WO2005057725A1

WO2005057725A1 - Radio frequency antennae

Info

Publication number: WO2005057725A1
Application number: PCT/GB2004/005127
Authority: WO
Inventors: Paul Dennis Camper
Original assignee: Telegesis (Uk) Ltd
Priority date: 2003-12-06
Filing date: 2004-12-06
Publication date: 2005-06-23
Also published as: GB0328359D0

Abstract

The invention relates to an antennae which includes a framework which defines a loop. As part of the framework, there are provided movable electrical contacts which can be selectively moved by the user of the antennae to allow adjustment of the electrical characteristics of the antennae. The antennae according to the invention achieves excellent tag reading performance at relatively large distances and the capacitance skin effects which commonly compromise both the efficiency and accuracy of the antennae do not affect the antennae in accordance with the invention.

Description

Radio Frequency Antennae

This invention relates to radio frequency antennae, and more specifically to radio frequency antennae which are adapted to be located so as to provide detection information or an alert. Common locations include on either side of, or to one side of, a passageway, entrance or walkway, through which the passage of products having passive or active tags attached thereto is desired to be restricted, controlled or detected.

The antennae are provided to detect the presence in a definable proximity of a transponder, hereinafter referred to as a tag. The tags may be either passive (not powered) or active (powered) . The system including the antennae can be designed to alert an operator or other security system that a tag and hence the item to which the same is attached, has come within the detectable proximity of the antennae and hence as passed through the passageway, or the like. In addition, the tag may not only be detected but information may be transmitted from the antennae to the tag for storage in memory thereon. Similarly, information already stored in said memory on the tag may be transmitted to the system including the antennae by either virtue of the activation of the tag caused by the incident radio-frequency radiation transmitted from the antennae or by virtue of the tag being powered independently e.g. by a battery..

Although the following description is provided with exclusive reference to RFID antennae and tag systems, such as those falling within International Standard ISO 15693 or the Philips® I-Code system, it should be appreciated that the antennae hereinafter described are operable in providing significantly enhanced detection ranges for so-called EAS (electronic article surveillance) systems. Some known systems are made and sold under the trademarks Checkpoint® and Sensormatic®. This type of system is often employed in retail premises where it is desired to detect the movement of tagged articles past the antennae which are typically located at the exit from the premises . Thus, if an article still has a tag thereon, an alarm is created when detected by the antennae to alert the retail premises of the attempted theft of the article as the tags are normally only removed when the article is purchased within the retail premises.

RFID antennae typically operate in the frequency ranges of 2.45GHz, 900MHz or 125KHz. However at operating frequencies of 2.45GHz and 900MHz, features such as metal and moisture have deleterious effects on performance, metal being exceedingly field reflective and moisture being exceedingly field absorptive, and therefore the interaction of metal objects within the field or the presence of a human body near the field is disruptive to the operation of the system. Accordingly, a more robust operating frequency of 13.56MHz is employed. Although operation at this frequency achieves good environmental performance, the operating range at this frequency is limited to approximately a maximum of 1.9m.

The antennae are generally constructed by injection or blow moulding insulating plastics rectangular frames and wrapping copper or other conducting material around the frames a suitable number of times to provide the antennae with the required inductance. As the mouldings are typically of the order of l -2m in height, the amount of copper wire required is accordingly large and hence the antennae of the prior art can be expensive to manufacture.

Those skilled in the art of antennae manufacture will understand that the "Q" factor (a measure of the frequency bandwidth over which an antennae is effective) is an important consideration. Conventional antennae designs have resulted in frequency response characteristics which have a high Q value, i.e. the antennae is very receptive to the presence of a suitable tag within the radiated field at its particular predetermined operating frequency (i.e. 13.56MHz in the present case) , but far less sensitive or more importantly not sensitive at all to a tag present in the field when the frequency marginally changes. It is also known that modern antennae are lossy devices, typically losing l OdB of power, and, with their inherent susceptibility to stray capacitance depending on the environment and location in which the antennae are disposed, these factors alone, or in combination tend to limit the effective range of antennae to optimally 1.9m, but more realistically 1.2m.

A further consideration in antennae design is the shape of the same, and in particular those portions around which the copper wire is conventionally looped during manufacture. For example, from experiments conducted by the applicant, a rectangular loop with dimensions of less than l m x 300mm is too small because the magnetic performance is inadequate to achieve repeatable tag recognition and detection, whereas a loop size of 1.6m x 500mm is too great because the effective radiated power (ERP) increases above the statutorily enforced legal maximums (in the UK -37dB at 10m) . Also, any antennae must comply with European legislation (EN300-330 and EMC Regulations) .

The antennae includes a reader module which reads information from the tags and the power output levels from the reader module is limited to a maximum of 4W per antennae, in order to achieve the best performance. The shape considerations for the antennae ultimately result in a trade off between magnetic performance and inductance. The design of the copper windings coupled with the conventional wisdom on antennae design usually results in antennaee having an inherently high value for Q, and indeed an unnecessarily high value that makes the antennae too sensitive over too narrow a frequency range.

Known systems are described in patent applications WO02/ 15139, US6255949 and 099/ 19851 , and the reader is referred to these documents as they provide a detailed description of the tags and the operation of the same. The tags are required to possess some electronic or magnetic capability to allow resonance at a particular radio frequency, and it is this resonance which allows the system as a whole to detect the presence of the tag within the RF field which defines the proximity area around the antennae.

With respect to antennae for RFID systems, patent application W099 /21 144 describes an antennae configuration adapted for use in a conveyor system along which boxes or packages with tags attached thereto are adapted to travel. WO00/ 14694 describes a multi-direction RFID read/write antennae for providing multi-directional RF communication to a source such as a RF tag. The conventional antennae typically comprises a plurality of copper wire coils which generate an inductance. As all materials have an inherent capacitance and resistance, such an antennae is capable of functioning as an electrical resonant circuit radiating electromagnetic energy at a particular frequency, ideally a radio frequency. The RF antennae can include a switch for selecting at least one of the RF antennae coils for transmission of the RF signals and receipt of the RF response signals whereby the RF signals can be directed toward and received from a plurality of different directions from a fixed position. Another known patent application WO02/31879, describes a modular RFID antennae system including a plurality of modular RFID antennae segments that can be selecteively connected together to allow multi-directional RF communication with at least one RF tag.

It is therefore an object of this invention to provide an antennae of a novel construction which achieves the required performance criteria and does not suffer from the disadvantages abovementioned.

According to the invention in a first aspect there is provided an antennae for the detection of a transponder or tag within a detection range of the antennae, said antennae including a framework, at least part of which defines a loop having an aperture therethrough, said framework including at least a pair of spaced parallel members formed of metal or metal alloy forming opposing sides of said framework, and an electrical contact on each of said spaced parallel members, said electrical contacts selectively locatable along at least a portion of said respective spaced parallel members, to allow adjustment of the electrical characteristics of said antennae.

In one embodiment the framework is rectangular and further preferably, the framework includes one or more cross-members which is disposed between and connected to the spaced parallel members and define the loop.

Typically the adjustable electrical contacts are in the form of bar-like members which, in one embodiment are locatable in channels formed in the spaced parallel members. In one embodiment each electrical contact is slidable along the channel to allow the selective positioning of the same.

In one embodiment the spaced parallel members are of a cross section which includes a body with a central bore and remote peripheral flange sections connected thereto by webs provided on at least three sides of said bore such that the flange sections, said webs and the outer surfaces of said central bore together define a channel along the length of said member in which the slidingly adjustable electrical contacts can be received.

In one arrangement of the invention the cross-section of the spaced parallel members and the cross members is the same, being generally cruciform in shape but with each extremity of the cruciform shape being provided with flange elements extending on either side thereof towards respective flange elements which extend from adjacent extremities of the cruciform shape.

The electrical contacts can be arranged generally symmetrically within the loop defined by the geometrical frame such that a source of electrical power may be applied uniformly and in balanced fashion to each of the spaced parallel members and thus to the loop as a whole.

Typically one of the cross members includes an electronic component enclosure which is electrically insulated from the remainder of the frame and which contains the electronic components required for providing the antennae with its desired resonant frequency. Ideally this box contains an adjustable capacitance of around l OpF to allow for minor adjustments in the electronic resonant characteristics of the antennae.

The dimensions of the loop can be selected to suit specific requirements but in one embodiment the dimensions are approximately 1 100mm x 460mm, and the space between the spaced electrical contacts is in the region of 100mm, said electrical contacts being mounted on the end of the tubular components which are slidingly received in the spaced parallel members. Ideally, the spaced parallel members extend beyond the loop in one direction and serve to act as feet for the antennae to support the frame structure as a whole.

In one embodiment the connection of the cross members and the tubular electrical contact elements is achieved by means of bolts which form part of said cross members and the tubular electrical contact elements and which are slottingly and slidingly received in the channels defined in the sides of the spaced parallel members .

The advantages of the present invention include the fact that the geometry (in particular the specific dimensions of the loop being 1 100mm x 460mm) achieves excellent tag reading performance at detection ranges of 3m and beyond, which exceeds known antennae of this type. It is also found that capacitance skin effects which commonly compromise both the efficiency and accuracy of antennae are pervasive only in the internal bore of the member sections and do not affect external flange elements and therefore metal or moisture effects are limited. A much lower Q value than that achieved with conventional antennae is achieved due to the use of aluminium as the primary loop conductor as opposed to copper.

It is also known that the use of bolts which are located in the aluminium sections to allow the connection of the cross members and the electrical contacts to the spaced parallel members allows discontinuity-free electrical connection which improves the overall accuracy and performance.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1A shows a schematic prospective view of an antennae according to the present invention,

Figure IB shows a cross sectional plan view along line AA of the a member of the antennae;

Figure 2 shows a further schematic perspective view of the antennae of Figure 1 with a quarter wave impedance matching cable connected thereto,

Figure 3 schematically represents the electronic circuit diagram for the antennae of Figures 1 and 2,

Figure 4 shows a cross section of one of the spaced parallel members from which the antennae of Figures 1 and 2 is constructed,

Figure 5 shows an enlarged cross-sectional view of one of the spaced parallel members and the means by which connection of the cross members and electrical contacts is achieved, and

Figure 6 provides a schematic representation of the manner in which the quarter wave impedance matching cable is electrically connected to the electrical connectors.

Referring firstly to Figure 1A there is shown an antennae 2 constructed from a framework which in this embodiment includes a pair of spaced parallel members 4, 6 and cross members 8, 10 which form a loop with an aperture therein. The cross member 10 includes an electronic component enclosure and structure bracing box 12 which is typically centrally located.

The antennae 2 is further provided with a pair of electrical contact members 14, 16 which are adjustable up and down the spaced parallel members 4, 6 between the cross members 8,10. The electrical contact member has a free end 15 which depends inwardly of the aperture defined by the loop. Typically the opposing end 17 of the electrical contact is connected to be slidable along a channel 63 defined along the said spaced parallel member as hereinafter described.

As can be seen from Figures 1A and 2, the spaced parallel members 4, 6 extend below the cross member 10 to provide leg sections 18, 20 which are further provided with buttress style feet 22, 24 by means of which the antennae is supported in its upright and conventional orientation.

Figure I B shows an enlarged cross section of one of the spaced parallel members 4 generally indicated at reference numeral 26, and it can be seen from this enlargement that the cross sectional shape of the spaced parallel members is generally cruciform with a body 27 having a central bore 28 surrounded by four cruciform elements 30, 32, 34, 36, each of which has flange elements 30A, 32A, 34A, 36A, which extend from said cruciform elements towards respective flange elements on adjacent cruciform elements and which form a channel 63 via which the cross member and/or electrical contacts can be located and slidingly moved..

The electrical contacts 14, 16, commonly referred to in the industry as gamma matching elements are constructed of copper, whereas the spaced parallel members 4, 6 and the cross members 8, 10 are constructed of aluminium sections as generally represented at 26.

Referring to Figure 2, the antennae of Figure 1A is shown as having a "quarter wave" impedance matching RG58 cable 40 insulatingly secured to the spaced parallel member 4 and electrically connected to either free end of the electrical contacts 14, 16 so that a balanced electrical feed to the antennae as a whole is achieved on one hand at the free end of electrical contact 14, and on the other hand at the free end of electrical contact 16. In this manner, electrical power flows along the electrical contacts 14, 16 and around the remaining rectangular framework of the antennae 2 which defines an aperture 42 through its centre and in effect creates an antennae loop. As mentioned above, the geometry of the antennae is linked to the antennae's tag reading performance, and in Figure 1A the dimensions of the rectangular framework are shown as being 460mm in width at 44 and 1100mm in height at 46.

The aluminium section shown generally at 26 is a proprietary product known as "Rexroth Profile System" aluminium beam, and is 30mm square with an 8mm channel defined between each of the flange elements 30A, 32A, 34A, 36A. Such a section is widely available, and can be obtained from the Bosch Group through RS Components Ltd, part number 389-9796.

The cable 40 in Figure 2 is additionally provided with a BNC connector to allow for easy electrical connection to a source of electrical power.

Also in Figure 2, it can be seen that the dimensions of the electronic component enclosure/structure bracing box 12 are 100mm by 100mm by 56mm, the latter most dimension being the depth of the box 12.

Referring now to Figure 3 there is schematically show , the electronic circuit which represents the antennae, and in particular, within the box 12 there is shown a parallel circuit consisting of one or more capacitances 50, 52, one or more of which may be adjustable, and a resistance 54. Figure 4 provides a further representation of the shape of the cross section of the spaced parallel members 4, 6 of the antennae, and this cross section is further represented in Figure 5 which also demonstrates the manner of connection which is most desirable for the cross members 8, 10, and the electrical contacts 14, 16 to said spaced parallel members. Specifically, the electrical contacts 14, 16 are in the form of tubular copper pipe, into which end is inserted a torque head threaded bolt 60 which first passes through a hexagonal nut 62 before entering one end of the electrical contact 14. The bolt head 64 is of such a size and shape that it is received in the channel 63 defined behind adjacent flange elements 32A, 34A, and therefore the connection of said electrical contact 14 is both slidably adjustable in said channel, and in a form of a slot-like connection which can be secured by means of tightening the hexagonal nut 62 against the outer surfaces of the flange elements 32A, 34A. The connection of the cross members 8, 10 and the remaining electrical contact 16 is achieved in identical fashion. Ideally, the copper pipe which forms the electrical contacts 14, 16 has shortcuts 66 provided in their ends (typically in the order of 5mm in length) and such allows the shank of the bolt 60 to be threadingly received therein. Once the shank is fixedly received in the free end of the copper pipe forming the electrical contacts 14, 16, the assembly may be soldered to complete the electrical connection.

Referring finally to Figure 6, there is schematically shown the pair of electrical contacts 14, 16, both being 15mm diameter copper pipes having their free end separated by a 100mm cap represented generally at 70. The electrical connection of the quarter wave impedance matching RG58 cable 40 which terminates in a female BNC connector is electrically connected to the electrical contacts 14, 16 by means of a simple solder tag and self tapping screw assembly 72, and in accordance with conventional practise, the outer coaxial section of the cable 40 is connected to one of the contacts as generally represented at 74, whereas the inner coaxial section of the cable 40 is connected to the alternate solder tag and self tapping screw assembly as generally indicated as 76. The electrical connections may be made permanent by the use of a simple solder as is also conventional in the industry.

The adjustability provided by the modular construction of the antennae described allows the electronic characteristics of the antennae to be altered so that it can be tuned to the exact frequency of the tags it is required to detect, and in particular the frequency of the tags can often vary between 13.1 and 13.9 MHz. The profile of the aluminium sections from which the various components of the antennae are formed are not affected by metal or moisture in the field on account of the remoteness of the flange elements 30A, 32A, 34A, 36A, and capacitance skin effects are generally limited to the internal bore 28 of the section as hereinbefore described. The inherently simple construction of the antennae also allows for easy and quick installation, and the overall impedance of the antennae can be accurately determined as 50 ohms + /-5%.

Claims

1. An antennae for the detection of a transponder or tag within a detection range of the antennae, said antennae including a framework, at least part of which defines a loop having an aperture therethrough, said framework including at least a pair of spaced parallel members formed of metal or metal alloy forming opposing sides of said framework, and an electrical contact on each of said spaced parallel members, said electrical contacts selectively locatable along at least a portion of said respective spaced parallel members, to allow adjustment of the electrical characteristics of said antennae.

2. An antennae according to claim 1 wherein the loop formed is rectangular in shape.

3. An antennae according to claim 1 wherein the framework includes at least one cross member disposed between the spaced parallel members.

4. An antennae according to claim 1 wherein two cross members are disposed between the spaced parallel members and define the loop.

5 An antennae according to claim 3 or 4 wherein at least one cross member carries an electronic component enclosure which is electrically insulated from the remainder of the framework.

6. An antennae according to claim 5 wherein the electronic component enclosure includes an adjustable capacitance of around l OpF.

7. An antennae according to claim 1 wherein the electrical contacts are slidingly disposed on the respective members. 8 An antennae according to claim 7 wherein the electrical contact is received in a channel formed in the member and movable along said channel.

9 An antennae according to claim 1 wherein the spaced parallel members are of a cross section including a body portion which includes a central bore and remote peripheral flange sections connected thereto to form a channel along the length of said parallel member in which the adjustable electrical contact can be located.

10 An antennae according to claim 9 wherein the cross section of the spaced parallel members and the cross members are the same.

1 1 An antennae according to claim 1 wherein the electrical contacts are positioned symmetrically on the parallel spaced members and electrical power is applied uniformly and in balance to each of the spaced parallel members via the electrical contacts .

12 An antennae according to claim 1 wherein the free ends of the electrical contacts depend inwardly into the aperture defined by the loop .

13 An antennae according to claim 12 wherein the free ends of the electrical contacts are separated by a gap.

14 An antennae according to any of the preceding claims wherein the electrical contacts are connected to a power supply.

15 An antennae according to claim 14 wherein the power supply is a coaxial cable with a first electrical contact connected to the inner coaxial section of the cable and the second electrical contact connected to the outer coaxial section of the cable.

16 An antennae according to any of the preceding claims wherein the antennae is as part of an RFID system and provided to use RFID to detect the presence of a tag in the detectable range of the antennae.

17 An antennae according to any of the preceding claims wherein the framework members are formed of aluminium.

18 An antennae according to any of the preceding claims wherein the framework members are extruded.

19 An antennae according to any of the preceding claims wherein the cross members and/or electrical contacts are located on the parallel spaced members via securing bolts, the heads of which are located and retained in the channel formed along the said member.