AU2007100554A4 - Hook antenna - Google Patents

Description

P/00/009A Section 29

AUSTRALIA

Patents Act 1990 INNOVATION PATENT SPECIFICATION Invention Title: Applicant: HOOK ANTENNA Tagsys SA The invention is described in the following statement: 1 7019 2 HOOK ANTENNA (-i SFIELD OF THE INVENTION The present invention relates to an antenna suitable for reading radio frequency identification (RFID) tags. In particular the present invention relates to an antenna for a hook to hold items of inventory with RFID tags attached to them so that they may be read. The items may reside on the hook for purposes such as storage or display for sale.

BACKGROUND OF THE INVENTION A transmission line comprising parallel wires or conductors in a form of a hook for an item may be utilised as a near field creation device for an interrogator or reader to communicate with an RFID tag or electronic label. Electric field lines just above the plane of the transmission line are in a direction perpendicular to the length of the line, and parallel with the plane of packaging associated with the item placed on the hook to hold the item. A tag placed on the packaging may couple to the electric field. Figure 1 shows a typical transmission line containing two parallel wires or conductors 11, 12, two feed terminals at a feed point 13 and a line end termination 14.

Loading transmission line 10 with its characteristic impedance creates a uniform distribution of current along and electric field between conductors 11, 12. In order to allow electric field lines to exist over some distance for coupling to an RFID tag, a twin or two wire line with wide spacing is desired. However, this structure has a large characteristic impedance making it difficult to achieve a suitable magnitude of line current and hence electric field strength. For example a line formed of 2 mm diameter conductors spaced 15 mm apart has an impedance of around 400 ohms. Reducing the spacing between the conductors to 5 mm still results in an impedance of around 275 ohms, along with a reduced region between the conductors for the electric field, which reduces possible locations for the tag with adequate coupling.

C \DOCOnte "i SoflrwssdjuppXo-1 So \IoTpurary Iroono Fi\OLK9ISNOOK ANTENNA doc Loading a line of such a high impedance with a low impedance placed between the conductors at a suitable position along the line may allow the high impedance line to be matched to a typical 50 ohm impedance antenna feed line, the matching providing improved magnitude of average current in the line conductors. The resulting current distribution on the line is in a form of a standing wave that contains maxima of increased (higher than the average) magnitude and minima of lesser magnitude. Electric field distribution between the line conductors also has these maxima and minima, and while more than adequate tag coupling may occur at a position of a maxima, less than adequate tag coupling may occur at a position of a minima.

SUMMARY OF THE INVENTION The present invention may provide an antenna including a transmission line for identification of RFID tags placed on item packaging wherein the packaging may be used to hold the item on a hook. The transmission line may include twin or two wires or conductors. The wires or conductors may be used to form at least part of the hook. As the wires or conductors used in the line are made larger in diameter or widened and flattened to effectively become conductive strips while width dimensions of the hook are maintained, spacing between the conductors may become smaller, and the line may be considered as a slot in a conductor. This may be especially so if a conductive strip line is terminated with a relatively low impedance or with pieces of a similar conductor material (short circuits). A conventional slot antenna, however, typically suffers from an absence of useful electric field at terminated ends.

The present invention may provide a folded slot antenna that may exhibit a useful electric field substantially along its entire length including the ends of the antenna. A co-linear array of folded slot antennas may be employed to create a long hook with little or no reading nulls for inventory of static items.

According to one aspect of the present invention there is provided an antenna for reading RFID tags, said antenna including a transmission line having two conductors and distributed series capacitance to at least minimize or eliminate C 0 e a o SettgsdJppLoC I So eng&XTenporrer Iyturnel FI,\OLK9I\HOOK ANTENNA doc zero-valued minima in a current distribution along said line, wherein said line is terminated such that null regions of electric field between said conductors are at least minimized or eliminated.

According to a further aspect of the present invention there is provided an antenna for reading RFID tags, said antenna including a transmission line having two conductors in a first plane wherein each conductor includes a loop in a second plane, and wherein said line is terminated such that minima in electric field distribution between said conductors occur in said second plane.

Each loop may include a cross over region such that the phase of said electric field between said conductors in said first plane is preserved across said loops.

According to a still further aspect of the present invention there is provided an antenna for reading RFID tags, said antenna including a transmission line having two conductors in a first plane wherein said conductors are folded at each end such that they continue in a second plane, and wherein said line is terminated such that minima in electric field distribution between said conductors occur in said second plane According to a still further aspect of the present invention there is provided an array of antennas, wherein each antenna is constructed as described above and wherein said antennas are arranged such that no minima in the electric field between said conductors occur in at least one plane.

DESCRIPTION OF A PREFERRED EMBODIMENT Preferred embodiments of the present invention will now be described with reference to the accompanying drawings wherein: Figure 1 shows a typical transmission line containing two parallel wires or conductors; C \DO enits and SethQ\djppPoci S.Mlng.1Te.-or.ry Internal FI..kOLKgNlOOK ANTENNA doo Figure 2 shows a transmission line wherein series capacitance has been inserted along its length; Figure 3 shows a transmission line wherein loops are introduced at regions of electric field minima; Figure 4 shows a transmission line wherein the conductors are crossed over in a region wherein loops have been introduced; Figure 5 shows a shorted two wire transmission line or slot antenna which is folded at its ends; and Figure 6 shows a co-linear array of antennas.

Although an item is usually suspended from a hook with the bulk of the packaging below the hook, in the figures it may be assumed that the field "above" the lines is utilised for tag coupling. A hook line for an application may possibly be inverted from a line as drawn in the figures, depending upon position of a tag on the item.

Figure 2 shows insertion of series capacitance including capacitors 21, 22 along the length of a transmission line 20 such that, when the electrical distance 23 between capacitors 21, 22 is less than one half wavelength, the minima in the magnitude of current distribution of a conductor 24 between capacitors 21, 22 are not zero-valued. Thus the electric field that can be achieved between the conductors of line 20 can be made to contain no nulls. As the separation 23 between capacitors 21, 22 is made decreasingly less than one half wavelength, the difference in magnitude between maxima and minima of the current distribution may be lessened, resulting in a flattened standing wave on line Thus a more uniform electric field, again with no nulls, may be achieved between the conductors of line Figure 3 shows a transmission line 30 which improves tag coupling to the electric field between conductors of the line by adding loops 31, 32 in line 30 in C MDOo,,,nts and Sethngssupp LoCal SoflnngsTenporary Iltenet F i..\OLKVSHOOK ANTENNA doc a region of electric field minima. The loops 31, 32 may allow the electric field minima to occur in a different plane to that of the plane of line 30 used for tag coupling. The magnitude of electric field that exists between conductors of line before and after the loops 31, 32 is adequate for tag coupling, however the field changes phase by 180 degrees across the loops 31, 32 so a tag in an immediate vicinity of loops 31, 32 may not adequately couple to the electric field of line 30 due to destructive superposition of oppositely phased electric fields.

Figure 4 shows a further improved transmission line 40 wherein loops similar to loops 31, 32 also contain cross-overs 41, 42 of conductors of the line such that the 180 degree phase change in the electric field between conductors in line across the loops is eliminated. The electric field has the same direction on each side of the loops so that superposition in the electric field in the immediate vicinity of the loops is constructive, providing adequate tag coupling.

Figure 5 shows a slot antenna comprising a transmission line 50 including two wires 51, 52 that is folded to form physical ends 53, 54 and terminated in low impedance or short circuits at electrical ends 55, 56. A zero in magnitude of the electric field that occurs at shorted (or low impedance) electrical ends 55, 56 exist both in a different plane (in a "down" direction as shown in Figure 5) to that of the electric field of a tag coupling region, and at a position on the line 50 in from physical ends 53, 54 so that as one moves from electrical ends 55, 56 towards physical ends 53, 54 respectively the magnitude of electric field between conductors of line 50 increases. This results in a line 50 with an electric field adequate for tag coupling at physical ends 53, 54. As one moves from physical ends 53, 54 along line 50 the electric field at this tag coupling side increases in magnitude and adds in-phase with the diminishing electric field that exists in the plane below up to the point above where the electrical ends 55, 56 are located. This constructive superposition allows the folds at the physical ends 53, 54 to occur closer to the electrical ends 55, 56 as compared to a point on line 50 where a non-folded line has an electric field which adequately couples to a tag. This results in a hook line usable over more of its physical length, including the physical ends 53, 54 where the item is placed onto the hook, compared to a non-folded line which may not be usable at such an end.

C Docnents and Seflangs djupp\Loo-I SatungsT porary Intoanot F IeW\OLKQ1\HOOK ANTENNAdoc Figure 6 shows a co-linear antenna array 60 including folded slot antenna elements 61, 62 and 63 which, when each antenna element is fed in-phase, may create a long hook suitable for storage or display of many items. The electric fields from physical ends 64 and 65 may combine in-phase providing adequate coupling to a tag present on an item in the vicinity of a gap 66 between antenna elements 61, 62.

A transmission line including twin or two wires or conductors may be constructed in a form of thin strips such as a conductor clad circuit board. The circuit board may itself provide a supporting structure. Alternatively or additionally materials may be used to support the transmission line including embedding the line into a support material. Short sections of transmission line connecting various planes thereof may be electrically connected by use of elements such as wires, vias, or strips of copper within small slots, or may be non-electrically connected (isolated from direct current) by use of elements such as plates of conductor which may capacitively couple between planes of construction. For a case of a conductor clad circuit board wherein wires or conductors are in a form of strips, the structure may be considered as a slot antenna as the strips become wider.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.

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Claims (5)

1. An antenna for reading RFID tags, said antenna including a transmission line having two conductors and distributed series capacitance to at least minimize or eliminate zero-valued minima in a current distribution along said line, wherein said line is terminated such that null regions of electric field between said conductors are at least minimized or eliminated.

2. An antenna for reading RFID tags, said antenna including a transmission line having two conductors in a first plane wherein each conductor includes a loop in a second plane, and wherein said line is terminated such that minima in electric field distribution between said conductors occur in said second plane.

3. An antenna according to claim 2 wherein each loop includes a cross over region such that the phase of said electric field between said conductors in said first plane is preserved across said loops.

4. An antenna for reading RFID tags, said antenna including a transmission line having two conductors in a first plane wherein said conductors are folded at each end such that they continue in a second plane, and wherein said line is terminated such that minima in electric field distribution between said conductors occur in said second plane.

5. An array of antennas, wherein each antenna is constructed in accordance with claim 1, 2, 3 or 4 and wherein said antennas are arranged such that no minima in the electric field between said conductors occur in at least one plane. C \Doo-fnts -d Settbg %djlpp SW sporry IfltflO Fi"oXOLK91MHOOK ANTENNAdoc