Case 2. For this extended element, the side length of each lower strip is equal to that of the upper side length of 4.8 mm and the height of each via is equal to 0.8 mm (substrate height). The total length of the extended U5 becomes 26.5 mm.
Due to this extension, the resulting resonant frequency in the RCS response reduces to 7.79 GHz, as shown inFig. 2.
A comparison between the results of Cases 1 and 2 shows that the line extension has reduced the resonant frequency, while, the substrate surface size has remained the same. It can be shown that for element U5, with the extended line to have the resonance at 7.79 GHz requires almost 50% substrate surface size as compared to the case in which strips are only placed on top of the substrate.
The length of the strips below the substrate can be further increased according to the caption ofFig. 2, Case 3. In doing so we obtain a closed loop in which almost similar results to that of Case 1 would be obtained, whereas, if a slot is placed in the middle of the strip on the lower side of the substrate the result of Case 3 of Fig. 2would be obtained. It can be shown that close loops need a higher overall length to resonate at the frequency equal to the case in which slot is present. It is seen that the resonant frequency is lowered to 5.31 GHz for the slot lengthSslotequal to 0.3 mm. For Sslotlower than 0.3 mm, the RCS magnitude response at higher res- onant frequencies reduces, soSslot0.3 mm is considered. With this Sslotthe highest length of strip is obtained. As can be seen, this
Ws Ss
Lsub
Se
Z Y X
U1U2 ... UN
Rv h h U1
...
LS1 LS2
LSi
...
Fig. 1.Layout of the proposed linear polarized compact N-1 bit chipless RFID tag based on extended U-shaped strips.
U5
Ls5=5.7 mm
U5
Sslot = 0.3 mm
U5
Fig. 2.RCS magnitude response of element U5: Case 1, strip placed on top of the substrate, Case 2, strips on both top and bottom surface of the substrate, Case 3, closed strip at bottom with slot.
extra length, shown as Case 3, for element U5, results in more than 70% substrate surface reduction as compared to that of Case 1.
For the Cases 2 and 3 shown inFig. 2, it can be seen that the 2nd harmonic related to the maximum point in RCS response occurs at twice the relevant fundamental resonant frequency while in Case 1
the 2nd harmonic occurs at a higher frequency. It needs to be men- tioned that, although not shown, simulation results for a 12-bit tag with surface area 0.12kg0.18kgand for a 16-bit tag with surface area 0.13kg0.18kgboth based on structure ofFig. 1, shows band- width of 2~6.5 GHz and 1.4~4.1 GHz, respectively. This shows that 2nd harmonics for the 8-bit, 12-bit, 16-bit tags occur at higher frequencies outside the bands quoted and hence are not important in the workability of the tags.
(B) Design of an 8-bit Compact Chipless RFID Tag
In the following the design of an 8-bit linear polarized compact chipless RFID tag based on the above U-shaped strips is given.
The RCS response of an 8-bit tag withN= 9 U-shaped strips is shown inFig. 3.Fig. 3(a) shows the response for U-shaped strips (similar to the element in Case 1 ofFig. 2) that are placed on the top surface of the substrate, Fig. 3(b) shows the response for extended U-shaped strips based on the element in Case 2 of Fig. 2, andFig. 3(c) shows the response for extended U-shaped strips with a slot in the middle of the bottom strips, similar to Case 3 ofFig. 2. It can be seen from the results ofFig. 3that there are 8 resonances in each case and the frequency range of 4.7–12 GHz of Fig. 3(a) has reduced to 2.96–6.9 GHz in Fig. 3(b) and to 2.4–
5.9 GHz inFig. 3(c). It can be seen that the high resonant frequency is related to the shortest resonant length and low resonant fre- quency is related to the longest resonant length.
3 4 5 6 7 8 9 10 11 12
Frequency (GHz) -70
-60 -50 -40 -30 -20 -10
|RCS| (dBsm)
(a) (b) (c)
2 3 4 5 6 7 8
Frequency (GHz) -70
-60 -50 -40 -30 -20 -10
|RCS| (dBsm)
2 2.5 3 3.5 4 4.5 5 5.5 6 6.5
Frequency (GHz) -70
-60 -50 -40 -30 -20 -10
|RCS| (dBsm)
Fig. 3.RCS magnitude response of an 8-bit tag withN= 9 for (a) U-shaped strips placed on the upper surface of substrate similar to Case 1 ofFig. 2, (b) extended U-shaped strips similar to Case 2 ofFig. 2, (c) extended U-shaped strips with slot similar to Case 3 ofFig. 2.
(a) (b) (c) (d)
CR1 LR1
CL1 LL1
L
t1=Lp1+Lp2+Lp3COR1
COL1
Cc1
CR2 LR2
CL2 LL2
L
t2COR2
COL2
...
Lp1
Lp2 Lp3
COL1 COR1
CR1 LR12 43CL1 LL1
U5
following of current
1
2 4 6 8 10 12 14 16 18 20
-60 -50 -40 -30 -20 -10 0
Frequency (GHz)
|RCS| (dBsm)
|S11|
-35
-75 -45
-85 -55
-95 -65
RCS response
S11
Fig. 4.(a) The equivalent circuit of each part of Case 2 of element U5 ofFig. 2, (b) the complete equivalent circuit model for element U5,(c) circuit model with element U4 present, (d) RCS and S11response for element U5. The parameters used in ADS are: COR1= 1.4 pF, LR1= 0.43 nH, CR1= 1 pF, LL1= 0.055 nH, CL1= 2.5 pF, Lt1= 0.5 nH, and COL1= 1.4 pF.
W
s
5=4.8mm Ws
5=4mmW
s
5=3.2mmW
s
5=2.4mm Ws
5=1.6mmFig. 5.Change in resonant frequency with change in the length of bottom strip Ws5.
Position 1Position 2Position 3Position 4
(a)
2 4 6 8 10 12 14 16 18
Frequency(GHz) -75
-70 -65 -60 -55 -50 -45 -40 -35
|RCS| (dBsm)
Position 1 Position 2 Position 3 Position 4
(b)
Side length
Fig. 6.(a) Various position of placing vias, 1–4 (b) Effect of various via position on resonant frequency.
(a)
(b) (c)
(d) (e)
1.39E-2 1.27E-2 1.14E-2 1.02E-2 8.94E-3 7.68E-3 6.43E-3 5.17E-3 3.92E-3 2.66E-3 1.41E-3
J (A/m)
Fig. 7.Current distribution of the structure (a) Case 1 ofFig. 2, (b) Case 2 ofFig. 2with via at position 1, (c) position 2, (d) position 3, (e) position 4.
Comparison of the results ofFig. 3(a) and (b) shows that the line extension has reduced the resonant frequency, while, the substrate surface size has remained the same. This shows that almost 50%
substrate surface size is required as compared to the case in which strips are only placed on top of the substrate. Also fromFig. 3(c), the frequency response shows that increase in the length of the bottom strips reduces the resonant frequency even more in com- parison to that ofFig. 3(b). In doing so, the surface area of the sub- strate can be reduced even further to more than 60% for operation over the same bandwidth if compared to the conventional case in which the strips are placed on one side of the substrate.
Nested resonators due to the neighbouring elements with reso- nant frequency close to each other suffer from mutual coupling between resonators. In such structures, to increase the number of resonators and reduce the gap between the elements mutual cou- pling effect increases. It can be shown that mutual coupling effect exists in all three cases ofFig. 3and that mutual coupling does not change appreciably if the bottom surface of the substrate is used.
FromFig. 3(c) it is seen that the RCS magnitude response is lower than that ofFig. 3(b) while the resonances shown inFig. 3 (b) (structure ofFig. 1) are better defined, as such in the following sections RCS of the structure shown inFig. 1would be covered.
One can also provide an approximate LC circuit model related to the design of the proposed tag. This model is simulated in ADS soft- ware and the parameters are optimized. For a single U-shaped ele- ment, e.g. Case 2 inFig. 2, one can show that the equivalent LC circuit of each strip making up the element is as shown in Fig. 4a.Fig. 4b shows the overall equivalent circuit of the element U5. If a second element e.g. U4is added, a second LC circuit similar to the previous one should be added, as shown inFig. 4c. In this multiple element structure the parameters of L and C should be re-considered every time that an element is added. In this figure, CC1is used to control the magnitude of S11between any two ele- ments.Fig. 4d shows the RCS as well as the S11magnitude response for the single U5 element. It is clear that the equivalent circuit shows the same results as that of the RCS response.
(C) Parametric study
In this section, the results of the parametric study of important parameters for the structure ofFig. 1are presented.
In the first case, the effect of changing the side length of the bot- tom strip,Wsi, while the side length of the top strip is kept constant at 4.8 mm is studied. In Fig. 1we have 9 bottom strips (on one side). Just to show how resonant frequency is affected by changing the length of the strips, the case of only one U-shaped strip, Case 2 inFig. 2(e.g. element U5with bottom strip lengthWs5), is consid- ered in which the lengths are reduced by steps of 0.8 mm.Fig. 5 shows that asWs5increases from 1.6 mm to 4.8 mm the resonant frequency decreases from 10.2 GHz to 7.79 GHz. This confirms that we can use the bottom surface of the substrate, extending the length of the U-shaped strips, decreasing the resonant frequency without increasing the surface area of the substrate. InFig. 5, the RCS response for each value ofWs5is shown.
It can be shown that ifLs5increases from 1.7 to 8 mm the band- width related to the maximum points on the RCS response reduces.
In the second case, the best position of placing a pair of vias along the side lengths of the extended U-shaped strip U5is studied.
InFig. 6(a), four possible via positions with a spacing of 1 mm from position 1 to position 4 is considered. The RCS response and reso- nant frequency for each of the via positions are shown inFig. 6(b).
Moving the vias from position 1 to position 4, the resonant fre- quency increases from 7.79 GHz to 11.4 GHz. As expected, by changing the position of via changes the resonant length and hence its resonant frequency. A parametric study is also done for various vias radius sizeRvand heighth. Although not shown with change inRvfrom 0.05 mm to 0.14 mm (this range is considered based on the width of the stripWs= 0.2 mm inFig. 1) the RCS response is almost constant. Height of the vias is related to the height of
(a)
(b)
Top Bottom
2 3 4 5 6 7 8
Frequency (GHz) -70
-65 -60 -55 -50 -45 -40 -35 -30
|RCS| (dBsm)
Measured RO4003 Simulated RO4003 Simulated FR4
Fig. 8.(a) Fabricated extended U-shaped chipless RFID tag and (b) measured RCS response based on RO4003 substrate and simulated RCS responses based on RO4003 substrate and a low cost FR4 substrate.
Table 1
Comparison of resonant frequencies for some IDs.
IDs Resonant Frequency (GHz)
f1 f2 f3 f4 f5 f6 f7 f8
111111111 2.96 3.32 3.66 4.14 4.72 5.42 6.26 6.86
011111111 – 3.21 3.6 4.09 4.64 5.41 6.3 7.04
111101111 2.95 3.3 3.82 – 4.75 5.42 6.22 6.87
111110111 2.96 3.3 3.68 4.32 – 5.44 6.24 6.88
111001111 2.96 3.42 – – 4.74 5.42 6.18 6.86
110101111 2.98 – 3.84 – 4.76 5.4 6.2 6.9
111011011 2.96 3.38 – 4.18 4.96 – 6.22 6.78
101010101 – 3.38 – 4.32 – 5.64 – 6.76
111010100 2.96 3.38 – 4.36 – 5.72 – 6.84
the substrate used. The result of parametric study for varioush(in- creasing up to 1.6 mm), shows that the resonant frequency reduces.
Fig. 7shows the current distribution on the U-shaped strip and the extended U-shaped strip for various via positions when ay directed plane wave is incident from above the substrate. From Fig. 7(a), it is seen that the current at the end of the U-shaped strip is minimum. From Fig. 7(b)–(e), current distribution for the extended U-shaped strip for the various positions of via is shown.
ForFig. 7(b) with via at the end, plane wave at the frequency of 7.79 GHz is incident and it can be seen that current also exists on the lower strips leading to a lower resonance frequency. By moving the via position,Fig. 7(c)–(e), it can be seen from the cur- rent magnitude that the active resonance length reduces and reso- nant frequency increases. Thus, the best position to add the vias is at the end of the U-shaped strips which results in maximum reso- nant length. Accordingly, in the proposed tag the vias for all the elements are placed at the end of each U-shaped strip, position 1 inFig. 6(a), to provide the largest increase in the resonant length.
3. Results
Based on the parametric study above, the structure ofFig. 1 with 9 extended U-shaped strips for an 8-bit tag is considered and a prototype is fabricated. The dimensions of the final proposed tag simulated and fabricated are:Wsub= 7.3 mm,Lsub= 10.2 mm, Ws= 0.2 mm,Ss= 0.3 mm,Rv= 0.085 mm (for fabricationRv= 0.1- mm) andSe= 0.25 mm. The substrate is RO4003 with a surface size equivalent to 0.09kg0.13kg, thickness of 0.8 mm, relative per- mittivity 3.55 and loss tangent 0.0027. This tag is designed to oper- ate over 2–8 GHz frequency range. The fabricated tag ofFig. 1is shown inFig. 8(a). In the measured result, the plane wave of a lin- ear polarized horn antenna operating over 2–8 GHz is incident on the tag and the scattered response received by the same horn antenna is measured through a Network-Analyzer. In the measure- ment process, the tag is placed in front of the horn antenna in the far field region. The measured and simulated RCS response of the tag is shown inFig. 8(b).
The proposed tag has also been checked on a low-cost FR4 sub- strate. The simulated RCS response is shown inFig. 8(b). It can be seen that the relevant resonant frequencies take place similar to those of the RO4003 substrate.
To verify the performance of the extended U-shaped chipless RFID tag, a set of states with some different cases of IDs are simu- lated. It should be noted that since no optimization on strips was carried out for various states, there are slight differences in reso- nant frequencies.
For the 8 bit tag ofFig. 1, if all of the strips are present it means that ID is 111111111. This case has eight resonant frequencies that are equal to 2.96, 3.32, 3.66, 4.14, 4.72, 5.42, 6.26 and 6.86 GHz, as shown inFig. 3(b) andTable 1.
Next, consider the following 6 states with IDs: 011111111, 111001111, 110101111, 111011011, 101010101 and 111010100.
Any bit being 1 means that the relevant strip is present in the tag while 0 bit means the strip is not present. The RCS response in each of the states is shown inFig. 9. InFig. 9(a), U1 is removed hence the first resonant frequency inFig. 9is also removed. In the case ofFig. 9(b), U4 and U5 are removed which leads to removal of the 3rd and 4th resonant frequencies. InFig. 9(c) U3 and U5 are removed hence the related two resonant frequencies (2nd and 4th) are removed. InFig. 9(d) U4 and U7 are removed leading to the removal of 3rd and 6th resonant frequencies. InFig. 9(e) all the even-numbered strips are removed leading to removal of four
Frequency (GHz)
(a)
(b)
(c)
(d)
(e)
(f) Frequency (GHz)
|RCS|(dBsm)
0 1 0 1 0 1 0 1
0 1 1 1 1 1 1 1
1 1 0 0 1 1 1 1
( )
1 0 1 0 1 1 1 1
1 1 0 1 1 0 1 1
( )
1 1 0 1 0 1 0 1
Fig. 9.Simulated RCS response of the 8-bit proposed tag for various combination of presence or absence of any of the extended U-shaped strips, with 1 meaning the strip is present and 0 means absent. State (a) 011111111, (b) 111001111, (c) 110101111, (d) 111011011, (e) 101010101, (f) 111010100.
resonant frequencies (1st, 3rd, 5th and 7th). Finally, in the case of Fig. 9(f) U4, U6, U8 and U9 are removed causing 3td, 5th, and 7th resonant frequencies to be removed.
Table 1 shows numerically the effect of presence/absence of strips on resonant frequencies.
The angular stability of the proposed tag ofFig. 1w.r.t the inci- dent plane wave is shown inFig. 10. It can be seen that the RCS response for different elevation and azimuth angle does not change very much up to 50°. Results confirm that resonant frequencies are not affected while RCS magnitude reduces. Although not shown, the angular stability response
does not exist above 50°. InFig. 10, the results are related to change ofhandUfrom 0 up to 50°in which the best and the worst magnitude response is related toh=U= 0 andh= 0,U= 50, respec- tively. The responses related to other h and U’s within the 50° range fall within the curves shown inFig. 10. One can conclude that above 50° angles the RCS response of the tag falls rapidly and for an x-polarized plane wave the response changes consider- ably, thus, this y-polarized tag would then not be workable.
Although not shown, the simulation results of 12 and 16 bit tags version ofFig. 1show that the proposed tag performs very similar to those published in the literature and with a little optimization on the widths of strips and distance between strips can be even better.
InTable 2, the proposed tag is compared to some of the chipless RFID tags reported in the literature to show the compactness of the present structure.
4. Conclusion
A compact chipless RFID tag is presented in this paper, in which extended U-shaped strips are used to lower the resonant frequency in the RCS response of the tag. Strips on both the top and bottom surface of the substrate are connected through vias. In doing so, the resonant length is increased without increasing the size of the tag and hence it results in a compact structure. It is shown that the overall substrate size is reduced by 60% compared to the usual case in which the relevant strips are placed on the top surface of the substrate only. This tag is linear polarized and orientation of up to 50°does not affect the performance. The equivalent LC circuit of the proposed tag verified through software ADS was also given.
The proposed tag can also be placed on many products such as PET and papers but it is not suitable for placement on metal surfaces.
The concept has been proved through both simulation and fabrication.
Declaration of Competing Interest
The authors declare that they have no known competing finan- cial interests or personal relationships that could have appeared to influence the work reported in this paper.
Appendix A. Supplementary material
Supplementary data to this article can be found online at https://doi.org/10.1016/j.aeue.2020.153129.
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Table 2
Comparison of the proposed tag with those reported.
Ref. Chipless RFID type Substrate surface
area corresponding tok2g
This paper Proposed tag, 8-bit 0.09kg0.13kg
[6] C-like, 5-bit 0.36kg0.72kg
[11] Square ring, 9-bit 0.21kg0.21kg
[12] Three ring, 19-bit 0.58kg0.58kg
[13] Trefoil,10-bit 0.34kg0.34kg
[14] I slot, 21-bit 1.71kg1.71kg
[15] Half-wave slots, 7-bit 0.38kg0.69kg
[16] Dipole, 20-bit 0.82kg0.82kg
[20] U-shaped, 4-bit 0.21kg0.26kg
[21] Circle ring, 8-bit 0.19k2g(radius is 0.25kg)
[22] C-shaped, 4-bit 0.21kg0.29kg
Fig. 10.Angular stability of the proposed tag ofFig. 1for different elevation and azimuth angles up to 50°.
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