READ RANGE REDUCTION IN UHF RFID DUE TO ANTENNA DETUNING AND GAIN PENALTY
J.Lorenzo,D.Girbau,A.La´zaro,and R.Villarino
Department of Electronics,Electrics and Automatics Engineering, Universitat Rovira i Virgili(URV),Av.Paı¨sos Catalans26,Campus Sescelades,43007Tarragona,Spain;Corresponding author: david.girbau@urv.cat
Received16April2010
ABSTRACT:This article propos an analytical study of the link budget in UHF RFID which takes into account the effects of materials where the tags are adhered,and which permits to analyze the impact of detuning and gain penalty on the read range.The analysis is performed for two typical tag antenna topologies:a dipole antenna and a slot antenna.V C2010Wiley Periodicals,Inc.Microwave Opt Technol Lett 53:144–148,2011;View this article online DOI10.1002/mop.25625
寒假记事Key words:UHF RFID;read range;detuning;gain penalty;dipole; slot antenna
1.INTRODUCTION
Passive RF tags are transponders that communicate with an inter-rogator or reader.In recent years,RFID tag usage is significantly increasing and its cost is being reduced.However,much rearch is still needed to improve RFID system performance[1,2].
红谷滩湿地公园The most common UHF RFID systems u passive(battery less or remotely powered)tags.Tags communicate with the reader by changing(modulating)their reflection coefficient to incoming radiation from the reader,that is,modulating their scattering/radar cross ction[2].The main performance criterion for RFID tags is read range,defined as the maximum distance at which the tag can be read by the reader.For many applications,such as warehous, the read range should be maximized to reduce the number of read-ers required and reader to reader interference[3].
RFID system performance does not only depend on propaga-tion.A key issue to be considered is the tag ud for a particular final application.Since antenna characteristics change when the tag is placed on different materials or when objects are prent in its vicinity,tag detuning,and gain penalty are two important factors,which degrade tag antenna performance,with direct impact on the read range[4].However,little literature has been published on this topic[4–6].Tag detuning address the varia-tion in operation frequency and,in conquence,impedance mismatching at the RFID band.Gain penalty address the vari-ation in the antenna available gain.
This study focus on all the aspects related with tag detun-ing and gain penalty when tags are adhered to materials.To this end,Sections2and3analyze the link budget reader-tag-reader, where tho terms that depend on materials are pointed out. Two antennas are propod in Section4to study detuning and gain penalty:a dipole antenna and a slot antenna.The are two well-known topologies in the UHF RFID band[7,8].In Section 5,a wide comparison between the two topologies is provided and conclusions on their nsitivity with regard to the materials are given.Finally,the impact of detuning and gain penalty on read range is studied for both antennas in Section6.
2.LINK BUDGET
The power received in the tag,P r,tag,is given by a modification of Friis transmission Eq.(6):P r;tagðdBmÞ¼P readerðdBmÞþG readerðdBÞþG tagðdBÞ
þ10logð1Àj q j2ÞþD GðdBÞÀL sysðdBÞÀL pðdBÞ;ð1Þwhere P reader is the power transmitted by the reader,G reader and G tag are the available gains of the reader and tag antennas in free space,respectively,L sys is the cable loss and L p is the path loss.Since the gain and the impedance of the tag antenna depend on the material where it is adhered,a gain penalty factor D G is introduced to take into account the change in the antenna available gain(which only takes into account directivity and ef-ficiency),and it is defined as:
D GðdBÞ¼G tag;materialðdBÞÀG tagðdBÞ:(2)
q is the reflection coefficient of the tag given by:
q¼
Z antennaÀZÃload
Z antennaþZ load
;(3)
where Z antenna is the input impedance of the antenna and Z load is the input impedance of the integrated circuit(IC)or chip.The tag nsitivity is defined as the minimum power(also known as power up threshold,P th)that it needs for rectification of the incident RF power and it responds by modulating the radar cross ction.For instance,for the tag Impinj Monza Gen2,nsitiv-ity is aboutÀ11dBm.
The power received in the reader in the backscatter commu-nication radio link budget,P r,reader,is a modification of the monostatic radar equation:
P r;readerðdBmÞ¼P readerðdBmÞþ2G readerðdBÞÀ2L sysðdBÞ
þ20log j q0jþ2G tagðdBÞþ2D GðdBÞÀ2L p
(4)
where q0is the differential reflection coefficient of the tag[q0¼q1Àq2,where q1and q2are the0and1states of the chip reflection coefficient q,which depend on the chip load(2)][9]. An extensive study on propagation to characterize the path loss L p can be found in[6].
3.DETUNING AND GAIN PENALTY ANALYSIS
UHF tag antennas for low-cost applications are usually dipoles with a matching network(bad on T-match techniques or shorted stubs)or slot-bad tags(Fig.1).The tags are highly nsitive to the material where they are attached.Two effects are obrved.Thefirst is due to the change between
the
Figure1Simulated return loss and layouts of the dipole and slot antennas(not to scale)
permittivity of the air and that of the material.The material pro-duces a change in the antenna impedance,which results in a mismatch or detuning.The cond effect is a change in the gain,efficiency,and radiation pattern becau of currents induced in the material.
When the radio link between reader and tag is considered, the mismatch effect caus a reduction of the available power in the tag due to the term10log(1À|q|2)in(1),since now the impedance of the antenna is no longer Z antenna.In addition,the difference in the gain between free space and a material affects the power received in the tag a quantity D G(dB).Thus,the dif-ference due to material in the power received in the tag is:
D L tagðdBÞ¼D GðdBÞþ10logð1Àj q mat j2Þ;(5) where q mat is the reflection coefficient between the chip input impedance(Z load)in the power on state and the antenna(Z an-tenna
)adhered to the material.The effect of antenna mismatch in the differential coefficient q0is difficult to evaluate.For infor-mation transmitted from the tag to the reader,ASK modulation is widely ud due t
o its simplicity and can achieve good effi-ciency in a low duty cycle system[10];here,the0-state is obtained from the matched condition,Z load¼Z chip,whereas the 1-state is obtained from the short-circuit condition Z load¼0.Then,the difference between the power received in the reader (in dB)with and without the material is the RCS difference,but since q0is unknown,it can be approximated by:
D L readerðdBÞ¼D RCSðdBÞ¼2D Gþ20log j q0j%
%2D Gþ10logð1Àj q mat jÞ2:ð6ÞEquations(5)and(6)can be interpreted as the fading margin due to material.The first term models the change in the tag antenna gain(gain penalty)and the cond the change in the antenna impedance(detuning).
The u of a spacer is a good option to overcome the effects of gain penalty and detuning and,in fact,it has been successfully applied as a solution in real cas[11].This option permits to u the same tag for a number of applications and materials but,at the same time,it implies an additional cost(which could be im-portant for low-cost applications)and it also implies that the tag has not near-zero thickness,with the conquent problem this may cau due to chafing.Alternatively,another solution is to design antennas oriented to the end application(and material where being adhered).In next ctions,a study of detuning and gain penalty is provided to analyze the effect of materials on tag performance using electromagnetic ADS-Momentum
鳕鱼的做法simulations. Figure2Return loss of the dipole and slot antennas when on contact with wood
4.ANTENNAS DESIGN
A dipole antenna and a slot antenna matched to the commercial tag chip Impinj Monza Gen 2with input impedance Z chip %40-j100X have been designed to be compared.The PET substrate is 100l m height,with permittivity e r ¼2.26,loss tangent t g d ¼0.0003and 10l m-thick metallization.For the strip antenna,an impedance matching network bad on short-ended stubs is ud to complex match the antenna impedance to the chip con-jugate input impedance.Return loss and layouts of the dipole and slot antennas are shown in Figure 1.The dipole antenna center frequency is 866.6MHz,with return loss of À25.03d
B and gain of 2.186dB.The slot antenna centre frequency is 867.2MHz,with return loss of À24.75dB and gain of 1.44dB.The physical dimensions of the antennas are:170Â15mm for the dipole and 95Â52mm for the slot antenna.
5.CHARACTERIZATION OF THE EFFECTS OF DETUNING AND GAIN PENALTY IN DIPOLE AND SLOT ANTENNAS
The problem aris when the antennas are in contact with mate-rials.Two typical materials where RFID tags can be adhered,wood or cardboard,with very similar dielectric properties,are considered (e r ¼1.7and tg d ¼0.036).In Figure 2(a)the effect of detuning of the dipole and slot antennas when on contact with materials is shown.Concretely,results shown in Figure 1(nominal free-space design)are compared to the return loss when the tag is adhered to two wood pieces of thickness 5and 10mm.Figure 2(b)shows the radiation patterns at 865MHz for both antennas under the same conditions,where the decrea in their gains can be obrved.Figure 3shows both problems for the free-space antenna designs:detuning and gain penalty for a wood thickness between 0an 20mm.Three conclusions can be drawn from Figures 2and 3.The first one is that the bandwidth of the slot antenna is narrower than the dipole;in addition,whereas the bandwidth of the slot remains nearly constant with detuning,the bandwidth of the dipole increas with detuning,which can be explained from their field distribution.Second,detuning is similar for both antennas.Third,gain penalty is im-portant,but this parameter increas at a similar rate for both dipole and slot antennas.However,the most important conclu-sion is that due to its larger bandwidth and its increa with detuning,the dipole return loss are less affected than in the slot
antenna,and remain at a more suitable value at the operation frequency (865MHz).
5.1.Tag Design Optimized for Wood and Cardboard
It comes out from Figures 2and 3that detuning increas rap-idly with thickness of the material;however,for large thick-ness detuning and return loss remain nearly constant.This trend approximately takes place at 10mm for the dipole antenna and at 5mm for the slot antenna.This suggests that a good strategy for designing a tag which could be effectively ud in wood,independently of its thickness,could be to optimize the antenna design taking into consideration the thickness of wood.This has been done and the results are also shown in Fig-ure 3(e optimized design).It is shown that the antennas now have return loss under À10dB for wood thickness above 1mm and 2mm for the dipole and slot antennas,respectively;in the latter,return loss are not as good as in the ca of the dipole antenna,again due to its smaller bandwidth.In conclusion,this design strategy demonstrates good behaviour and permits the design of tags which work properly for one material,independ-ently of its thickness.
祝福的近义词The difference in the power received at the tag (5)and the reader (6)due to material is given in Figure 4for both antennas.A comparison between the free-space designs (Section 4)and the optimized designs (Section 5.1)is shown.It can be obrved that when zero-thickness material is considered,the designs shown in Figure 4have much better behavior,as expected;how-ever,when th
e wood thickness increas,the optimized antennas are much better in both cas.
5.2.Dependence on the Dielectric Properties
It has been assumed in Section 5.1that the dielectric properties of wood or cardboard are constant.However,this can change between types of wood or cardboard,since it depends specially on its density.In addition,nonhomogeneities can also be impor-tant in the materials (especially in wood).To this end,a study of the effects caud by the variation of the dielectric permittiv-ity has been carried out.The two antennas which have been ud are the ones designed in Section 5.1(with 10and 5mm of wood for the dipole and the slot,respectively).Figure 5shows return loss and gain for both antennas.The most important
issue
Figure 3Radiation patterns of the dipole and slot antennas when on contact with
wood
Figure 4Return loss and gain of the dipole and slot antennas,for the free-space design and the optimized design (Section 5.1)as a function of the thickness of the wood piece
is that it can be obrved that return loss is under À10dB for 1.2<e r <2.4(71%)in the dipole and for 1.5<e r <1.98(30%)in the slot antenna.Clearly the dipole antenna prents much less nsitivity to th
e variations of dielectric permittivity;this means that it will be much less nsible to changes in the material,especially to density variations.
6.EFFECT OF DETUNING AND GAIN PENALTY ON READ RANGE
As already introduced,the main effect of detuning and gain pen-alty is the decrea of the read range.A study of the real impact is provided here.To this end,typical RFID reader and tag nsi-tivities of S reader ¼À70dBm and S tag ¼À11dBm (Impinj Monza Gen 2)are considered.For this calculation,the power received at the tag and reader as a function of distance obtained from expressions (1)and (4),respectively,are compared to the nsitivities.In conquence,two parameters are obtained:the read range in the reader-tag link (uplink)and the read range in the tag-reader link (downlink).Of cour,the most restrictive one shall be considered.
Figure 6shows the read range for the dipole and slot anten-nas as a function of the wood thickness.The behavior of the free-space antenna designs (Section 4)are compared to the opti-mized designs (Section 5.1).It can be obrved that the read
range in the optimized design is incread in the order of 1m in the uplink and 3m in the downlink.Figure 7shows the read range for the optimized dipole and slot antennas (designs done in
Section 5.1)as a function of the relative permittivity of the material.The most interesting conclusion is that while the dipole antenna has better performance for low permittivity,the slot antenna is less nsible for higher permittivity,both having an optimum point at the design value (e r ¼1.7).
Finally,in Figure 8(a)comparison between power received at the dipole and slot tags designed at Section 4,with and with-out material (20mm-thick wood)and their nsitivity,and the power received at the reader (with and without material)and its nsitivity is shown.Two tags are here considered:Impinj Monza Gen 2and Alien Higgs Gen 3(with nsitivity À18dBm).A typical reader nsitivity of À70dBm has been assumed.Table 1summarizes the read ranges in meters obtained in each situation;the read ranges obtained for the
脑溢血的症状optimized
Figure 5Difference in the power received at the tag and at the reader as a function of the thickness of the wood piece for the dipole and slot antennas.Comparison between the free-space design and the optimized
design
Figure 6Return loss and gain of the dipole and slot antennas (opti-mized design with 10mm-thick and 5mm thick wood,respectively),as a function of the relative
permittivity
Figure 7Read range for the dipole and slot antennas as a function of the wood thickness.Comparison between the free-space design (Section 4)and the optimized design (Section 5.1)for uplink and
downlink
Figure 8Read range in the reader-tag link (uplink)and tag-reader link (downlink)as a function of the relative permittivity for the dipole and slot antennas.Comparison between the powers received at the dipole (up)and slot (down)tags and at the reader and their respective nsitivities for two tags (Gen 2and Gen 3)as a function of distance.Ca without material and with a 20mm-thick piece of wood
dipole and slot antennas(designed in Section5.1)are also pro-vided.It is important to note that,for both dipole and slot tags, the uplink(reader-tag)limits the read range when Gen2tags are ud,while for Gen3tags,the downlink is more restrictive. It is also shown that by using optimized antennas,designed tak-ing into account the application,read ranges can be well improved when the tag is on contact with material.
7.CONCLUSION
仰卧臂屈伸This article has dealed with the effects of detuning and gain penalty experienced in UHF RFID tags when they are attached to materials.An analytical study has been provided and tho terms which depend on the material have been identified and their effect quantified.It has been shown that important detuning and some gain penalty are experienced in two typical tag antenna topologies(dipole and slot antenna)and the read range is considerably reduced.This reduction must be taken into account becau in veral applications could not be permitted. ACKNOWLEDGMENT
This article was supported by the Spanish Government Project TEC2008-06758-C02-02.
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V C2010Wiley Periodicals,Inc.
NOVEL BROADBAND DUAL-POLARIZED DIPOLE ANTENNA
Hua Huang,Zhongqi Niu,Bing Bai,and Jinfeng Zhang
School of Electronic Engineering(SEE),Xidian University,Xi’an, Shaanxi710071,People’s Republic of China;Corresponding author:hhuang@mail.xidian.edu
Received22April2010
ABSTRACT:A novel broadband dual-polarized cross-pair dipole antenna is prented in this article.Two pairs of a folded dipole antenna are ud as645 polarization element,respectively.With su
ch a cross pair of folded dipoles,a polarization diversity antenna is constructed.A prototype antenna has been fabricated and tested.Results of simulation and measurement agree well and show the antenna exhibits encouraging ,10-dB impedance bandwidth of 87.3%from800to2040MHz,which is much wider than that of conventional cross-pair antennas,as well as a half-power beamwidth within6565 in the horizontal plane.V C2010Wiley Periodicals,Inc. Microwave Opt Technol Lett53:148–150,2011;View this article online DOI10.1002/mop.25659
Key words:broadband;dual polarization;ba station antenna
1.INTRODUCTION
With the development of mobile communication,higher demands have been raid for ba station antennas,such as broadband or multiple bands,higher gain,good patterns,and so on.At the same time,a compact structure is required for the ea of installations and lightning wind pressures.The designed antenna in this article is introduced to resolve this issue.Several ba station antennas are available by many predecessors.In Ref.1,a kind of broadband dual polarization bad on patch ba station antenna is constructed for mobile communication. In Ref.2,the design topology of a wideband
L-probe stacked patch ba station antenna is prented.In Ref.3,a cross pair of folded dipoles is already ud in a broadband polarization diver-sity antenna.Due to an increa in the subscribed capacity of the mobile station system,a six-ctor wireless zone configura-tion with the beamwidth of65 has been the main configuration. The frequency range ud in2G communication system such as
TABLE1Read Ranges(in meters)in the Uplink and Downlink for the Dipole and Slot Antennas(Free-Space Designs of Section4 and Optimized Designs of Section5.1)in Free-Space and Placed on20mm-thick Wood,Using the Chips Impinj Monza Gen2and Alien Higgs Gen3
Impinj Monza Gen2Uplink Ahen Higgs Gen3Uplink Downlink Dipole(free-space design,Section4)Free-space 4.29.49.35
20mm wood 3.1 6.8 5.6 Dipole(optimized design,Section5.1)Free-space 4.179.39.38
20mm wood 3.687.75 Slot(free-space design,Section4)Free-space 4.159.39.35
20mm wood 2.6 5.78 4.49 Slot(optimized design,Section5.1)Free-space 4.179.299.38
20mm wood 3.477.75 6.9
