JPH0676185A - Individual power-feeding multiplex loop antenna for electronic guard system - Google Patents

Abstract

PURPOSE: To establish exact phase and amplitude balance, and to extend a valid frequency range by individually connecting each loop of a transmission antenna with a divided circuit network, connecting each loop of a reception antenna with a connected circuit network, allowing an article to pass through the antennas for detection, and controlling each loop independently from the other loops. CONSTITUTION: Antenna systems 14 and 16 are positioned in mutually separated parallel relation, and the presence of a resonance tag circuit 18 which can pass through a space between those antennas is detected by a security system. A divided circuit network 20 is arranged between a transmitter 10 and the transmission antenna 14, and a connected circuit network 22 is arranged between a receiver 12 and the reception antenna system 16. The transmission antenna 14 is constituted of four coplanar loops 24, 26, 28, and 30, and the arbitrary number of coplanar loops can be provided. Also, each loop can be properly driven by making the valid total loop areas of the loops driven in one phase equal to the valid total loop areas of the loops driven in an inverse phase.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to antenna systems for use in electronic security systems, and more particularly to such antenna systems including individually fed multiple loops.

[0002]

Electronic guards for detecting unauthorized removal of articles with a resonant circuit, a saturable magnetic wire strip or a detectable target containing mechanical resonant magnetic material.
Anti-theft systems are widely known. The basic concept of such a theft detection system is US Pat. No. 3,810,147.
No. 3,973,263, 4,016,553
No. 4,215,342 and 4,795,995
And many other patents.

A wide variety of antenna configurations are designed for anti-theft systems. Practical transmit antenna designs typically have wires in one or more loops that carry an alternating current to generate an electromagnetic field. The receive antenna is also usually a wire of one or more loops that undergoes small distortions or disturbances in the electromagnetic field caused by the detectable target as it passes through the interrogation region between the transmit and receive antennas. Is. A preferred feature of the receiving antenna system is that it is sensitive to signals generated in the interrogation area or at a distance that is relatively small with respect to the size of the antenna and at a distance from the interrogation area, i.e. the size of the antenna. Compared with the above, it is insensitive to noises and unnecessary signals generated at a large distance or cancels them.

Similarly, it is desirable for the transmitting antenna to create a strong local field in the interrogation area and to minimize or eliminate the electromagnetic field generated at great distances from the interrogation area. The remote electromagnetic field cancellation of such a transmitting antenna is F
Preferred for meeting RF emission levels as required by CC or other similar regulatory authorities.

Remote field cancellation is described by Heltemes in US Pat. No. 4,135,183 for a receive and transmit antenna for an hourglass or figure eight design. Lichtblau in U.S. Pat. No. 4,243,980 proposes twisting one conductor to form multiple loops in a remote field cancellation configuration. U.S. Pat. No. 4,251,808 adds a conductive shield enclosing a twisted loop to provide an electrostatic field shield. And U.S. Pat. No. 4,751,516 proposes driving a symmetrical portion of a twisted loop.

[0006]

All of the remote field-eliminating multi-loop antennas in the above cited patents are inherently capable of achieving good amplitude balance and accurate phase position at high frequencies. I can't. In essence, the twisted loop is
When a person moves away from the drive source, causing an imbalance in the loop furthest from the drive source, it shifts the current phase relative to the drive source. Shielded loops exaggerate this problem. In addition, the inherent phase imbalances described above can have the undesired effect of appearing as distortions on the signals normally associated with electromagnetic field disturbance targets or markers in certain frequency swept detection systems.

[0007]

The present invention is designed to overcome the above-mentioned drawbacks of the prior art. The antenna system of the present invention, useful in electronic security system transmitters or receivers, has two or more loops.
Each loop of the transmit antenna system is individually connected to the split network, while each loop of the receive antenna system is individually connected to the coupling network at the receiver.

By connecting each of these loops individually, each loop can be controlled independently of the other loops. As a result, a more accurate phase and amplitude balance can be achieved as there is minimal phase shift in the loop remote from the drive source. In addition, such a discrete drive configuration allows for a wider range of useful frequencies for a given antenna geometry by using a larger number of smaller loops of discrete drives. In addition,
The detection pattern can be more easily optimized because the current and data can be adjusted independently and indefinitely in each loop. A flatter frequency response and well-matched linear phase characteristics in each loop that minimizes unwanted distortion in the received marker signal can also be achieved. The improved configuration also allows for independent signal processing of each receive loop, independent pulsing or time multiplexing of the transmit loop, which results in improved immunity to false alarms or improved detection range. can do.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The drawings will now be described. Like reference numerals in the figures refer to like elements. FIG. 1 illustrates an electronic security system using the improved antenna system of the present invention. This security system includes a transmitter 10 and a receiver 12,
These are connected to the transmitting antenna system 14 and the receiving antenna system 16, respectively. antenna·
The systems 14 and 16 are located in a parallel relationship spaced apart from each other, which allows the resonant tag circuit 18 to pass through the space between the antennas 14 and 16.
The security system can detect the presence of (or other marker tags such as magnetic markers or other target circuits).

The actual arrangement of antennas 14 and 16 with respect to each other is known in the art. Similarly, transmitter circuit 10 and receiver circuit 12 are also known. Therefore, these features will not be described in detail. Transmitter 10
A divided circuit network 20 is disposed between the transmission antenna 14 and the transmission antenna 14. Similarly, a coupling network 22 is arranged between the receiver 12 and the receiving antenna system 16. These networks 20 and 22 are described in detail below.

As can be seen in FIG. 1, transmit antenna system 14 preferably includes a plurality of coplanar loops 24, 26, 28 which preferably include conductive shields such as those described in US Pat. No. 4,251,808. And 30 are included. The loops 24-30 are continuous along the vertical axis of the antenna. However, this is just one example,
It is also possible to arrange the loops to be coplanar and along the horizontal axis. For reasons well known in the art, two of these loops are driven out of phase with each other.

Loop 24 of transmit antenna system 14
Are located remotely from the loop 24. It includes a pair of leads 32 extending through the split network 20. Similarly, the loops 26, 28 and 30 include a pair of leads 34, which also extend to the split network 20.
36 and 38, respectively. In a practical application of the transmit antenna system 14, the loop 30 is physically located closer to the split network 20 or other common point where these leads are interconnected. Thus, the lead 32 is longer than the lead 38 as will be described in more detail hereinafter.

Four coplanar loops 24, 26, 28 and 3
Although 0 is shown as constituting the transmit antenna 14, it is readily apparent that any number of coplanar loops is possible. However, of course, if equal currents are used in each loop, it is required that the effective total loop area of the loop driven in one phase be equal to the effective total loop area of the loop driven in the opposite phase. This is achieved by simply choosing the geometrical dimensions of these loops appropriately, but the present invention also accomplishes this by driving each loop appropriately (as will become more apparent hereafter).

In the above description of the antenna system, particular reference has been made to the transmit antenna system 14.
However, it should be appreciated that the receive antenna system 16 including the coplanar loops 40, 42, 44 and 46 may also be arranged and function in a substantially equivalent manner.

FIG. 2 will now be described. In Figure 2,
A more detailed schematic diagram of a transmission system and antenna according to the present invention is illustrated. The transmitter 10 of FIG. 2 comprises a swept signal generator 48, a voltage controlled oscillator 50 and an RF amplifier 52, which are well known in the art.
As should be noted here, although one RF amplifier 52 is shown, it is possible to use multiple individual RF amplifiers, one for each of the antenna loops.

The antenna loops 24, 26, 28 and 30 of FIG. 2 have their respective pairs of leads 32,3.
It is shown as being connected to the split network 20 via 4, 36 and 38. These leads 32-3
Reference numeral 8 is a shielded cable, and as described above, the lead wire 32 is longer than the lead wire 34, and the lead wire 34 is longer than the lead wires 36 and 38. That is, these leads are progressively shorter because the loops 24-30 are progressively closer to the split network 20.

The split network 20 comprises a plurality of toroid transformers 54, 56, 58 and 60. Each of these transformers has a 1: 1 primary to secondary winding ratio and includes a center tap on the grounded secondary winding. The secondary winding of the transformer 54 is the lead 3 of the antenna loop 24.
Connected to 2. Similarly, transformers 56, 58 and 60 are connected to loops 26, 28 and 30, respectively.

The primary winding of the transformer 54 has one side grounded and the other side connected to the voltage / current resistor R1.
R1 is connected to the output of the RF amplifier 52. The primary winding of transformer 54 is directly connected to the RF amplifier through resistor R1, while the primary windings of transformers 56, 58 and 60 include a delay line circuit. For example, the delay line circuit associated with transformer 56 includes an inductor L1 arranged in series with a primary winding and a capacitor C1. L1 and C
The contact No. 1 is connected to the RF amplifier 52 via the resistor R2. Similarly, the primary winding circuit of the transformer 58 is
The transformer 6 includes an inductor L2 and a capacitor C2 connected to the RF amplifier 52 via a resistor R3.
0 includes an inductor L3 and a capacitor C3 connected to the amplifier via a resistor R4.

As will be readily apparent to those of ordinary skill in the art, these delay line circuits include leads 32, 34, 36 and 3.
8 is needed to compensate for the difference in length. Thus,
The inductance of inductor L3 is selected to be equal to the inductance of lead 32 minus the inductance of lead 38. Similarly, the value of capacitor C3 is selected to be equal to the parasitic capacitance of lead 32 minus the parasitic capacitance of lead 38. Inductors L1 and L2
And the values of capacitors C1 and C2 are similarly selected to compensate for the lead length differences. Moreover, although these delay line circuits are shown as being on the primary side of the transformer, as is readily apparent, they may be placed on the secondary side to achieve similar results.

As pointed out above, half of the total effective loop area is in one phase and the other half is these 180
Loops 24, 26, 28, 3 so that they are out of phase
0 is driven. This is easily accomplished by simply choosing the polarity of the transformer. Thus, in FIG. 2, it can be seen that transformers 54 and 60 have the same polarity, while transformers 56 and 58 are driven to opposite polarities.

Furthermore, each of the loops 24, 26, 28 and 30 is driven independently of the other, so that the total current is 1
By simply increasing or decreasing the current to one or more of these loops, provided that the area of the loops of one phase is equal to the total current multiplied by the area of the loops of the opposite phase, It is also possible to have loops of unequal area and to achieve remote field cancellation. This flexibility allows the detection pattern to be optimized because the current in each loop can be adjusted independently and indefinitely. Furthermore, by achieving a flat frequency response and a matched linear phase characteristic in each loop, undesired distortion in the received marker signal is minimized, improved immunity to false alarms and improved A detection range can be achieved.

The present invention also eliminates high frequency limitations. This is accomplished by increasing the number of loops and making each loop smaller. Thus, as can be seen from FIG.
Each of the loops 24, 26, 28 and 30 can be reduced to half their size and replaced with a corresponding pair of loops 24A and B, 26A and B, 28A and B and 30A and B. The combined loop of loops 24A and B will be substantially equal to the area of loop A. Each of these sub-loops is connected to a division network similar to the division network shown above so that it is driven independently of each of the other sub-loops.

Although the above description is primarily for transmit antenna systems, it will be readily apparent that this applies substantially to receive antenna systems as well. However, in the receiving system, it is preferable to add the individual reception signals from the individual loop circuits in series. These signals are then provided to the RF amplifier, detector and signal processor as is known in the art.

The present invention may be embodied in other specific forms without departing from its spirit or its essential characteristics.
Therefore, reference should be made to the following claims as indicating the scope of the invention.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an electronic security system illustrating an antenna system according to the present invention.

2 is a schematic diagram illustrating the transmit antenna system of FIG. 1 in more detail.

FIG. 3 is a schematic diagram of a modified form of an antenna system according to the present invention.

[Explanation of symbols]

10: transmitter, 12: receiver, 14: antenna system, 16: antenna system, 18: resonance tag circuit,
20: division circuit network, 22: coupling circuit network, 24: coplanar loop, 26: coplanar loop, 28: coplanar loop, 30: coplanar loop, 32: lead wire, 40: coplanar loop, 42: coplanar loop. Surface loop, 44: coplanar loop, 46: coplanar loop, 4
8: Sweep signal generator, 50: Voltage controlled oscillator, 52: R
F amplifier, 54: toroid transformer, 56: toroid transformer, 58: toroid transformer, 60: toroid transformer.

Claims (5)

[Claims] 1. An antenna system used in an electronic security system for detecting unauthorized removal of an article containing a marker tag, comprising: a transmitting antenna coupled to a transmitter of the security system and a receiver of the security system. Comprising coupled receiving antennas, the antennas being arranged in spaced apart parallel relationship and between which the article passes for detection, each of the antennas being continuous along an antenna axis. There are at least two coplanar loops present, each loop having a pair of leads extending to a common point for connection to said transmitter or said receiver, said common point being said An antenna system characterized by being closer to one of the loops than the other. 2. Each antenna comprises at least three coplanar loops, each loop comprising a coplanar loop having a pair of leads extending to a common point, the common point comprising the three coplanar loops. The antenna system of claim 1, wherein the antenna system is continuously distant from each of the loops. 3. The antenna system of claim 1, further comprising a delay line circuit associated with each loop other than the loop furthest from said common point. 4. An antenna system used in an electronic security system for detecting unauthorized removal of an article containing a marker tag, comprising: a transmitting antenna coupled to a transmitter of the security system and a receiver of the security system. Comprising coupled receiving antennas, the antennas being arranged in a spaced parallel relationship and between which the article passes for detection, each of said antennas being present continuously along the antenna axis At least two coplanar loops, each loop of the transmit antenna being individually connected to a split network at the transmitter, and each loop of the receive antenna being individually connected to a combining network at the receiver; Antenna system characterized by 5. Each loop of the transmit antenna and each loop of the receive antenna have a pair of leads, a lead from at least one loop of the transmit or receive antenna being another lead of the loop. The antenna system of claim 4, wherein the antenna system is shorter and includes a delay line circuit associated therewith.