CA2047559C

CA2047559C - Magnetically-coupled, two-resonant-circuit, frequency-division tag

Info

Publication number: CA2047559C
Application number: CA002047559A
Authority: CA
Inventors: Fred Wade Herman; Ming Ren Lian
Original assignee: Security Tag Systems Inc
Current assignee: Security Tag Systems Inc
Priority date: 1990-08-03
Filing date: 1991-07-22
Publication date: 1999-07-06
Anticipated expiration: 2011-07-22
Also published as: AU635654B2; EP0469769A2; NO912997L; EP0469769A3; ATE152848T1; JP3224564B2; CA2047559A1; AU8132591A; EP0469769B1; DE69125985D1; DE69125985T2; NO912997D0; JPH04233696A; ES2100934T3

Abstract

A batteryless, portable, frequency divider includes a first resonant LC circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency; and a second resonant LC circuit that is resonant at a second frequency that is one-half the first frequency for transmitting electromagnetic radiation at the second frequency. The first circuit is coupled only magnetically to the second circuit to transfer energy to the second circuit in response to receipt by the first circuit of electromagnetic radiation at the first frequency. At least one of the resonant circuits includes a variable reactance element, such as a variable capacitance diode or varactor. In a variable reactance element of the first circuit, the reactance varies with variations in energy received by the first circuit for causing the second circuit to vary in reactance due to mutual reactive coupling to cause the second circuit to transmit electromagnetic radiation at the second frequency in response to the energy transferred from the first circuit at the first frequency. In a variable reactance element of the second circuit, the reactance varies with variations in energy transferred from the first circuit for causing the second circuit to transmit electromagnetic radiation at the second frequency in response to the energy transferred from the first circuit at the first frequency. Both resonant circuits include inductance coils that are disposed on a ferrite rod, for enhancing the magnetic coupling. The frequency divider is included in a tag utilized in a presence detection system.

Description

MAGNETICALLY-COUPLED, TWO-RESONANT-CIRCUIT, FRE~UENCY-Dlvl~lON TAG

r ~ ROUND OF T~ L.~l .lloN

The present lll~,e,lliull generally pertalns to frequency dividers and ls par-ticularly directed to portable. batteryless, frequency dlviders of type that are lncluded ln tags that are used in pl~_sel~cc ~let~ Cl lo~ ~ systems.

Portable, batteryless, frequency dlviders are descrlbed In U.S. Patent No.
4,481,428 to Llncoln H. Charlot, Jr. and ln U.S. Patent No. 4,670,740 to Fred Wade Herman and Lincoln H. Charlot, Jr.

The frequency divider descrlbed in the 428 patent inclll-l~ a resonAnt flrst clrcuit that ls resonAnt at a first frequency for receiving electroma~lot~-~ rArllAt~on at the first frequency, and a second 1~3u..A..l circuit that ls resnnAnt at a second frequency that ls one-half the first frequency for tr~n~...~lll..~ ele~t~ n~tlc radiation at the second frequency; and the two resonAnt circults are electrically connecte-l to one Anothçr by a sp~ ~o~ uctor switching device having gain coupling the first and second 1~s.~..A..l clrcults for causing the second circuit to transrnit elc~lr~..Aen. l~- rA.-liAtlnn at the second frequency solely in ~ ,ollse to unrectified energy at the first frequency provided in the first circuit upon receipt of ele~l~u...A~netic r~ t~on at the first fre-quency. Each 1~ circuit lnrl~ os a fixed c~p~ e C~J~ d in parallel with an inductance coil. In order to ~--I--I---I,e dif~icultles due to mA~net~-~ coupllng between the colls when tuning the rç~onAnt clrcuits to their respective 1~30l~ t frequencies the coils are disposed perpen~ lAr to each other so that the maenet1~ flelds of the two coils are orthogonal to each other. In one current embo~l'ment of this frequency divlder t~

that utll~zes an air core coil for the ~Irst i~a~ l clrcult and a ferrlte core coil for the second resonant circuit. the inside dlameter of the air core coil is mueh larger than the diameter of the ferlqte core coll to fi~rther ~ ,~ the magnetic coupl~ng between the co~ls.

~e frequency divlder descrlbèd in the 740 patent conslsts of a slngle resonant clrcuit conslsting of an lnductor and a dlode or varactor crnnf ~tt~d In parallel with the diode or varactor to derlne a resonant clrcult that detects ele:l-u...~radlation at a first predetermined rretluency and responds to sald detection by trans-mitt~ng electr--~n~nf t~c radiat~o~ at a second fi~ lu~ y that is one-hatf the flrst fre-quency wherein the clrcuit is l~sol~dlll at the second frequency when the voltage across the diode or varactor is zero.

Althougll the frequency dilrider described ln the 740 patent ls less complex than the r~ u~.lcy dlvlder described in the 428 patent. whereby the former may be manuractured less expensively and packaged more compactly In a tag for att~- hment to an artlcle to be detected by a presence detectlon system. the former also Is less elllcient In ~nitlattng frequency dlvlslon ~rom the ener~ of the detecled electromagnetlc radla-tlon since the frequency dlvlder clrcult 19 re~onant at orlly the second frequency.

8VI~Y C~F r~ l~V~A~ o~

The present l lv lltloll provldes a frequency dlvlder that 15 less complex and 2U ~IJCnS;IVG to m~mlfaeture and that may be packaged more coml-actly than the fre-quency dlrlder descrlbed In the 428 patent u~ithout a s~gnific~nt decrease ln perfor-mance.

~, ~ f~ ~J rj ''j r,~

A batteryless portable r ~ e~ dlvider according to one aspect of the present invention includes a first I~30~ l circult that Is resonant at a first ~requency for receivlng eleul ul-lagnetlc radlatlûn at the nrst fic lueIiuy; and a second ~~o~ l clr-cuit that Is I~sul-ant at a second r~u~.Icy that ls one-half the ~;t frequency for trans-mltting electr~lm~n~Hr radlation at the second r.~ en-;y wherein the fl st clrcuit is coupled only m~netic~lly to the second clrcult to transrer ener~y to the secorld circuit at the ~Irst frequency in response to receipt by the flrst circult ~ de~L~ eneti ~ radla-tion at the flrst frequency; and where~n th~ second circuit includes a variable le~ e element in whlch the reactance varies with variatlons in ener~r transferred frorrs the flrst circult for causing the second clrcult to tIdIIS IIiL eleul-v~ etic radlatlon at the second frequency in .eslucllse to the energy transferred from the first cLrcuit at the flrst frequency.

A batteryless. portable frequency divider according to another aspect of the present Inventlon includes a flrst ~usoIlalll clrcult that is resonant at a f~t ~requency for ~cce~ g electrorn~n~t~e radlation at the flrst freq~ency: and a second resonarlt cir-cuit that Is resonant at a second frequency that is one-halr the nrst frequency ~or trans-mltting ele~llv~ eti~ raciiatlon at the second rrequency; where-ln the hrst clrcuit is coupled only magnetIcally to the second clrcuit to transfer ener~y to the second circuit in response to receipt by the first circult of ele~l-o~I~agnetic radiatIon at the first fre-quency: anc-i wherein the first circult inchldes a varlable reaclance element ln whlch the reactance varies wlth varlatlons ln encrgy recelved by the nr~t clrcult for cau~ing the second circult to valy in rea~t~-n ~e due to mutLIal reactlve coupllng to cause the second clrcuit to L~dn~---lL electrom~gn~t1c radlatlon at the second frequency in response to the ener~ transrerred from the flrst clrc~llt al the first ~requency. In the prererred embodl-ment of lhis aspect of the present Lnvenllon~ the second clrcIlIt 1nrll~de~ a varlable reac-tance element in which the l~a~ ce varles with varlatJons in e~er~y transferred from the flrst circuit for causing the second c~rcuit to trarlsrnlt ele~ g. ~I'r radlatlon at the second frequency ln ~ ullse to the ener~y transferred from the Qrst clrcult at the flrst frequency.

Preferably each clrcuit tncludes a e~rac~t~nre and an inductance coll, wlth the coUs being dilsl~osed on m~gnf~t~e clrcuit means for enh~n~ng said m~ne~l~ cou plln~.

By utilizing only m:~ne~ic coupling between the resonant circults, co~tly ~d/or energy rll~s1pat~ng elem~nt~ that are used for ~le~triez~lly connect~ng the resonant ckcults in such a manner as to produce frequeney division in the prlor art frequency dividers are e~ n~t~f1 The present inventlon also provldes a tag inel-lA~n~ the frequency dlvider of the present Illv~llllc)n and a pl~sellce detectlon system ineluding such tag.

~ rlr1lt~nn~1 features of the present ~ Uoll are described in rclation to the descrlptlon of the prefer~ed embodiments.

BRIEF DE~3~;ku~ll~. C3F TH~ D~lVlNG

Flgure 1 is a diagrarn of a pl-,re.,~,d embodlment of the frequency dlvlder of the present inventlon.

Figure 2 ~ a dlagram of an allernative prefelTed embodiment of the frequency divlder of the present ll~ nUc,ll Figure 3 is a diagram of anolher a}temative preferred embodiment of the fre-queney divider of the present invention.

7 ~

Figure 4 ts a diagram of still another alternative preferrcd embodiment of the frequency di~lder of the present invent~on.

Flgure 5 ~5 a d~agram of yet another alternative preferred ernhocl'rn~nt of the frequency dlvider of the present invent~on.

Flgure 6 ts a diagram of a further alten~attve preferred P.ml~ofl~ . l of the rre-quency dlvider of the present invention.

Figure 7 is a diagram of another ~IGfitll~d embodiment of the frequency div~der of the present invention.

Flgure 7A ls a s-~hem~tlc clrcu~t dlagram of the frequency dlvlder of Flgure 1.

Ftgure 8 ts a diagram of an alternatlve preferred embod~nent of the fi'~lu~ncy divlder of the present invention.

Figure 9 i9 a dlagram of a presence tl~tect1on system accordin~ to the present Invention tncluding a tag accordlng to the present Inventton.

T~F*S;~ a.~ OF T~E ~r ~E~ED ;E:~Ol)l~qlENT8 Referrtng to ~Igure 1 a preferred e~nbod~nent of a frequency dlvlder accord-ing to the present invention includes a flrst resonant cJrcuit 10 co~ g of a capacitor Cl connected in parallel with an inductance coil Ll wound about a strai&t ferrite rod 12: and a second .cso.lal-l clrcuit 14 cul~slsllllg of a varlable c~rac1t~nre dlode or varactor D2 c-3nnect-od in parallel with a second inductallce coil L2 thal is also wound about the fe~ite rod 1~.

~J ~

The f~rst resonant circult lO is resonant at a ~Irst frequency fl for l~c~1~illgele~t~ lagnetic radlatlon at the first rl~,quc~lc~ fl; and the second resonant circult 14 Is resonant at a second frequency f2 tllat is one-half the flrst r,~ en~y fl ~or lla~ ~
eleel.u "s~nP~c radiation at the second rrequency f2. The first clrcult IO is coupled only magnetically by the ~errite rod 12 anci air to the second circult 14 to transfer energy to the second circuit 1~ at the ~irst frequency fl ln response to recelpt by the first circult lO of electrorn~gnPtlc radtallon at the rlrst frequency fl. The va~able capacitance diode or varactor D2 In the second clrcult 14 is a variable rea~t~nce ele-ment In which the react~n-~e varies with varlatlons In ener~y transferred rrorn the first circuit lO for causing the second circult 14 to L.dn~lllt cle~ ,",a~nei1c radiatlon at the second frequency f2 in ~ SC to the ener~y tran~Çerred Çrom the first clrcult lO at the flrst rrequency fl.

It is believed that the coll Ll of the first resonant clrcuit IO enh~nrPq the ele.;LIu~ n~tlr radlatlon at the flrst frequency fl that Is Induced in the coli L2 of the second resonant circult 14, and thereby decreases the requlred fielcl strength of electromagnetic radiation at the flrst rrequency fl necessary for accompllshlng fre-quen~ divlslon.

Because the values of the in~ cl;mces In each of the resonant circults lO, l4 are arrected by the respectlve posltions Or lhe colls Ll and L2 on the ferrlte rod 12 In relatlon to each other and in relation lo the enci~ Or lhe rod 12, the resonant clrcuit~ lO, 14 are tuned to their respectlve resonant frequencles fl and fa by adJusLIng the posi-tlons of the colls Ll and L2 on the rod 1~.

In order that the coiis Ll and L2 are not so highly coup]ed to each olher that ad~usting the posltion of a coli In one resonant circult so greatly af~ects the l~so~
frequency of the other resonant ctrcuit as a result o~ the interactive coupling between the h,vo coils as to make tur~ng Or bolh resonant circuits dlllicult, the co~Js Ll, L2 are wound wtth an inslde .l~ u l d' that ~s somewhat larger than the the cros~-se~t~n~l dimension d" of the ~errite rod 12. The coils Ll, L2 are wound on a non-m~1~et~c spac-ing element 16 that is adlustably mounted on the fe~ite rod 12. In the pr~f~rred em-bodLment, the rod 12 ha~ a rll~m~ter d" of a~ ely 0.125 ~ich (0.31 ~n.); and theco~ls Ll, L2 each have an Inside diameter of ~ uJ~ ately 0.1~ inch (0.38 cm.).

It has been determined that in order to al~comr1~h ~requency dlYislon, the coupling coefficient '~" between the inductance coll L1 of the first reson~nt circuit 10 and thç inductance coil L2 of the second ~e30n~l1 circuit 14 should be wlthin a range of zero to 0.6; and that conversion of the ener~fy of ele~ ,a~ radiatlon at the first l~sollalll frequency fl recelved by the flrst resonant circult 10 ~nto e~ net~f~rad~ation }adiated by the second resonant circuit 14 at the second frequency f2 ls most emctenl when the coupling co~clc~-L k is about 0.3.

In one example of the preferred emhQ~11mPnt of Flg~re 1, the colls L1 and L2 are wound on opposite ends of a 1.25 inch (3.2 cm.) long straight ferrlte rod 12 having a diameter of 0.125 inch ~0.3 cm.). Each coll L1, L2 Is ~p~ tely 0.375 inch (0.95 cm.) long, with the ends of the coils Ll, L2 adJacent the l~ cu~ ends of the rod 12 being posttloned +0.125 lnch from the ends of the rod 12. The coils should be at least 0.375 ~nch apart to prevent such interactlve cûupling as would ~ ke tuning of both resonant circults 10, 14 difflcult. Each coll Ll, 1,2 should not be longer lhan ap-proximately 35 percent of the length or the rod 12.

The frequency dlvider of this example is aetivated at signal levels that are several orders oi m~ tl~de below those of prlor art irequency dlviders of similar s~e.
Even more Irnportant the frequency dhtlsion emclency of this freq~lenc~ divider as deter-mined by its energy transfer function is very high, thereby enabling Ll~ ,s~ of elc~ u...~n~t1c ld~lUv.l at the f.c:~ue..~ dlvlded second ~ nalll rl~ e~ 2 having the same order o~ magnltude as provlded by prlûr art i'requency divlder~ that are mar~y tLmes larger.

In this ~Y~mp1P, the carac~t:~nce CI Is a 6~0 plco-farad c~racl~Gr and the dlode or varactûr DI has a varactor ~unctlon e"l~ar~ rc of ~p~ le~y 600 pico~
farads.

A variable rP~rar~t~nce c-iiode or varactor DI, which has one or a pluralit~
para]lel varactor 1.1.~. ll....~ that exhlbit a large and nrn1int~r change in capacitance with small level~ of applied alternating voltage, such as a zener dlode, is utilized as the voltage-responslve-variable-re~ct~n-~e elemerlt in the second .~,30.la,ll clrcuit 14 be-cause of its low cost. In nther embodl~ some other device ~Yh~httin~ the required Iarge and nr)n~ P~r r~lla~ ce varlatlon with applied alt,L~ lltlg voltage, and having sumc~ently low Ioss. and a hlgh Q factor, could be substltuted for a variable capae~t~nce diode or varactor.

Low-Tn~E~nPtlc-lûs3 ferromagnetlc materlals other than ferrite can be utili~ed in the rod 12 of the m~n.ot~r circuit means.

In an alt~---alh~ embodl~ nl (nût ~hown), the magnetlc clrcuit means used to couple the colls of the different .-,sollt~tlL clrcults ls merely alr. ~is cm~otllment ls the least compl~Y; and adequate maanellc coupling can be attained to provide a presence ~1~tec~1~n tag that is practlca1 ror sûme appllcatlon~ by cllsl)osl.~ the colls in close P~ [nllY to one another. However, this embodiment may be more dl~icult to tune to the ~ e~ ,s~l~alll irequencles in the absence of a feIrlte core whlch en-ables flne adJ~t~nPntq of t}le lGsc~n~lll firequencles by adJL~ ellL of the positlons of coils on the core, a~ dlscussed above.

In Irarlous other preferred embodlrntJnt~ the rnagnet3c circult mean~ for cou-pllng the colls of the different l~sonal~t c~rcuits are ferrlte el~ment~ hav~ng conflgura-tlons other than that of a straight rod. By changing the shape of the magnetic circult ~neans, the or~ent~tlon of the le~ JOnSe Or a tag c.~ ng the frequency dlvider rnay be tailored to specl~ic appl'c~l~n.c and conflguratlons of e~citing electromagnetlc fields at the first resonant frequency fl.

In one such embof~mPnt, as shown in Figure 2, the Tn~gn~tlc ci~ult means includes an ~shaped ferrlte element 20. In this embodiment, the ~requency divlder i:n-cludes a first resonant clrcuit 22 col,~bll-~g of a capacitor Cl C(J~lllf~ l~'cl In parallel w~th an inductance coD Ll wound about one end of the ~shaped ferrlte elernent 20; and a second It:so~ t circult 24 co~ ..e of a variable c~ra lt~n~e diode or v~ra- tor D2 conn~-~ted ln pa~llel with a second inductance coil L2 that ls wound about the other end o~ the l,shaped ferrlte element 20. In other respects the constructlon of the fre-quency divider of Flgure 2 ls ~ub~ect to the col1dlllolls stated above wlth respect to the constructlon of the frequency tllvlder of Ftgure 1, such that the ~l~el~UolJ of the fre-quency divider of Flgure 2 ls the same as the operatlon of the frequen~ divlder of Flgure 1.

In another such embo(l'merlt~ a~ showrl In Flgure 3, more than two magnetlc poles are l~lco~ ted into a mz~enetlr clrcuit element 30 ror controlllng thc or~entatlon and amount of coupllng of the flrst l~,s~ allt frequency r~ and the ~econd re~;onant ~re-quency f2 to the ~urrountllng space. In thls embodlment, the frequency dlvider ln-c]udes a flrst resonant clrcult 32 con~lstlng of a capacltor Cl connected in parallel with an Inductance coil Ll wound about one end of the ferrlte element 30; and a second ~solla-~L clrcult 34 consisting Or a varlable r~rac~t~n~e diode or varactor D2 i ~nnect~ in parallel wlth a second ~ductance coll L2 that ~s wound about the other end ~ the ferrite element 30. In other respects the constructlon of the rrequency divider or Flgure 3 is sub~ect to the condi~lons stated ab~ve with re~spect to the con-struction of the r~ , y divlder Or Flgure 1. such that the upeldLlon oî the rr~u~n(:y dh~tder of Figure 3 is the same as the c,p~. dlkJll of the frequency d~ider of Figure 1.

The ,..~ t~ circuit rmeans may i~clude two or Drlore separate ferrlte rods that are closely m~enetlc~lly coupled to each other to optlmlze perfc,l l-al-cc and/or provlde a m~netlc circuit with a larger aperture than can be achleved wlth a single ferrlte rod of the m~m1m~ manufacturable length. Currently ferrite rods cannot be cheap1y manufactured with le~th-to-dlameter ratlos greater than ten or twelve. By dlsl~osl"g a plurality of straight ferrtte rods end to end, the ape~ture of the ~ tl-~
circuit can be enlarged.

Also by providing an air-gap in the m~netlc clrcuit between separate ferrite rods upon which the coils of the separate resonant circuits are l~s~)ec~ disposed, the interactive m:l~netlf coupling between the coils is de~-~ased by decreasing the reluctance between the coils, thereby m;~king the separate l~solla--l circuits easler to tune by adJusting the posltions of the coils on t}le rods.

In one embodiment utilizlng a plura]lty of ferrom~ ne~lr rods In the magnetlc circuit, as shown in Flgure 4. the magnetlc clrcult meaws includc two str~lght ferromag-netic rods 40, 42 disposed end to end wlth an alr gap 44 th~l-;bct~ l-. In this embodl-ment, the frequenc~r dlvlder lnchldes a rirst resonant circult 46 c~ nq'qtlnl~ of a capacitor C L connected in parallel wlth an iTlductance coll l,l wound about one of the ferrlte rods 40; and a second resonallL circult 48 consistiTlg of a variable c~r~rltanre diode or varactor D2 conn~eted in parallel with a second inductance coii L2 that is woundabout the other of the ferrite rods 42. In other respects the constructlorl of the fre-quency dlvider of Figure 2 is subiect to the contiltions stated aboYe with respect to the construction of the frequency diYider of Figure 1, such that the operation of the fre-quency dlvider of Figure 4 ls the same as the operatlon Or the rr~,~u~.lcy divlder of Flgure 1.

In another emho-limPnt of the present inventtoFI, as shown in Flgure 5, the var~able reactance element of the second le3~ ll clrcu~t is a varlable induc~nct ele-ment rather than a variable capacltance ele~nent~ as in the Pmho~lment describedabove. In thls emboAlment. the frequency divider includes a fkst ~~s~la-ll circult 50 conslstll,g of a capacitor C1~ connected in parariel wlth an inductance coil L1~: and a second resonant clrcuit 52 cv~ g Or a second capacltance C2~ connected tn parallel w~th a variable inductance element L2~. The flrst l'l,S;)11211t circuit 50 and th~ secs~nd resonant circuit 52 are coupled only magnetlca]ly by such m~nf~tlc circult means as described above in relatlon to the dic~l llJllu-l of the other embo~ e~ , The variable inductance element La~ incllldes an inductance coil 56 and a low-loss rerrom~en~t~r matertal 58 that exhibits a large change ~n perrneabllity wlthin the desired voltage ra~e of the incident ele~ aen~tlc radiatton at the first predetermlned frequency fl. The low-~oss fenom~netl~ material S8 is placed in the magnetic circuit of tl7e lnd~ n~e coll 56. Tn thls embodirnent, not only are the bulk magnetic characteristics of the fer-romagnetlc matertal 58 l~ o-lanl, but also the phy~ical shape of the rerroma~netl~
mater~al 58 has proÇound eifects upon the frequency divlslon characteristlcs of the second resonant ctrcult 52. Ferrite materlals are preferred for the ferr~Tnz-enet~c mater~al 58. The rnaterial formulation Js selected to glve the deslred characterlstlcs at the chosen operatlng rl~ u~l~.y. Wllh the proper design of resonant circults 5Q, 52, operatlon is pos~lble from the low kllohertz region through the mlcr)w~v., reglon. In other respects the construction of lhe frequency dlvlder of ~lgure 5 is subJect to the condll~ons stated above wlth respect to the constructlon of the frequency dlvider of Figure 1, such that the operatlon of the rrequency divlder of Figure S is the same as the operatlon of the ~ ucllcy dlvider of ~tgure 1.

In the emborllmPntc~ ur the fi ~ue~lcy dhider of the present invention described above, the l~sona~ll circults have been de~cribed a~ including i ~ lallce coils and capac~t~n~es because the described embodiments are deslgnPd for use at rela-tively low frequencies. In en~ho~ n~.q of the frequency divlder rlP~ nPd l~or use at hlgh irequencles, such as those in the ~I.h~ reglon. the ~esl;~nal~l circult~ include ele-ments embodylng rniero-strlp, strip-line, and/or cavlty technolo~y.

Also, in iurther ernbodiments of the frequency divider of the present inven-tion, the sccond leS(Il~alll circuit may be a device that mechanically resQn~tPs at the second frequency. A ~--tch~ 11y ~~so~lal.L device is equivalent to a parallel LCresonant circuit.

In one such c.llbodl~ l, as shown in Flgure 6, the r~ ucl~cy divlder in-cludes a rirst ~escl~alll circult 61) c.,nsblillg of a capacltor Cl~ connected ln parallel with an inductarlce coil Ll-; and a second resonant circult 62 CO~ of strlp 64 of saturable magnelo~ l;U;~ m~net~r materia1 that i9 magnetomPrhzmlr~lly resonant at a frequency f2 that is one-half the resonant rrequency r, Or the rirst resonant circult ~0.
The coil Ll of the first resonant clrcult 60 Is Tn~nPlk~a11y coupled to the mag-neto.l.~rha~ al1y resonant strlp 64 by belng wound around the strlp 6'~. The lnslde dimenslon of the coD Ll- Is spaced from the strlp 64 so as not to be so lightly wound around the strlp 64 as to make tuninL~ of lhe first resonant clr~ul~ 60 diffl~ult, The strip 64 is met~hzm~cz~lly re_o~alll in the length e~tens~n~l mode and functlons as a varlable rea~t~nce core of lield level variable permeabillty materlal to con~ert elccll4~ netlc radiatlon recelved by the ~irst resonant circuit 60 at the fre-quency fl into electrom~n~tl~ radlation at the ~requency f2 that i9 one-half the ~al l frequency fl of the ilrst resonant circuit 6~.

In the preferred embodlment t:he strlp 64 i~ a satur~le mag~ o~l.lclhc, arnorphous fc~ a~ c materlal such as descrlbed in U.S, Patent No, 4,727,360 to Luclan G. Ferguson and Lincoln H. Charlot, Jr.

In other respects the constructlon of the fi~luen~y dlvlder of Flgure 6 is sub--S ~ect to the ronflltlr~n~: stated above wlth respect to the con~ ;U~l of the fi~uen~;y ~vider of Figure l, sucb that the operation of the fiequency dlvlder of Flgure 2 1~ the same as the operation of the frequency dlvider of Figure 1.

Refe~ring to Figure 7, a pre~erred embodiment of a frequen~y dh~ider accord-ing to the present Illvelllloll ~n-~h1-lPs a ~irst l~sonal,l clrcult 70 co~ of a variable ca~ re dlode or v~idcLfJI Dl~ connf~cted in parallel u~lth an 1.~7~ c coil Ll-wound about a straight ferrite rod 72; and a second I~JndllL CiTCUlt 7~ C~J~IS~ g of a varlable capac1t~n~e diode or v~ld~Lol D2- connPcted in parallel with a second induc-tance coil L2- that is also wound about the ferrite rod 72.

~he first resonant circuit 70 is resonant at a first frequenc~ fl for recel~ing ele~ . ., enPtlr rad~ation at the flrst frequency fl; and the second resonant circu~t 74 Is -,s~,llanl at a second frequency f2 that is one-half the first frequency fl ~or ll,...~...lil1~.g clc~ l,agnetlc radiation at the second frequency f2. The first clrcult 70 is coupled on~ k~11y by the i~errlte rod 72 and air to the second ckcu~t 74 to ~ransfer ener~y to the second circult 7a, in response to recelpt by the nrst clrcult 70 of elc~ n~t~-~ radlatlon at the flrsl frequency fl. The varlable capac~ n~e d10de or varaetor Dl- in the first clrcult 70 Is a varlable reac~zlnne element In whlch the reac-tance varles wlth vzlr1~t1nnq In energy recelved by the first circuit 70 for causing the second clrcuit 74 to vz~ In rezlctz~n~e rnutual reactive coupling thereby causing the second circuit to l~ ll ele~ll.,...zl~ne11c radiation at the second frequency f2 in l~S~C nse to the ener~r transferred from the Q~;t circuit 70 at the flrst frequency fl. The - i3-variable ca~)a~ e dlode or varaotor D2- in the seconcl circult 74 15 a varl~ble reac-tance element in which the re~ct~n-~e varies wlth variations in ener~y transferred :fiom the first circult 70 for causing the second circult 74 to il~n~ lL elccl~ nel1c radia-tion at the second frequerlcy f2 in response to the energy transferred ~rom the first clr-C~lit 70 and also aided by the mutua1 reactlve coupling of the first clrcuit at the flr3t frequency fl.

AS best shown in Figure 7A, the sense of the windings of the coils LI~ oi the first and second l~s~ L circuits 70, 74 is such that the start of the wlndlng of the coil Ll~ of the fl~st re30~ t c~rcult 70 Is co~ rcl~ d to the anode of the var~able c~rarlt~n--e diode DI~, and the start of the wlnding of the coil L2~ of the second resonant clrcuit 74 is co~ P~Ietl to the cathode of the varlable ~r~ nce diode D2C.
This manner of co~ cl~-~n achicves a power limiting action by reducing overl~ 11n~ ~-fects at high input fleld le~els as the va~iable cap~ t;~.1ee diodes Dl~, D2' tend to son-duct in the forward diode region of their conducti~ity and thereby shunt some cur~ent across the ~ ecU~,~ coils Ll- and L2-.

It Is believed that the coil LI- of the first resonant circult 70 enh~n(~e5 the elc~Lrv.--~net1e radiation at the ilrst frequency fl that l.s Induced in the coll L2- of the second lt;sol~lJl clrcult 74, and thereb~ decreases the requ:lred fleld strength of elc~ net~c radlatlon at the fJrst frequency fl necess~ry for accornr11qh~ng ~re-quency dlvlslon and also aldes the varylng of the re~t~nce of the ~3econd resonant ck-cuit by mutual coupllng due to the varylng reactance of the f:lrst .~son~.nt &Ircuit.

Because the values of the lnd~tct~nces In each of the resonant circuits 70, Y4 are ~ected by the ~ e~:lh ~. pc slt1()n~ of the coils LI- and L2~ on the ferrlte rod 72 in relation to each other and ~ relation to the ends of the rod 72, the ~sw~ l c~uits 71), 74 are tuned to thelr lt;~lJc~:Lh~, resonant frequencies fl and f2 by ad~usU~ the ~si-tions of the coils Ll' and L2~ on the rod 72.

In order that the coils Ll- and L2- are not so hlghly coupled to eacl~ other that adjusting the position of a coil In one resonant circult ~o greatly af~ects the l~una~lt rl~u~ ;y of the other l~,so~ l clrcuit as a result of the ~ Jd~~ SOUPI~ng between the two coils as to make tuning Or both resonant circuits difficult, the coils L1-, I2- are wound wlth an inside ~l~men.e~t)n d' that is sv~ a~ larger than the the cross-sectlonal tll~ .ll d" of the ferrite rod 72. The c~ils Ll-, L2- are wound on a non-m~gnPt~ spacing element 76 that ls ad~ustab~r mounted on the ~e~lte rod 72.

It has been deter~ined that in order to ~ct.~cmrll~h r~ u~l.y dlvlslon, the coupling co~: m~ l "k" between the 1nt~ e coll Ll~ of the first l~svll~lL c1~cuit 70 and the 1n~lct~n~e coii L2~ of the second resonant circuit 74 should be within a range of zero to 0.6; and that conversion of the ener~y of cle~ u,llagnetic radiatlon at thç first l~:sona.ll frequency fl received ~y the ilrst resonant circult 70 into ele~Llu,..dgnetic radiation radiateci by the second l~s~,nallL c~rcuit 74 at the second Ç~que~ y ~2 is most efficient when the coupling coPm~ nt k is about 0.3.

In one example of the preÇerred emhod~ment of Flgure 7, the coils I,lg and L2 are wound on opposlte ends of a 1.2~ inch (3.2 cm.) long strai8ht ferrlte rod 72 havlrlg a .~ mete~ of 0.125 inch (0.3 cm.). Each coil Ll-, 1,2- ls appl~ ~-at~1y 0.37~ inch (0.95 cm.) long, with the ends of the coils Ll-, L2- a~lJar~n~ the respective ends of the rod 72 being posl~ lo.~Pd ~0~125 inch from the ends of the rod 72. ~e coils should be at least 0.375 inch apart to prevent such lnteractive coupliIlg as would make tunlng of both c,l~al.L clrcuits 70, 74 difficult. Each coil Ll-, La- should not be longer ~han ap-proYIm~t~1y 35 percent of the length of the ro~i 72.

The frequency divider of this example is activated at signal levels that are scveral orders o~ ...a~ cle below those of ~lor art frequency divlders oi slmDar size.
Even more illlpolL~il the firequency divislon emcll,~y of this ~requency divider as deter--1~

mined by its ener0r transfer funct~on Is very hlgh, ~ by ~n~h1~r~e l~ L~I(.
cle~ 1 Ir radiation at the Çle~u~ vided second 1eSO1~At, &~ue~icy fa havlngthe same order of m~gnltude as plu.ldcd by prlor art frequency dh~lders that are m~ny times larger.

In thls ~mple the variable c~r~~ltanre diode or varactor Dl~ ha~ a varactor Ju~ o~l c~ra~1t~nre of appl.J l...a~ly 600 pico-fardds and the varlable cap~cltanre diode or va~d~:Lc~f D2- has a varactor )unction r~p~r1t~nre of ~ ately 800 plco-farads.

~n an integrated cir~uit ~ I,o~ both of the variable e~ e~t~nee diodes or ~ dcL(Jl~ Dl-, D2- are formed ~vlth a co~nn~nn cathode. In this ~ ho~ ent fre-quency division occur~ over a wider range because of l~niting action of the va}iable r~rar~t~nre dlodes crvaractors Dl~, D2~.

Varlable r~paclt~n~e dlodes or varactors Dl~, D2- which have one or a plurality of parallel v~ cl,ul ,lunrt!nn~; that exhibit a large and nonlJne~r change in rRrRr1tRnce v~th small le~rels oi applled ~lt~ t1n{~ voltage, such as ~ner diodes. are utilized as the voltage-responslve-varlable-reRrtRnre rl..~ I the f~rst and second 1e3Jila1ll circults 70, 74 because of thek low cost. In other embodirnents some other devlce ~h~h'~ng thc required large and nonlinf~Rr l'SI~ f ll~ll 'f'C variatlon wlth applled alternatlng voltage, and having st1mrlent~ IOW 1033, and a hlgh Q factor, could be sub-stltuted for a varlable rRr~feltance dlode or varactor.

Low-...1~nellc-loss ferromRgrl.ot1r materlals other than ferrNe can be utllized n the rod 72 of t~e maZ~netir clrcult means.

~ IJ~ 3 In an ~ltf~ ttve emholllmPnt (not shown), the magnetic circult means used to couple the coils of the different resonant ctrcuits ls merely alr. ~is e-.-hf"1l".. ,.1 Is the least cn~nrleY- and adequate ~ coupling can be attalned to provlde a presence d~Lf~l;n~- tag that is practlcal for some app~ tlon~ posl.lg the colls In close ~ ily to one another. Hovever, this r~mhr~ltment may be more dlfflcult to tune to the ~ ceth~ l~sullallL frequencles In the absence of a ferrlte core wlth enables f~ne ad~us~mlents of the resonant frequencies by ad~u~ ellt of the positions of CO~15 on the core, as ~ uqsed above.

In va~ious other i,l-,fel-~d r~ml-o~im~ntQ (not shown), the ma~r~t~ clrcuit meanQ fvr ~ouy]ll-g the coils of the different l~so~ L clrcults are i'e~rite ~oh~mr~ntQ
hav~g co~ul~Ll(,lls other than that of a straight rod. ~3y rh~n~in~ the shape of the mzl~net~n cirC~llt means, the n~ n~l of the l~yu~-se of a tag ~"..t~.t..ll,& the fre-q~lenqy divider may be tallored to speclfic appllratlon~ and configurations of excltlng elecllV~ P~ flelds at the flrst l~sol~alll frequeney fl. In one such embodl.. ~, the magnet~c clrcuit means includes an L~shaped ferrlte elempnt~ wlth the lnductance coil of one resonant clrcuit being wound about one end of the L,shaped fe~lte elemerlt; and the inductance coll of Ule other ~ ~S0llatl~ circult belng wound about the o ther end of the ~shaped ferrite elf~nent. In other respects the co~ uellon of such a freq~ency dlvider is su~ect to the co~ ---4 stated above wlth re~pee~ to the ~:o~ llon of the fre-quency dlvider of Flgure 7, so that the operatlon of sueh a frequenc~ dlvlder Ig the same as the operatlon of the frequency dlvider of Flgure 7.

In another such Pmho~lm~nt, more than two felTlte rods are lllc-w~c~l~led into a m~Er etle clrcult element for controlllng the orlf~nt:~t1rm and amount of coupling of the first ~~so~llL frequency fl and the second l~501l~ll frequency f2 to the sulluullflillg space. In other respects the cu~ Llu~:llull of the L~lu~ y dlvlder of such an eII~bodi-ment is sub~ect to the cQ~fl~ s stated above wlth respect to the const}uctlon of the frequeney dlvlder of FJgure 7, such that the operatlon of ~e frequency dh~lder of sueb an embodl~ L is the same as the ~ llu l of the rl~clucll~y cihrlder of Flgure 7.
The magnetie elreult mean~ may Jnelude tYvo or more separate ferrlte rod~
that are elosel~ ma~nPt~ 11y eoupled to eaeh other to optlmi~e p~lrO~ cc and/ôr provlde a ~ Ir~ ekclllt wlth a larger aperture than ean be achleYed wlth a slngle ferrlte rod of the ...~ m~nl-f~ctl1rable length. Currently ferrlte }ods eannot be cheap~ m~mlf~tllred with length-to-dlameter ratios greater than ten or tw~lve. By disposlng a plurallty of straight ferr~te rods end to end. the aperture of the magnetie clreuit can be enlarged.

lû Also by yluvl~ g an alr-gap in the m~ etle elrcult between separate fe~rite rods upon whlch the eo~ls of the separate resonant elreuits are l~ Je~U~ l4pç~erl the interactlve m~nPtle eoupling bet7veçn the colls i5 decreascd by deulca~lg the rPlllct~n-~e between the eoils. thereby ma~dng the sepa ate resonant circults easler to tune by adlustlng the posltlons of the eoils on the rods.

In one emhofl1m~pnt utDizing a pluralit~r of ferrrm~enetlc rods ln the magnetlc circuit, the mZ~etic~ clrcult means include two straight ferromagnetic rods dlsposed end to end wlth an air gap tllerebetween. In thls embodirnent, the ind,lcl~llct coll of the first l.,son~-~l circult ls wound about one of the ferrlte rods, and the ln,dllctance coll of the second l~,s~ alll clrcult is wound about the other o~ the ferrlte rod~3. In other respects the construction of the frequency dlvlder of such an embodlment ~ sub3ect to the c"~ stated above wlth respect to the construction of the rl~,qlle~luy dlvlder of Figure 7, so that the upeldlloll of the frequency divider of such an eïnbndiment is the s~}e as the operation of the frequer~ ivider of Figure 7.

" ~ j 1 3 In another emboditnent of the present inventivn, as shGwn in Figu~e 8, a fre-quency divider according to the present i..~ lloll includes a flr~t 51...30~ circult 80 ron~C;tlng of a variable cap~c1t~n~e diode or varactor Dl~- conne~ted in parallel with an inductance co31 Llfi- wound abollt a sLraight ferrlte rod 82; and a second resonant cir-cult 84 consi~ lg of a r:~r)arlt~nre C2-- conn~cted in parallel with a second 1nrll~ct~nre coil L2-~ that is also wound about the rerrtte rod 82.

The first resonant c~rcuit 80 is l~30~ L at a first frequency fl for ~tcclvlng electrom~gn~t1r radlation at the first fr~quency fl; and the second resonant circuit 84 Is resonant at a second frequency f2 that Is one-half the i~st frequency fl for lI 0~ e electrr,m~gn~-t1r radiation at the second frequency f2. The first circuit 80 lg coupled only m~nf~t~c~11y by the ierrlte rod 82 and air to the second circuit 84 to transfer energy to the second circuit 84 ln rc;,~ e to receipt by the first circuit 80 ofelectrr,m~n~t~-~ radiation at the first frequency fl. The variable r~r~C~t~nre diode or varactor Dl~- in the rlrst circuit 80 is a varlable reactance element ln which the reac-tance varles with variations In ener~fy received by the first circuit 80 for causing the second clrcuit 84 to vaIy in r~rtz-nre by mutual coupling to lransmit elcclro~ g.l~t~r ra~ t1r,n at the second frequency f2 in response to the ene-r~y transferred from the first circult 80 at the first rrequency fl.

Although the embodiment of Figure 8 is very Inerflclen~ in relation to the ern-bodiments dtscussed above, it does functlon as a frequency divlder beca1lse .scme var1-able reactance is reflected Into the second resonant circuit 8~ by reason of the mz)~netlc coupling of the two resonant circults 80, 84.

In other respects the constn1ction of the frequency divider of Figure 8 Is sub-~ect to the cnnr11t10n~: stated above with respect to the construction of the r~q,len.y divider of Figure 7, such that the operation of the L~ y dividier of Figure 8 ls the same as the operation of the irequency divider of Figure 7.

In other etnho~l1ment~ of the Jl~,qu~ ;y divider ~f the present im~ention, the ln(l... ~ re coils of the ~irst and/or l~,so~l~-i circults may aLso be varlable l~a~ n~e el~ ".- .~t~ Such varlable ~ lct~nce ~1-"........... t~ are plu~ldc~l in a(l~Hon to the varlab3e ~ra~t~nne dlode or varactor in the flrst l~sc,nal-l circult in the r~llh~lh~ of Flgure 7, or ln additl- n to the varlable e~ diodes or varactors in the flrst and second l~so~ lL circuits in the ~A.Il~odllllcllL of Flgure 7. A variable inductance element ls formed by windlng a coil about a low-loss ferrorn~gnrttr materlal 58 thal; e~iblts a large change in prnnP~hlllty within the desired ~voltage range of the incident e3e.:L-u--lag-netic radlation at the resonant frequency of the respective resonant c3rcult. In these ~ .~,h4-~l.. ,.t~, not only are the bulk ",a~.. .Itr chara~ .bUcs of the ferrom~t~r m.~st~-.rl~ ul Las~t, but also the physlcal shape of the ~ ag- ~etlr m~3terl~l has profound ef~ects upon the fre~uerlcy dhlslorl characterlstics of the l~Aonallt clrcults.
Fe~Tite materlals are preferred ~or the ferromz~f~nPt1-~ mat~ z,l The mater~al forrnt~ nn Is selected to give the cles~red characteristlcs at the chosen operatlng frequency. Wlth proper deslgn, u~J~.dU~ ls posslble from the low kilohertz regJon through the llllUIUWdl,~, region.

In the ~;111~/0~ i of the frequency divlder of the present Inventlon descrlbed above, the l.,sonallL clrcults have been descrlbed as Irl~?t~ rl~ Indltct~nre coJls and C~rJ'AC~ f es because the descrlbed embo-1l-ne--1q are desl~ed for use at rela-tlvely low frequencles. In embc(11ment~ of the frequencsr dl,rlder ~e~lgned for use at hlgh frequencles, such as those In the Illlc~uwdv~, region, the resonant clrcults Includc ele-rnents e.l-bo~lylng mlcro-strlp, strlp-llne, and/or c~vlty tenhnnlcgy.

Ihe frequency dlvider of the present InventloII Is utillzed in a preferred em-bodlm~nt of a presence detectlon system acculdl~-g to the present InYention, as shown in Flgure 9. Such ~;ystem Int~ a t~ .4~ 90, a ta~ 91 and a tietect1nn system 92.

~2û-~1 r~ / ? ~J ~~

The l~ f~ lld~ -lt4 an elcclr~ ttr radiation signal ~4 of a i~rst predetermlned r, ~ n.;y ~nto a survelllance zone 96.

The tag 91 is ~t~-~hed to an article (not shown) to be detected within the sur-ve~llance zone 96. The tag 91 includes a batteryless. portable f~l ~uL~ r dtvider in ac-C~ i3LllCC with the p~esent ~ention, such as the h~uc,n~:y dlvlder de_~lllbcd above with lef~ ncc to Figure I or Figure 7.

'rhe ~ tect~r~n system 92 detects elecllu~ g,~ llc radiation 98 in the su~veil-lance ~one ~8 at a second ~.~d~ h-ed irequency ~hat is one-half the first predeter-mirled ~ u~ney, and thereby detects the plesc.lce of the tag in the su~veillance zone 96.

The presence (1Ptpctlon system utilizing a tag lnrl~ ng the f ~ Jell~y dh~lder of the present Illvr.-l-Uu-- is used for various app~ tlnTIr~ that take ad~irantage of the si~
and ef~iclency of such Ç.~,q~ ;y d~rider, Including appllcS~tl~nq utDlzing longer range tags, and appl~ tlon.c utll~zing small tags requirlng onlyr a short co~ L~ r~tton range.

In one example, small tags including the frequenc~r dlvrlder of the present ln-ventlon a~ subcl-laneously Implz-nted ln anlmals and such anlmals are counted by the y~s~ ce ~letectinn system.

In anûther ~ 1~ le, small tags lncludlrlg the f~equency clivlder of the present In~,~.-llùll are lmrl~nted ln non-metallic canlsters of exploslves and sueh c~n1qs~rs a}e ~tected by the ~ cc s~e~ect1on system.

In st~ll another ~rnrle. tags 1m~1url~ng e~nh~ -e~l.q of the L~u~ ;y di~der of the present lnv ~.lUc,n that are relatlvely large in one ~11. . " . .~ are imrl~ntf~d ~n non-metallic gun stocks and the guns are detected by the presence detect~on ~qystem.

Claims

1. A batteryless, portable, frequency divider, comprising a first resonant circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency; and a second resonant circuit that is resonant at a second frequency that is one-half the first frequency for transmitting electromagnetic radiation at the second frequency;
wherein the first circuit is coupled only magnetically to the second circuit to transfer energy to the second circuit at the frequency in response to receipt by the first circuit of electromagnetic radiation at the first frequency; and wherein the second circuit includes a variable reactance element in which the reactance varies with variations in energy transferred from the first circuit for causing the second circuit to transmit electromagnetic radiation at the second frequency in response to the energy transferred from the first circuit at the first frequency.

2. A batteryless, portable, frequency divider, comprising a first resonant circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency; and a second resonant circuit that is resonant at a second frequency that is one-half the first frequency for transmitting electromagnetic radiation at the second frequency;

wherein the first circuit is coupled only magnetically to the second circuit to transfer energy to the second circuit in response to receipt by the first circuit of electromagnetic radiation at the first frequency; and wherein the first circuit includes a variable reactance element in which the reactance varies with variations in energy received by the first circuit for causing the second circuit to vary in reactance due to mutual reactive coupling to cause the second circuit to transmit electromagnetic radiation at the second frequency in response to the energy transferred from the first circuit at the first frequency.

3. A frequency divider according to Claim 2, wherein wherein the second circuit includes a variable reactance element in which the reactance varies with variations in energy transferred from the first circuit for causing the second circuit to transmit electromagnetic radiation at the second frequency in response to the energy transferred from the first circuit at the first frequency.

4. A frequency divider according to Claims 1, 2 or 3, wherein each circuit includes a capacitance and an inductance coil, with the coils being disposed on magnetic circuit means for enhancing said magnetic coupling.

5. A frequency divider according to Claim 1, wherein each circuit includes a capacitance and an inductance coil, with the coils being disposed on magnetic circuit means for enhancing said magnetic coupling, and wherein the magnetic circuit means consist of a pair of separate straight ferromagnetic rods that are aligned end to end.

with the coil of one resonant circuit being disposed on one rod and the coil of the other circuit being disposed on the other rod.

6. A tag for use in a presence detection system, comprising a frequency divider; and means for fastening the frequency divider to an article to be detected by the presence detection system;
wherein the frequency divider comprises a first resonant circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency; and a second resonant circuit that is resonant at a second frequency that is one-half the first frequency for transmitting electromagnetic radiation at the second frequency;
wherein the first circuit is coupled only magnetically to the second circuit to transfer energy to the second circuit at the first frequency in response to receipt by the first circuit of electromagnetic radiation at the first frequency; and wherein the second circuit includes a variable reactance element in which the reactance varies with variations in energy transferred from the first circuit for causing the second circuit to transmit electromagnetic radiation at the second frequency in response to the energy transferred from the first circuit at the first frequency.

7. A tag for use in a presence detection system, comprising a frequency divider; and means for fastening the frequency divider to an article to be detected by the presence detection system;
wherein the frequency divider comprises a first resonant circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency; and a second resonant circuit that is resonant at a second frequency that is one-half the first frequency for transmitting electromagnetic radiation at the second frequency;
wherein the first circuit is coupled only magnetically to the second circuit to transfer energy to the second circuit in response to receipt by the first circuit of electromagnetic radiation at the first frequency; and wherein the first circuit includes a variable reactance element in which the reactance varies with variations in energy received by the first circuit for causing the second circuit to vary in reactance due to mutual reactive coupling to cause the second circuit to transmit electromagnetic radiation at the second frequency in response to the energy transferred from the first circuit at the first frequency.

8. A tag according to Claim 7, wherein wherein the second circuit includes a variable reactance element in which the reactance varies with variations in energy transferred from the first circuit for causing the second circuit to transmit electromagnetic radiation at the second frequency in response to the energy transferred from the first circuit at the first frequency.

9. A presence detection system, comprising means for transmitting an electromagnetic radiation signal at a first frequency into a surveillance zone;
a tag for attachment to an article to be detected within the surveillance zone, comprising a frequency divider and means for fastening the frequency divider to an article to be detected by the presence detection system; wherein the frequency divider comprises a first resonant circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency; and a second resonant circuit that is resonant at a second frequency that is one-half the first frequency for transmitting electromagnetic radiation at the second frequency;
wherein the first circuit is coupled only magnetically to the second circuit to transfer energy to the second circuit at the first frequency in response to receipt by the first circuit of electromagnetic radiation at the first frequency; and wherein the second circuit includes a variable reactance element in which the reactance varies with variations in energy transferred from the first circuit for causing the second circuit to transmit electromagnetic radiation at the second frequency in response to the energy transferred from the first circuit at the first frequency; and means for detecting electromagnetic radiation t the second frequency in the surveillance zone.

10. A presence detection system, comprising means for transmitting an electromagnetic radiation signal at a first frequency into a surveillance zone;
a tag for attachment to an article to be detected within the surveillance zone, comprising a frequency divider and means for fastening the frequency divider to an article to be detected by the presence detection system; wherein the frequency divider comprises a first resonant circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency; and a second resonant circuit that is resonant at a second frequency that is one-half the first frequency for transmitting electromagnetic radiation at the second frequency;
wherein the first circuit is coupled only magnetically to the second circuit to transfer energy to the second circuit in response to receipt by the first circuit of electromagnetic radiation at the first frequency; and wherein the first circuit includes a variable reactance element in which the reactance varies with variations in energy received by the first circuit for causing the second circuit to vary in reactance due to mutual reactive coupling to cause the second circuit to transmit electromagnetic radiation at the second frequency in response to the energy transferred from the first circuit at the first frequency; and means for detecting electromagnetic radiation at the second frequency in the surveillance zone.