GB2376801A - Antenna device having suppressed far field radiation - Google Patents

Abstract

A RF antenna 100 has a first radiative portion 104 and a second radiative portion 106. The portions each generate an electromagnetic field, the fields having magnetic vector directions 112 and 110 which are substantially opposing, so as to suppress far field radiation. The two radiative regions, which each define a plane, may be superimposed, or may be displaced from one another. Preferably the radiating portions comprise one or more loops, which may have a rectangular shape. The antenna may be in the form of microstrip lines on a printed circuit board (PCB). The antenna may be connected to a transmitter 102. Applications include near field probes for identification detectors/electronic readers.

Description

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Title of the Invention R. F. RADIATORS AND TRANSMITTERS Field of the Invention This invention relates to r. f. radiators and transmitters. In particular, the invention relates to, but not limited to, r. f. radiators and transmitters including them for use as near-field probes.

Background of the Invention R. f. radiators radiate electromagnetic energy in the radio frequency part of the spectrum. As such, they are the basic components of any electronic system that uses free space as an r. f. propagation medium. An antenna is a device that conventionally provides a means for radiating or receiving radio waves in many r. f. systems.

Ir is a transducer converting between an electrical signal generated in a r. f. transmitter and/or received in an r. f. receiver and an electromagnetic wave which propagates in free space.

The physical form and dimensions of a radiator such as an antenna dictate the radiation pattern of the transmitted electromagnetic energy. There are certain basic properties that define the function and operation of an antenna. The properties most often of interest in the design of an antenna are: radiation pattern, antenna gain, polarisation and impedance. For a linear, passive antenna, these properties are identical for the

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transmitting operation and the receiving operation of the antenna, by virtue of the reciprocity theorem as known to those skilled in the art.

The form of an r. f. radiator determines the spatial distribution of the radiated energy. For example, a vertical wire antenna gives uniform coverage in the horizontal (azimuth) plane, with some vertical directionality. As such, a vertical wire antenna is often used for broadcasting purposes.

As an alternative to a radiation pattern providing a uniform coverage, the radiation pattern can be made directional. The directional properties of antennas are frequently expressed in terms of a gain function. The gain of an antenna is defined as the ratio of the maximum radiation intensity from the antenna to the maximum from a reference antenna having the same input power. The reference antenna for this purpose is usually a hypothetical loss-less isotropic radiator and the gain is subsequently expressed in dBi (dB level with reference to an isotropic radiator).

Conventional antennas employed in telecommunications applications are designed to radiate energy to the farfield. The far-field is generally understood, by those skilled in the art, to be indicative of a region which is at a distance from the antenna of the transmitting device which is equal to or greater than a distance which defines the boundary of the so called near-field. The near-field is generally understood, by those skilled in the art, to be a region which is at a small distance from the transmitting device antenna or radiator. Such a

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distance may be defined as any distance within one wavelength of the antenna (at its radiative centre).

Conventional antennas that radiate energy to the farfield, inherently emit some radiated energy in the nearfield. For telecommunications applications radiation patterns produced in the near-field are deemed undesirable.

In some applications, such as where a radiator is to be used as an r. f. probe, e. g. for use in an electronic reader or interrogation device, it may be necessary to provide a radiation pattern only in the near-field. The inventors of the present invention have recognised that conventional antennas do not provide acceptable performance in some such applications since they radiate also to the far-field. Furthermore, the inventors of the present invention have recognised that there is a significant disadvantage in emitting both near-field and far-field radiation patterns in such applications since far field transmissions may have to meet specifications laid down by regulatory authorities. Every device that radiates to the far-field at a power level above a specified minimum must be approved. The specified minimum power level is-ISdBmw. Ensuring that devices and transmissions from them comply with such specifications adds to the cost of antenna design and manufacture.

The purpose of the present invention is to provide an r. f. radiator that may be used to produce a near-field radiation pattern without having the disadvantages described earlier associated with conventional antennas.

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Summary of the present invention According to the present invention in a first aspect there is provided an r. f. radiator which includes a first radiative portion and a second radiative portion, wherein the portions are arranged such that in operation the first radiative portion generates a first electromagnetic field having a first magnetic vector in a first direction and the second radiating portion generates a second electromagnetic field having a second magnetic vector in a second direction substantially opposite to the first direction, such that the first and second magnetic vectors substantially cancel each other.

The first and second portions may each comprise a radiative track or region substantially defining a plane and each of the magnetic field vectors may be substantially perpendicular to the plane defined by the radiative track or region which has produced it. The first and second portions may be arranged such that the planes which are defined by their radiative track or region in this way co-incide, i. e. the first and second portions are superimposed one on the other.

Alternatively, the radiative portions may arranged such that the said defined planes are displaced from one another. Even where displaced, the said planes are preferably parallel or in the same overall plane.

In any event, the radiating portions of the r. f. radiator according to the invention may beneficially be arranged such that in operation substantial radiation transmission to the far-field is eliminated.

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In the r. f. radiator according to the invention each of the radiative portions may comprise one or more loops which in operation are current-carrying loops. Each loop may be formed in a region which approximates to an associated defined plane. The loops may be or may approximate to rectangular-shaped current-carrying loops, e. g. wire loops. Each of the current-carrying loops may conveniently be of shape and dimensions substantially the same as for the other loops. The radiative portions may comprise wire loops although they may alternatively be printed on a printed circuit board and/or are microstrip or stripline portions.

The r. f. radiator according to the invention may be such that the portions are arranged so that in operation they form a single electric current path. In operation, where the portions comprise one or more loops, the instantaneous sense (i. e. clockwise or anticlockwise) of current flow in the loop (s) of the first portion may be opposite to that of current flow in the second portion.

According to the present invention in a second aspect there is provided an r. f. transmitter or transceiver including an r. f. radiator according to first aspect described earlier. The transmitter or transceiver may be a probe for near field r. f. irradiation, e. g. near-field excitation, of a target, e. g. an article or sample to be inspected or interrogated for identity or authentication. The target or sample may be located so as to be strongly irradiated by r. f. radiation from the radiator in its near-field. A detector device to hold the sample or target and irradiate the same with r. f.

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radiation may be constructed so that a sample when held by the device is positioned to be suitably irradiated by the r. f. probe.

As noted earlier, if a device radiates in the far-field at a power level below that which requires compliance with regulatory specifications, namely-13dBmw, it can be designed and produced more cheaply than conventional antennas and r. f. probes. The present invention which can be operated at an output power level substantially less than lOOmw giving below the specified far-field radiation level of-13dBmw beneficially provides such a device.

In summary, the inventive concepts described herein provide an arrangement and method for producing a r. f. electromagnetic pattern in the near-field of a radiating device and at the same time suitably minimising or eliminating the radiation delivered to the far-field.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Brief description of the accompanying drawings Figure 1 is a front view of an r. f. transmitter and radiator arrangement in accordance with an embodiment of the present invention; Description of embodiments of the Invention As shown in Figure 1, a current-carrying wire radiator 100 is coupled 108 to a r. f. transceiver device 102. The transceiver device 102 generates r. f. signals for

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transmission via the radiator 100. The device 102 also processes returned signals received via the radiator 100 (acting as a receiving transducer). The radiator 100 is formed from a plurality of rectangular conducting wire loops 104. Notably, the rectangular conducting wire loops 104 (two loops in the FIG. 1 example) are formed and arranged so that when the loops 104 are energised with suitable electric signals from the transceiver 102, the resultant r. f. electromagnetic field has a magnetic vector in a direction 112 which is'out from the page'as indicated, i. e. perpendicular to the plane of the loops 104. In a preferred embodiment of the invention, the rectangles are narrow and long, to ease the manufacturing process. However, it is within the contemplation of the invention that alternative configurations can be used.

The wire employed to form the loops 104 forms a second set of rectangular conducting loops 106 (two shown in Figure 1), i. e. all of the loops being in one overall current path. The loops 106 are laterally displaced from the loops 104 by a distance (between nearest sides) approximately equal to the loop width of the loops. The instantaneous current flow in the loops 106 is as shown in Figure 1 in a clockwise sense when that in the loops 106 is in an anticlockwise sense. The rectangular loops 106 have been formed in such a manner as to generate a radiation pattern having a magnetic vector in a direction 110 which is'into the page'as shown in Figure 1, i. e. in a direction opposite to that of the magnetic vector of the field produced by the loops 104.

A strong near-field radiation pattern is achieved by ensuring the parallel wires in each side of each

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rectangular loop are formed very close together. Since the instantaneous currents are directed in an opposite sense in the two sets of loops 104,106 and the wires of each set of loops are close together, so the overall sense of the current flow is substantially summed to zero. A minimal electromagnetic far-field radiation pattern is thereby achieved by mutual cancellation of the magnetic vectors of the fields produced by the loops 104 on the one hand and the loops 106 on the other hand. achieved.

In an alternative embodiment of the present invention, the radiator arrangement shown in Figure 1 can be formed in microstrip or stripline form, rather than in wire form, on a suitable insulating substrate in a well known manner. For example, the radiator can be easily printed on a printed circuit board (PCB) in this form, with minimal cost.

In a further embodiment of the invention, the loops 104 and 106 may be substantially superimposed one on top of the other.

It will be understood that the radiator arrangement described above, provides the following advantages: (i) it enhances the near-field radiation pattern by arranging and operating the two radiating portions such that the radiation emitted to the far-field is minimised or eliminated; (ii) it avoids the need for users or manufacturers to apply for and receive regulatory approvals or licences, thereby saving time and money ; and (iii) it permits designers and manufacturers to develop a class of r. f. probe for use in authentication

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or identification detectors, e. g. to be used to detect forgery/counterfeiting of inspected samples, that benefit from such an improved radiation pattern that the radiator according to the invention can provide.

Claims (14)

1. Claims 1. An r. f. radiator which includes a first radiative portion and a second radiative portion, wherein the portions are arranged such that in operation the first radiative portion generates a first electromagnetic field having a first magnetic vector in a first direction and the second radiating portion generates a second electromagnetic field having a second magnetic vector in a second direction substantially opposite to the first direction, such that the first and second magnetic vectors substantially cancel each other.
2. 2. An r. f. radiator according to claim 1, wherein each of the radiative portions comprises a radiative track or region substantially defining a plane and wherein in operation each of the said magnetic field vectors is substantially perpendicular to the plane defined by the radiative track or region which has produced it.
3. 3. An r. f. radiator according to claim 2 and wherein the first and second portions are arranged such that the planes which are defined by their respective tracks or regions coincide, whereby the first and second portions are superimposed one on the other.
4. 4. An r. f. radiator according to claim 2 and wherein the radiative portions are arranged such that the planes which are defined by their respective tracks or regions are displaced from one another.
5. 5. An r. f. radiator according to claim 4 and wherein the radiative portions are arranged such that the said planes are parallel or in the same overall plane.

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6. 6. An r. f. radiator according to any one of the preceding claims and wherein the radiative portions are arranged such that in operation substantial radiation transmission to the far-field is eliminated.
7. 7. An r. f. radiator according to any one of the preceding claims, and wherein each of the radiating portions comprises one or more loops which in operation are current-carrying loops.
8. 8. An r. f. radiator according to claim 7, wherein the loops are or approximate to rectangular-shaped loops.
9. 9. An r. f. radiator according to claim 7 or claim 8, and wherein the loops are of substantially the same shape and dimensions.
10. 10. An r. f. radiator according to any one of the preceding claims and wherein the portions are arranged so that they form a single electric current path.
11. 11. An r. f. radiator according to any one of claims 7 to 10 and wherein the first and second portions are arranged so that in operation the instantaneous sense of current flow in the loop or loops comprising the first portion is opposite to that of the current flow in the loop or loop comprising the second portion.
12. 12. An r. f. radiator according to any one of the preceding claims, and wherein the radiative portions are printed on a printed circuit board and/or are microstrip or stripline portions.
13. 13. An r. f. radiator according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawing.
14. 14. An r. f. transmitter or transceiver including a r. f. radiator according to any one of the preceding claims.