Resonant tag circuits useful in electronic security systems

A resonant tag circuit is provided which is useful in conjunction with an electronic security system for preventing theft of articles from a protected area. The tag circuit comprises an electrically insulating substrate having a spiral conductive path on each surface of the substrate. The spiral conductive paths are positioned to overlap each other to effect distributed capacitance. The degree of overlap of the distributed capacitance portion of the conductive paths is such that it provides at least 70%, preferably 90% and most preferably 100% of the total overlapping portions of the circuit.

BACKGROUND OF THE INVENTION 
This invention relates to a resonant tag circuit design useful in 
conjunction with an electronic security system for reliably detecting a 
circuit within a controlled area. 
Inventory shrinkage as a result of shoplifting costs U.S. retailers in 
excess of $10 billion per year. To counteract shoplifting, electronic 
security systems have been utilized to detect the unauthorized removal of 
articles from a protected area. These systems utilize radiowaves, 
microwaves or a magnetic field generated within a confined area through 
which all articles from the store must pass. A special electronic tag is 
attached to the article which is sensed by a receiving system to signify 
the unauthorized removal of the article. If the sensing system does not 
sense the presence of this special electronic tag within the confined 
area, then the removal of the article is authorized by virtue of its being 
paid for and the tag has been either removed from the article at the 
checkout counter or has been deactivated at the check-out counter. 
Presently there are two basic types of tags commercially available. One 
type is a large reusable tag mounted in a plastic housing which is usually 
fastened to clothing articles; while the other tag is relatively small and 
disposable and is normally pasted on packages. The large reusable plastic 
tags are expensive, but can be reused. The small disposable sticker tags 
can be made at low cost. 
A preferred special electronic tag for both the reusable and the disposable 
applications, utilizes a technology based on tuned circuits that operate 
in the radio frequency range. To render the tuned circuit functional at 
the desired frequency, a discrete inductor (L) and discrete capacitor (C) 
are connected together. The reusable resonant tags use discrete capacitor 
and inductor components which are connected to form the tuned 
inductor-capacitor (LC) circuit. In the disposable resonant sticker tag, a 
discrete inductor and capacitor are formed on a dielectric substrate. 
Here, the capacitor and inductor are formed by conventional fabrication 
methods for forming printed circuits including selected use of laminated 
substrates having an interior dielectric layer laminated on both surfaces 
with a conductive composition such as aluminum or copper. The conductive 
layers are printed with an etchant resistant material in the form of the 
desired circuit and after etching, the remaining conductive material is 
now in the form of the desired circuit. Such a conventional process is 
disclosed, for example, in U.S. Pat. No. 3,913,219. Alternatively, the 
resonant tag circuits can be formed by an additive process whereby an 
activatable composition is printed upon a dielectric substrate in the form 
of the desired circuit. The activatable composition then is chemically 
activated so that, when placed in an electroless bath, it causes reduction 
of a conductive metal thereon selectively so that metal is not deposited 
on those areas which are not chemically activated. The electrolesscoated 
pattern then can be further coated with metal by conventional plating 
techniques to form a resonant tag circuit. Alternately, the resonant tag 
circuits can be formed by an additive process whereby the film is 
chemically treated to render it platable and a plating mask is used to 
prevent plating in noncircuit areas. Alternatively, the resonant tag 
circuits can be formed by stamping, dye cutting, precision fine blanking 
or other form of stamping the circuit out of thin metal sheets and 
laminating the two sides to the circuit on opposite sides of a film. 
Prior to the present invention, the resonant tag circuits were formed with 
discrete inductor and capacitor components. Such tags are shown, for 
example, in U.S. Pat. Nos. 3,967,161; 4,021,705; 3,913,219; 4,369,557; 
3,810,147 and 3,863,244. These prior art tags, by virtue of their use of 
an inductor and a capacitor as separate elements introduce inherent 
limitations in the disposable resonant sticker tag produced therewith. 
In these resonant tag circuits, it is desirable to produce tags that 
operate at specific frequencies. Specific frequencies can be obtained by 
varying L and/or C based on the equation: 
##EQU1## 
In general, it is also desirable to have a sharp resonance curve where 
there is a large change in impedance as a function of frequency over a 
narrow frequency range in order to provide the desired selectivity to 
discriminate between tuned circuits and environmental interferences. 
The sharpness of the resonance curve is usually determined by a quality 
factor called "Q" which can be defined as the ratio of the reactance of 
either the coil or the capacitor at resonant frequency to the total 
resistance. It is also a measure of the reactive power stored in the tuned 
circuit to the actual power dissipated in the resistance. The higher the 
"Q", the greater the amount of energy stored in the circuit compared with 
the energy lost in the resistance during each cycle. 
Therefore, it is generally desirable to have a resonant tag circuit with a 
high "Q" factor. 
Mathematically: 
##EQU2## 
Where X.sub.L =Inductive reactance 
X.sub.C =Capacitive reactance 
L=Inductance 
C=Capacitance 
f=Frequency 
R=Resistance. 
Combining equations 2 and 3: 
##EQU3## 
which indicates "Q" can be improved by: (a) Lowering the resistance (R) 
(b) Increasing the inductance (L) 
(c) Reducing the capacitance (C). 
The "Q" factor is also related to the power stored in the resonant tag 
circuit which means, the dielectric loss of the substrate should be 
minimized to improve the "Q" factor. This dielectric loss is normally 
referred to as the dielectric dissipation factor of the capacitor. 
Assuming the dielectric dissipation factor for a particular class of 
resonant tag circuits is constant, then lowering the series resistance, 
increasing the inductance and/or lowering the capacitance, are three 
possible variables that can be changed to improving the "Q" factor for a 
resonant tag circuit tuned at a specific frequency. 
The most obvious approach for improving "Q" is to reduce the resistance 
(R). Difficulty in improving "Q" is increased when "Q" is to be improved 
by adjusting the L/C ratio because when L is increased, C must be reduced 
to maintain the desired frequency; and in most cases, the resistance (R) 
increases as L is increased because an increase in inductance is usually 
associated with a longer inductor. 
In many applications, it is desirable to have a resonant tag that is 
relatively small, inexpensive to made and functions as an antenna allowing 
it to be sensed by detecting equipment. Adding these three additional 
objectives to making a high "Q" factor resonant tag circuit that functions 
at a specific frequency further complicates the circuit design when using 
a discrete inductor in combination with a discrete capacitor. 
Presently available resonant sticker tag circuits are produced by an 
etching process. U.S. Pat. No. 3,863,244, issued Jan. 28, 1975, and U.S. 
Pat. No. 3,967,161, issued June 29, 1976, disclose resonant tag circuits 
which are fabricated by etched circuit techniques. The tag circuit 
comprises an insulatve substrate having one portion of the circuit formed 
on one side of the substrate and another portion of the circuit formed on 
the opposite side of the substrate. Electrical connection is made between 
the portions of the circuit on opposite sides of the substrate by means of 
a conductive pin or eyelet extending through the substrate, or by means of 
a spot weld joining confronting circuit areas. U.S. Pat. No. 4,021,705, 
issued May 3, 1977, discloses a similar type of resonant tag circuit. 
U.S. Pat. No. 3,913,219, issued Oct. 21, 1975, discloses a fabrication 
process for planar resonant tag circuits. in which both sides of a web of 
insulative material are provided with a conductive material to serve as 
conductive surfaces from which circuit patterns are formed by etched 
circuit techniques. Electrical connection is established between the two 
conductive patterns on opposing faces to the web by welding confronting 
conductive surfaces, such as by ultrasonic welding or cold-welding with 
the aid of a tool having a chisel-like tip. 
These resonant tag circuits require a relatively long and thin inductor 
line with many turns and a large capacitor plate (FIGS. 1-4) to properly 
tune the resonant tag circuit, but this fine circuit line with many turns 
is difficult to produce by etching techniques without having shorts or 
delamination of the conductive material. This same fine circuit line and 
large capacitor plate is equally, if not more difficult, to make by an 
additive process where electroplating may be required to reduce the 
resistance. The fine circuit line cannot carry a substantial amount of 
current, thereby requiring an extremely long time to deposit conductive 
metal onto this fine line. When plating is finished, the portion of the 
line closest to the electroplating connection would be plated to a much 
greater thickness than other portions of the line with the excessive 
amounts of conductive material being an undesirable increase in the cost 
of the overall circuit. This same fine circuit line and capacitor plate 
would be equally difficult to produce by stamping and laminating 
techniques because the inductor coil is long and thin and a relatively 
large capacitor is located along or at the end of this thin coil making it 
difficult to handle after stamping and equally difficult to place on an 
insulative substrate and properly align prior to lamination. 
Accordingly, it would be highly desirable to provide a resonant tag circuit 
which is resonant at a desired frequency, has a high "Q" factor, is 
relatively small in size, can be detected by existing detection equipment 
and can be made economically and quickly by etching, additive plating and 
stamping/laminating techniques. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a resonant tag circuit design 
for use in electronic security systems, which can be manufactured in high 
volume at very low cost, by minimizing the area the conductive patterns 
utilize to make a resonant circuit with specific frequency response 
characteristics. 
It is a further object of this invention to provide a resonant tag circuit 
design with conductive patterns on opposing sides of an insulative 
substrate which have substantially equal areas for optimum speed and ease 
of processing by various methods of manufacturing such as etching, plating 
or die cutting. 
A further object of this invention is to provide a resonant tag circuit 
design with tapered conductive patterns on opposing sides of an insulative 
substrate forming the inductor coil, whereby the width of the pattern 
decreases towards the inner portion of the coil pattern in order to 
facilitate the electroplating of the patterns to a more uniform thickness, 
for example, by an additive process and to allow more space for additional 
coil winds to increase the inductance, thereby allowing the frequency to 
be lowered and improving the "Q" factor. 
A further object of this invention is to provide a resonant tag circuit 
design with wide patterns and a minimum number of inductor coil windings 
on each side of an insulative substrate to facilitate ease of processing 
during manufacturing such as by etching, die cutting and plating 
techniques. 
A further object of this invention is to provide a resonant tag circuit 
design wherein the conductive pattern forming one half of the inductor 
coil portion and capacitor portion of the resonant tag circuit on one side 
of an insulative substrate is slightly wider than the conductive pattern 
of the opposite side of the insulative substrate to minimize the effect of 
undesirable misalignment of the opposing patterns on each side of the 
insulative substrate. 
In accordance with this invention, a resonant tag circuit is provided with 
inductor coil circuit paths on the two separate surfaces of a thin 
dielectric sheet such as a plastic sheet, whereby a portion or all of the 
inductor coil circuit paths cooperate with the inductor coil circuit paths 
on the opposing surface of the dielectric sheet to form the capacitor of 
the tuned circuit. The inductor coils and amount of capacitance achieved 
between the overlapping inductor coil circuit paths (distributed 
capacitance) is such as to tune the circuit at the desired frequency, 
yielding a high "Q" factor, and strong antenna effect. The inductor coils 
each are formed from a spiral conductive path that turns through at least 
360 degrees. In a preferred aspect of this invention, it is desirable to 
control the distributed capacitance provided between the opposing inductor 
coil circuit paths such as to eliminate the need for any discrete 
capacitance means in the circuit. In any event, the discrete capacitance 
means provides less than about 30% of the total capacitance required by 
the circuit, and preferably the discrete capacitance means is eliminated. 
By utilizing the resonant tag circuit design of this invention, it is 
possible to provide smaller resonant tag circuits than can be obtained 
with present circuit designs, the need for the fine lines in the inductor 
circuits required by present resonant tag circuits is reduced or 
eliminated and permits the use of etching (subtractive) means, additive 
means, and stamping techniques for forming circuits without the 
requirement of close tolerances in these processes. The distributed 
capacitor portion of the tuned circuit can be formed between the inner 
portions of the inductor coil circuit paths, the outer portions of the 
inductor coil circuit paths or intermediate portions of the inductor coil 
circuit paths.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
As used herein, the term "spiral conductive path" or "spiral path" means a 
continuous conductive path that turns through greater than 360 degrees. 
As used herein, the term "discrete capacitance" or "discrete capacitor" 
means a capacitor element formed from two conductive paths, each located 
on one surface of an electrically insulating substrate such as a plastic 
sheet and which conductive paths overlap each other a sufficiently great 
area as to function primarily as a capacitor which has little inductance. 
The relative contribution of capacitance or inductance can be estimated as 
a first approximation by measuring the overlap area for a specific element 
and the overlap area of the total circuit as compared to measuring the 
total length of a specific element in the direction of the spiral path(s) 
and the total length of the inductor coil(s) in the resonant tag 
circuit(s). When the ratio defined by Formula 1 is greater than 10, the 
circuit element is a discrete capacitor as that term is used herein: 
##EQU4## 
wherein A equals the area of overlap for a conductive element that 
contributes to capacitance and inductance, A.sub.T equals the total 
capacitor area of the resonant tag circuit(s), L equals the total length 
of the conductive element in the direction of the spiral path(s) that 
contributes to capacitance and inductance and L.sub.T equals the total 
inductor length in the direction of the spiral of the resonant tag 
circuit(s). The factors of Formula 1 can be directly measured. 
As used herein, the term "distributed capacitor" or "distributed 
capacitance" means a circuit element which functions both as a capacitor 
and as an inductor. The distributed capacitor is formed from two 
overlapping spiral conductive paths, one each on a surface of an 
electrically insulating layer. The relative contribution of capacitance to 
inductance is approximated by Formula 1. A distributed capacitor in 
accordance with this invention has a ratio according to Formula 1 of 10 or 
less. In accordance with this invention, the distributed capacitor portion 
of the circuit provides at least 70% of the area of overlap between the 
two opposing spiral conductive paths, preferably at least 90% of such 
overlap and most preferably at least 100% of such overlap. 
In accordance with this invention, a resonant tag circuit is formed by 
inductor coil circuit paths affixed to the opposing surfaces of a 
dielectric substrate. Each inductor coil is formed of a spiral such as a 
rectangular spiral, circular spiral, triangular spiral or the like wherein 
a portion of the spiral overlaps with a corresponding portion of the 
spiral on the opposite surface wherein the degree of overlap is controlled 
to form a distributed capacitor comprising the overlapping portions of the 
spiral and the interposed dielectric material between the two overlapping 
portions of the spiral. The remaining portions of the spiral can be offset 
from each other so that their function as inductor coils does not 
contribute to capacitance. One end of the spiral inductor coil may be 
electrically connected to the corresponding end of the spiral inductor 
coil on the opposing surface of the dielectric material. Using the 
distributed capacitance, it is possible to eliminate the need for any 
through hole connection provided the amount of distributed capacitance and 
inductance is sufficient to tune the circuit to the desired frequency. 
In one embodiment of this invention, a fusable link can be introduced into 
either one or both of the inductor coil circuit paths at any section of 
the spiral coil. This fusable link can be destroyed with electromagnetic 
energy at a specific frequency to deactivate the tag circuit. The 
distributed capacitor portion of each inductor coil can comprise the outer 
portion of the spiral path, the inner portion of the spiral path and/or an 
intermediate portion of the spiral path. 
In another embodiment of this invention, the inductor coil circuit paths 
are tapered, the outside spiral path being the widest and tapering down 
towards the inner spiral path to optimize current carrying capacity during 
the electroplating of the circuit. In another embodiment of this 
invention, the inductor coil circuit paths are tapered to minimize the 
amount of conductive material located near the center of the tag to 
improve the antenna effect of the tuned circuit. In still another 
embodiment of this invention, the inductor coil paths are tapered to 
increase the number of possible coil winds that can be added, thus 
increasing the inductance, reducing the frequency and/or increasing the 
"Q" factor. 
It is not necessary that the circuit paths on both surfaces of the 
insulating layer be identical. For example, the circuit path or coil on 
one surface of the insulating layer can be wider than the circuit path or 
coil on the opposite surface so that the desired overlap of circuit paths 
is achieved even though there is minor misalignment of the circuit paths 
during manufacture. This embodiment provides latitude during manufacture 
which minimizes the production of misfunctioning tag circuits. 
Referring to FIGS. 1 and 2, a prior art multifrequency resonant circuit is 
shown which includes the requirement of a separate capacitor means 
comprising a conductive area 22 which cooperates with conductive area 24 
with an interposed dielectric substrate to form the capacitor. The circuit 
is formed by conventional etching means and includes a first conductive 
path 10 arranged in a generally rectangular path on a surface of an 
insulative substrate 12 terminating at one end in a conductive area 14. 
The other end of path 10 terminates at conductive area 16. A second 
conductive path 18 is formed as a rectangular spiral on substrate 12 and 
terminates at junction 20 with area 14 and at its inner end at conductive 
path 22. A conductive path 23 connects area 14 and area 16 and is 
dimensioned to fuse upon flow therethrough of a predetermined current 
produced upon energization of the circuit by an applied energizing field. 
The opposite surface of substrate 12 shown in FIG. 2 includes a conductive 
area 24 aligned with conductive area 22 shown in FIG. 1 and a pair of 
conductive areas 26 and 28 in alignment with areas 14 and 16 on the 
opposing surface. The conductive areas 24 and 26 are interconnected with a 
conductive path 30 while the conductive areas 26 and 28 are interconnected 
by a conductive path 32. Conductive path 32 also is dimensioned to fuse 
upon energization by a predetermined electromagnetic field, thereby to 
alter the resonant properties of the tag circuit. Areas 16 and 28 are 
electrically connected by conductive pin 34 extending through the 
substrate 12. 
The conductive areas 10 and 18 serve as inductors of the resonant circuit. 
The conductive areas 22 and 24 spaced by the interposed substrate 12 serve 
as a first capacitor while a second capacitor is formed by the conductive 
area 14 cooperating with path 26 on the opposing substrate surface. In 
order for the conductive paths 10 and 18 to function as inductors, they 
must be spaced apart a certain distance from conductive areas 22 and 24 
which serve as the capacitive portion of the overall circuit. Thus, the 
overall minimum size of the circuit is much larger than a circuit wherein 
the separate capacitor means corresponding to the conductive areas 22 and 
24 is eliminated. The larger circuits are much more costly to produce 
since they require significantly more raw materials and chemicals and a 
longer manufacturing time. Accordingly, it would be highly desirable to 
provide an efficient tag circuit design wherein the interior capacitive 
portion corresponding to conductive areas 22 and 24 could be eliminated. 
Such a design would permit the use of a tighter spiral conductive path 
which would allow the size of the circuit to be reduced and be produced 
much faster than those produced at the present time such as those shown in 
FIGS. 1 and 2. 
FIGS. 3 and 4 show an alternative prior art resonant circuit tag which is 
used widely. One side of the tag is shown in FIG. 3 and comprises a 
conductive area 42 which functions as part of a capacitor which overlaps 
with conductive area 38 (see FIG. 4) and wherein an insulating layer such 
as a plastic layer is interposed between the conductive areas 38 and 42. 
The conductive area 42 is connected to conductive area 46 which extends 
through the insulating layer and is connected to conductive path 44 which, 
in turn, is connected to conductive area 38. Coil 36 is positioned on the 
same surface of insulating layer 40 as is conductive area 38 wherein one 
end of the coil 36 is connected to conductive area 38 while the other end 
of coil 36 is connected to connector 46. The coil 36 functions as the 
inductor while the conductive areas 38 and 42 together with insulating 
layer 40 function as the capacitor. 
Referring to FIG. 5, a resonant circuit comprising the present invention 
which has a center frequency of 8.1 MHz is shown. This circuit is formed 
by overlapping points 11 to each other and points 13 to each on opposing 
surfaces of an insulating layer. Conductive path 15 is provided on one 
surface and conductive path 17 is provided on the opposing surface. A 
conductive path extends through the insulating layer 19 and is connected 
to points 21 and 23 on opposing surfaces of the insulating layer 19 so 
that the conductive paths 15 and 17 are connected thereby. The conductive 
paths 15 and 17 overlap each other on windings 25, 27 and 28 which overlap 
windings 29, 31 and 32. Windings 33, 35 and 37 overlap windings 39, 41 and 
43, respectively. Conductive paths 45, 47 and 49 overlap conductive paths 
51, 53 and 55, respectively, while conductive paths 57, 59 and 61 overlap 
conductive paths 63, 65 and 67, respectively. Utilizing this design, a 
discrete capacitor can be eliminated and the conductive paths 15 and 17 
cooperate together to provide both the inductor function and the 
distributed capacitor function. On one side of the tag, the inductor line 
is only about 18 inches and the line width varies from between about 0.08 
inches and 0.06 inches. The total surface area utilized on one side of the 
tag is less than about 55%. Thus, by utilizing the design of the present 
invention, the discrete capacitor can be eliminated and the size of the 
overall tag can be made much smaller than that which can be made by the 
prior art designs. 
Utilizing the distributed capacitance design shown in FIG. 5 to manufacture 
a tag resonant circuit provides many significant advantages. Utilizing the 
present state of the art etching processes, the manufacturing time and 
cost is greatly reduced as compared to prior art tag resonant circuit 
designs having a discrete capacitor because the circuit line in the design 
of this invention can be made wider, thereby allowing thinner conductive 
paths to be utilized. By utilizing wider lines which are thinner, the cost 
is reduced and the possibility of breaks or short circuits also is 
reduced. Furthermore, if an additive manufacturing approach were utilized 
which involves forming a pattern of the desired circuit on an insulated 
substrate, which pattern is rendered chemically active so that it can be 
plated with an electrically conductive material, the use of the 
distributed capacitance concept of this invention also allows the use of 
wider lines and a short line width per side of insulating material. In 
addition, a balanced two sided design can be utilized in the present 
invention. For example, with the prior art tag circuit shown in FIGS. 3 
and 4, approximately 2 hours is required to form a plated copper 
conductive path to reduce the coil resistance to less than about 0.5 ohms. 
In contrast, in utilizing the pattern of the present invention shown in 
FIG. 5, the plating time can be reduced to less than about 20 minutes. In 
addition, if the manufacturing process used to form the desired circuit 
involves stamping, the use of distributed capacitance, as provided by the 
present invention, allows for the incorporation of a wide and short 
conductive path that has strength and form after stamping and that can be 
easily laminated with good registration from side to side. Accordingly, 
the present invention provides the natural advantages over the prior art 
tag circuit designs presently utilized. 
Referring to FIG. 6, a resonant tag circuit is shown which includes both 
distributed capacitance and discrete capacitance wherein the discrete 
capacitance comprises less than about 30% of the total capacitance of the 
entire circuit. This contrasts with prior art tag circuit designs wherein 
the capacitance is prior-provided essentially entirely by discrete 
capacitance. As shown in FIG. 6, the discrete capacitor is formed from 
conductive area 71 and conductive area 73 located on opposing surfaces of 
insulating layer 69. Conductive path 75 is formed from a rectangular 
spiral as shown in FIG. 6 wherein a portion of the conductive path shown 
in darkened area 77 overlaps with the darkened portion 79 of conductive 
path 81. The overlapping portions of couductive paths 75 and 81 function 
as a distributed capacitor as well as the inductor. Conductive paths 75 
and 81 can be joined through or around the substrate 69 by means of 
electrical connections 83 and 85. 
Referring to FIG. 7, a resonant circuit is shown wherein there is partial 
overlap of two separate conductive paths 87 and 89. The light portion 91 
of spiral 87 does not overlap with spiral 89 while the dark portion 93 of 
spiral 87 overlaps with the dark portion 95 of spiral 89. Similarly, the 
light portion 97 of spiral 89 does not overlap with spiral 87. Spirals 87 
and 89 can be connected through insulated substrate 99 at points 101 and 
103. 
Referring to FIG. 8, an alternative embodiment is shown wherein there is 
partial overlap between two conductive paths 105 and 107 over the entire 
length of each circuit. As is the case in FIGS. 6 and 7, the light 
portions 109 and 111 of the respective spirals do not overlap each other 
while the dark portions 113 and 115 of each spiral overlap each other. 
Electrical connections between the two spirals can be made through 
substrate 117 at points 119 and 121. 
Referring to FIG. 9, conductive paths 54 and 58 are shown which are 
provided with fusable link 80. The conductive paths 54 and 58 are 
connected through an insulated substrate (not shown) at points 70 and 78. 
The fusable link 80 can be rendered inoperative by means of radio 
frequency energy in a manner well known in the art. 
As noted above, the essential feature of this invention comprises 
distributed capacitance wherein at least a portion of the rectangular 
spiral conductive paths overlap each other to effect distributed 
capacitance so as to form a resonant circuit that resonates at the desired 
frequency while utilizing a minimum circuit area. The incorporation of 
distributed capacitance design allows the circuit to be made more 
economically using conventional printing and etching processes and allows 
the use of other unique cost-effective technologies such as plating and 
stamping which cannot be utilized economically with a design incorporating 
discrete capacitor and inductor elements. By comparing the resonant 
circuits shown in FIGS. 3 and 4 with the resonant circuit of FIG. 5, both 
tuned to the same frequency of about 8.1 MHz, the inductive path 36 of 
FIG. 4 consists of 7 lines with an overall length of 43 inches utilizing 
70% of the area on one side of the tag for only the inductor. When the 
capacitor area is added to that of the inductor, over 80% of the area is 
utilized. The tag shown in FIGS. 3 and 4 has many problems limiting it to 
manufacture by etching techniques. These limiting factors include the fact 
that the inductor line is too narrow and the length is too long, causing 
shorting problems and excessive plating time. If an additive approach 
incorporating electroplating were to be used, this requires over two hours 
to plate up this design with sufficient copper to reduce the total coil 
resistance to less than about 0.5 ohm. When this is done, the copper 
thickness on the outer portion of the coil 36 can exceed 2 mils while the 
inner-most portion of the coil will have only about 0.5 miles of copper. 
Furthermore, the narrow line and long line length make stamped parts 
difficult to make and extremely difficult to handle and register. The 43 
inch coil loses all integrity as soon as it has been stamped and the 
location of the discrete capacitor plate 38 at the end of the 43 inch coil 
is extremely difficult to register over the conductive surface 42 located 
on the opposite side of the dielectric substrate. Furthermore, the narrow 
lines can also cause major manufacturing problems when using conventional 
etching techniques. The etching solution can undercut the conductive metal 
that is laminated to the dielectric substrate, thereby causing breaks in 
the circuit line or short circuiting. In addition, a significant amount of 
surface area is taken up with the discrete capacitor and conductor and 
this reduces flexibility in making circuits that are smaller or can be 
tuned at lower frequencies. 
Referring to FIG. 10, the equivalent circuit of this invention is made up 
of a plurality of inductor portions L.sub.la -L.sub.na and L.sub.lb 
-L.sub.nb as well as distributed capacitor portions C.sub.l -C.sub.n. 
Referring to FIG. 11, the electronic security system utilized in the 
present invention includes a transmitter 96 coupled to an antenna 97, 
typically a loop antenna operative to provide an electromagnetic field 
within a predetermined area to be controlled. A receiving antenna 90, also 
typically a loop antenna, is arranged at the controlled area to receive 
energy radiating by transmitter antenna 97 and to couple received energy 
to an Rf front end which includes an Rf band pass filter 92 and Rf 
amplifier 100. The output of amplifier 100 is applied to a detector 94, 
the output of which is, in turn, coupled to noise rejection circuitry 102. 
Output signals from noise rejection circuitry 102 are amplified by 
amplifier 98 and applied to pulse shaping circuitry 104 and thence to 
digital processing circuitry 88, the output of which is operative to 
actuate an alarm 106 or other output utilization apparatus. 
EXAMPLE I 
FIGS. 12a and 12b represent a resonant tag circuit design that utilizes 
distributed capacitance along the entire length of the inductor circuit 
paths. This design has two conductive paths (110 to 111 and 112 to 113) 
located on opposite sides of a dielectric substrate (not shown) both of 
which have tapered line widths starting with a wider line at the outermost 
wind and progressively becoming thinner towards the center. This resonant 
tag circuit is formed by registering the following points: 114 to 115, 116 
to 117, 118 to 119 and 120 to 121. In this example, the conductive 
material is copper and the insulating material is polypropylene and the 
resonant tag circuit was made by an additive electroplating process. The 
polypropylene first is treated to render it receptive to electroless and 
electrolytic plating. The film is passed through a solvent to swell the 
film. Thereafter, it is passed through a chromic acid etching bath and 
thence through a palladium-tin activater solution. The dielectric is 
masked in the non-circuit area and thereafter plated with copper 
electrolessly and electrolytically. 
When the two conductive paths are connected with a through substrate 
connection at 111 to 113 (the innermost point), then the frequency is 6.4 
MHz. When the two conductive paths are connected at 110 to 113 (the 
outermost point), then the frequency is 8.7 MHz. When the two conductive 
paths are connected along the conductive paths at points 134 and 135, the 
frequency is 8.2 MHz. 
EXAMPLE II 
FIGS. 13a and 13b represent a resonant tag circuit design that utilizes 
distributed capacitance along a portion of the inductor circuit path. This 
design has two conductive paths (130 to 135 and 132 to 134) located on 
opposite sides of a dielectric substrate (not shown). The resonant tag 
circuit is formed by registering the following points --122 to 123, 124 to 
125, 126 to 127 and 128 to 129. The conductive paths have tapered line 
widths starting with a wider line at the outermost wind and progressively 
becoming thinner towards the center. Distributed capacitance is developed 
by inductor overlap of the outside wind 130 to 131 overlapping 132 to 133. 
In this example, the conductive material is aluminum, the insulating 
material is polyethylene and the tag is made by a masking and etching 
process. An aluminum foil laminate comprising polyethylene, laminated on 
each surface with about 2 mils aluminum was masked in the circuit area and 
then etched in an alkaline-caustic bath to form the circuit. Using a 1.00 
mil polyethylene dielectric and connecting the two conductive patterns at 
point 134 and 135, this tag circuit has a resonant frequency of 8.8 MHz. 
EXAMPLE III 
FIGS. 14a and 14b represent a resonant tag circuit design that utilizes 
distributed capacitance along a portion of the inductor circuit path. This 
design has two conductive paths (145 to 149 and 147 to 150) located on 
opposite sides of a dielectric substrate (not shown). The resonant tag 
circuit is formed by registering the following points --137 to 138, 139 to 
140, 141 to 142 and 143 to 144. The conductive paths have tapered line 
widths starting with a wider line at the outermost wind and progressively 
becoming thinner towards the center. Distributed capacitance is developed 
by inductor overlap of the two outermost winds 145 to 146 overlapping 147 
to 148. In this example, the conductive material is copper, the insulating 
material is polypropylene and the tag was made by the additive plating of 
Example I. Using a 1.25 mil polypropylene dielectric and connecting the 
two conductive patterns at point 149 and 150, this tag circuit has a 
resonant frequency of 8.2 MHz.