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Segmented Magnetic Antennas for Near-fieldUHF RFID

A simple loop antenna of less-than-resonant size is awkward to match and a

resonant antenna contains opposing current flows and makes a poor

magnetic antenna. Segmented antennas have been constructed that can

operate at resonant diameters while still p...Daniel M. Dobkin, Steven M. eigand and Nat!an I"e#$ne %&, '(()No Comments

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U! R!"# and $ag Antenna Scattering% &art ""' $heor(U! R!"# and $ag Antenna Scattering% &art "' )*perimental Results

Related )vents

"+),China /0ANS1S 2lobal )vents

*onventional radiativel" co$+led, $ltra-!ig! fre$enc" radio-fre$enc" identification UHF

RFID tags !ave certain limitations/ t!e" are +!"sicall" large and do not o+erate 0ell in or

close to a$eo$s fl$ids. Recentl", t!e $se of tags config$red to $se ind$ctive instead of, or

as a s$++lement to, radiative co$+ling !as been advanced as an alternative a++lication

model for UHF RFID. Ind$ctivel" co$+led tags re$ire reader antennas t!at +rovide

significant magnetic fields.

1!e m$t$al ind$ctance bet0een t0o loo+ antennas is strongl" infl$enced b" t!e antenna

diameter. A small loo+ +rod$ces a strong magnetic field along its a2is, b$t t!e field falls

ra+idl" 0!en t!e sensing coil is moved more t!an a++ro2imatel" one diameter above t!e

larger loo+. A large loo+ +rod$ces a more modest +eak field, b$t t!e e2tent of t!e field is

larger, scaling 0it! its diameter. 1!erefore, t!ere is an o+timal si3e for best read range for

an" given +eak loo+ c$rrent and co$+ling re$irement.% 10o obstacles are enco$ntered

0!en one attem+ts to design sim+le loo+ antennas for UHF a++ro2imatel" 4(( MH3 near-field readers. Antennas of intermediate diameter bet0een ' and 5 cm are a0k0ard to

matc! to a 5( 6 in+$t, d$e bot! to lo0 radiation resistance and !ig! in+$t im+edance/ loo+

antennas !ave a +arallel resonance in t!is si3e range. As t!e antenna diameter

a++roac!es t!e series resonant si3e a++ro2imatel" 789 t!e c$rrent distrib$tion s!ifts to

+rod$ce a c!ange in sign of t!e c$rrent flo0, and t!e antennas +rod$ce relativel" little

magnetic field on a2is.',:

Segmented Loop Antennas

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Model and Measurement Results

1o sim$late t!e loo+ resonance, a '.5D electromagnetic solver soft0are 0as $sed and an

N-+ort S-+arameter block 0as e2tracted. For e2am+le, for t!e %( cm loo+, a :'-+ort S-

+arameter file 0as e2tracted. 1!is file 0as $sed in a linear sim$lator to determine t!e

+ro+er ca+acitances bet0een segments, cascade t!e in+$t of t!e loo+ 0it! t!e S-+arameter res$lts of a bal$n and add matc!ing com+onents. Since t!e radiation resistance

is ver" small com+ared to t!e 5( 6 coa2ial feed line, t!e loo+ matc! is ver" sensitive to t!e

reactance val$es. 1o make t!e matc!ing more rob$st and attain better man$fact$rabilit", a

resistor 0as mo$nted on t!e o++osite side of t!e antenna feed to decrease t!e = of t!e

loo+ s$fficientl". *ommonl", a loo+ antenna 0it! a lengt! m$c! smaller t!an its e2citing

0avelengt! is referred to as a small loo+. Small loo+s !ave constant c$rrent +!ase along

t!e loo+ 0ire and !ave t!e ca+abilit" of generating a strong near-field magnetic field. >n

t!e ot!er !and, d$e to t!e small circ$mference, t!e magnetic field ta+ers off ver" $ickl"

be"ond a distance e$ivalent to one loo+ diameter. Increasing t!e coil si3e 0ill increase t!e

read-distance of near-field tags b$t !ave t!e disadvantage t!at, at UHF fre$encies and for 

+ractical read distances, t!e loo+ is not small com+ared to t!e 0avelengt!. A %( cm

diameter coil, for e2am+le, is ro$g!l" one 0avelengt! in +erimeter and 0ill !ave c$rrentn$lling and a +!ase-inversion along t!e circ$mference.

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Figure 3 s!o0s t!e s$rface c$rrent along a %( cm coil driven at 4%5 MH3. 1!e to+ and

bottom s!o0 c$rrent n$lls and t!e left +ortion is o$t of +!ase 0it! t!e rig!t.

Segmenting and combining t!e +arasitic ind$ctance of eac! section 0it! a l$m+ed

ca+acitor 0ill ca$se t!e large loo+ to be!ave electricall" like a small one.

1!e c$rrent 0ill remain constant along t!e loo+ and +rovide a strong magnetic field see

Figure 4. 1!is +!"sicall" large b$t electricall" small loo+ !as ver" little radiation resistance

and t!$s can be driven 0it! significant strong c$rrents 0it!o$t violating government

radiation limitations.

Distrib$ting t!e ca+acitances also rela2es t!e voltage breakdo0n re$irements allo0ing

stronger drive +o0er.

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1!e meas$red ret$rn loss for t!e 5 cm loo+ is s!o0n in Figure 5 , and for t!e %( cm loo+in Figure 6 , demonstrating t!e good band0idt! matc!ing obtained 0it! small and large

loo+s.

Inductive Coupling Improvement 

1!e ind$ctive co$+ling 0as meas$red $sing an ? mm diameter sim+le loo+, matc!ed to 5(

6. 1!e small loo+ 0as matc!ed to a : k6 terminating resistor +laced o++osite t!e loo+ feed

$sing a %.: +F ca+acitor.

 A loo+ of t!is diameter !as a radiation resistance m$c! less t!an % 6, so co$+ling bet0een

t!e small and large loo+s o$g!t to be almost +$rel" ind$ctive.

1!e loo+ incor+orated a 0ire-0o$nd bal$n to avoid common-mode co$+ling bet0een t!e

antenna $nder test and t!e s!ield of t!e coa2ial !ook$+ cable.

In eac! test t!e sense antenna 0as +laced above and +arallel to t!e +lane of t!e larger

loo+ antenna $nder test. A %( cm diameter segmented loo+ 0as com+ared to a sim+le

single-segment loo+ of t!e same si3e and s!a+e.

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1!e res$lts are s!o0n in Figure 7 . 1!e $se of t!e segmented loo+ res$lts in an

a++ro2imatel" '( d< increase in co$+ling to t!e small test loo+.

1!e ret$rn loss is sensitive to t!e environment, des+ite t!e fact t!at t!e antennas are

e2cited t!ro$g! a bal$n. For e2am+le, t!e antenna is det$ned if +laced directl" on a static-

dissi+ative s$rface.

1!e res$lts re+orted in t!e ne2t section are obtained 0it! t!e antenna mo$nted in a

+ol"et!"lene bo2 several centimeters a0a" from ot!er s$rfaces.

It seems likel" t!at t!e $se of a fairl" !ig! dielectric-constant base+late combined 0it!

a++ro+riate ad@$stment of t!e segment +arameters 0ill +rod$ce an antenna less affected

b" nearb" ob@ects, b$t t!is !as not "et been confirmed b" e2+eriment or anal"sis.

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Read Range

UHF RFID tags t!at co$+le ind$ctivel" can be constr$cted $sing a loo+ antenna rat!er t!ant!e conventional s!ort di+ole a test tag 0as constr$cted b" removing t!e di+ole from a

conventional 1e2as Instr$ments *lass % Beneration ' inla", leaving onl" t!e %( C %: mm

t$ning loo+.

Using t!is t"+e of tag, t!e read range of t!e segmented antennas 0as tested em+lo"ing a

conventional # *omm$nications MRE((( reader mod$le.

1!e 5 cm diameter segmented loo+ res$lts in a s!ar+l" defined read range of

a++ro2imatel" 5.5 cm on a2is, as s!o0n in Figure 8 .

1!e read +ercentage for t!e %( cm diameter segmented loo+ falls more grad$all" 0it!distance, b$t reads are +ossible o$t to a++ro2imatel" ?.5 to 4 cm along t!e a2is

see Figure 9.

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1!e detailed origins of t!e different be!avior of t!e small and large antennas !ave not "et

been e2amined, alt!o$g! since t!e +eak magnetic field on a2is is !ig!er and t!e fall-off

more abr$+t for a small antenna, some difference in be!avior is to be e2+ected.

1!e im+ortance of red$ced read rates is a++lication-de+endant. A single read $sing IS>

%?(((-E*-com+liant tags 0it! t!e inventor" +arameter = ( or = %, for instance,

re$ires a fe0 milliseconds.

In some a++lications, it is onl" necessar" t!at a tag be read once and an am+le amo$nt of

time ma" be available, 0!ereas in ot!er $sage models, a large n$mber of tags ma" be

+resent and m$st be read reliabl".

1!e a$t!ors !ave verified t!at at least in some circ$mstances reads are ro$g!l" oisson-

distrib$ted, so to ac!ieve 44 +ercent reliabilit" for a single read, for e2am+le, a tag s!o$ld

average E to ? +ossible reads.

1!erefore, t!e +ractical range res$lting from t!e be!avior s!o0n for t!e %( cm diameter

segmented loo+ 0ill var" de+ending on t!e e2act a++lication.

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It ma" also be +ossible to o+timi3e t!e antenna +erformance to im+rove reads at t!e E to ?

cm range.

Conclusion

Ind$ctive loo+ antennas can be constr$cted at si3es com+arable to t!e resonant diameter

b" dividing t!e loo+ into individ$all" resonant segments.

 Acknowledgments

1!e a$t!ors 0o$ld like to t!ank Gendall Gelsen of # *omm$nications for !is assistance in

constr$cting some of t!e antennas, and Ron >liver of Im+in@ for !el+f$l disc$ssions.

References

8/10/2019 Segmented Magnetic Antennas for Near-field UHF RFID

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%. G. Finken3eller, RFID Handbuch, *arl Hanser erlag, Mnc!en, Berman", :, A$fl.,

'((' ;nglis! translation available from ile" J Sons Ktd., *!ic!ester, UG, Second

;dition, '((:.

'. *. <alanis, Antenna Theor , 1!ird ;dition, *!a+ter 5, ile" Interscience, Hoboken,N#, '((5.

:. . St$t3man and B. 1!iele, Antenna Theor and Design, *!a+ter 5, #o!n ile" J

Sons Inc., Hoboken, N#, %4?%.