Circularly polarized compact helical antenna

Abstract

The present invention relates to a circularly polarized directional helical antenna that is capable of being used in RFID devices and more particularly in RFID readers. The antenna is intended to transmit or receive signals in a predetermined frequency band, being the wavelength associated with the minimum frequency of the predetermined frequency band. It includes a helicoidal radiating element made of conductive material extending along a longitudinal axis (A) and the axial length (H) of which is less than the wavelength , and a cavity made of conductive material having an open end and a closed end and having an axis of symmetry that coincides with the longitudinal axis of the radiating element, at least one lower portion of the radiating element being arranged inside the cavity so that its lower end is in contact with the closed end of the cavity.

Claims

1. A circularly polarized directional helical antenna configured to transmit or receive radio-frequency signals in a predetermined frequency band, , being the wavelength associated with the minimum frequency of the predetermined frequency band, comprising a helicoidal radiating element made of conductive material extending along a longitudinal axis (A), a cavity made of conductive material having an open end, a closed end, and having an axis of symmetry that substantially coincides with the longitudinal axis of the radiating element, with at least one lower portion of the radiating element being arranged inside said cavity so that the lower end of the helicoidal radiating element is in contact with the closed end of the cavity, wherein the axial length (H) of the radiating element is less than the wavelength A and the open end of the cavity is equipped with a periodic metal structure allowing the height of the cavity to be reduced, the periodic metal structure being in the form of metal wire netting comprising a plurality of square meshes, wherein a length of each mesh is between 0.27 and 0.3; and a thickness of the metal wires forming the netting being between 0.003 and 0.012.

2. The antenna according to claim 1, wherein the axial length of the radiating element is substantially equal to 0.288.

3. The antenna according to claim 2, wherein the cavity has cylindrical shape, the height of the cavity is substantially equal to 0.45, and the radius of the cavity is equal to 0.98.

4. The antenna according to claim 2, wherein the periodic metal structure is a wire mesh network having square mesh, the width (a) of the mesh being between 0.27 and 0.30.

5. The antenna according to claim 1, wherein the internal surface of the cavity is covered with a meta-material layer.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a schematic perspective view of a helical antenna according to a first embodiment of the invention with a cylindrical cavity;

(2) FIG. 2 shows the gain and axial ratio curves for the helical antenna of FIG. 1 and for a helical antenna with a circular ground plane as a function of frequency;

(3) FIG. 3 shows the RHCP and LHCP gain of the antenna of FIG. 1 and of the helical antenna with a circular ground plane as a function of its degree of aperture;

(4) FIG. 4 shows a schematic perspective view of a helical antenna according to a second embodiment of the invention with a frustoconical cavity;

(5) FIG. 5 shows the gain and axial ratio curves with a helical antenna of FIG. 4 and for a helical antenna with a circular ground plane as a function of frequency;

(6) FIG. 6 shows the RHCP and LHCP gains of the antenna of FIG. 4 and of the helical antenna with a circular ground plane as a function of its degree of aperture;

(7) FIG. 7 shows a schematic perspective view of a helical antenna according to a third embodiment of the invention with a cylindrical cavity and a periodic metal structure FSS;

(8) FIG. 8 is a schematic view of a portion of a periodic metal structure FSS of the antenna of FIG. 7;

(9) FIG. 9 shows the gain and axial-ratio curves for the helical antenna of FIG. 7 with and without a periodic metal structure FSS as a function of frequency;

(10) FIG. 10 shows the gain of the antenna of FIG. 7 with and without a periodic metal structure as a function of its degree of aperture; and

(11) FIG. 11 is a variant of the embodiment of FIG. 7.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT

(12) The invention will be illustrated by means of various exemplary embodiments of a circularly polarized helical antenna capable of operating in the frequency band [865 MHz-965 MHz] corresponding to the frequencies dedicated to worldwide ISM applications. RFID more particularly uses the 865-868 MHz band in Europe and the 902 MHz-928 MHz band in the USA.

(13) In the description which follows, denotes the wavelength associated with the frequency of 865 MHz. The dimensions of the antenna in the various embodiments are defined in relation to this wavelength.

First Embodiment

(14) According to a first embodiment that is illustrated by FIGS. 1 to 3, the helical antenna, referenced 10 in FIG. 1, has a helicoidal radiating element 11 made of conductive material extending along a vertical axis A and a cylindrical cavity 12 made of conductive material, the axis of symmetry of which coincides with the longitudinal axis A. The cavity has a bottom in the lower part and is open at the top. The lower end of the radiating element 11 is electrically connected to the bottom of the cavity.

(15) The radiating element 11 has the following features: height (axial length) H=30 cm=0.865, winding diameter D=11 cm=0.32, element width L=2 cm=0.057, and winding angle =12.5.

(16) The length of each winding of the element has a length substantially equal to the wavelength .

(17) The dimensions of the cylindrical cavity are: height H=21 cm=0.60, radius R=34 cm=0.98.

(18) The gain and axial ratio curves for the antenna 10 are shown in FIG. 2 and can be compared with those of an identical antenna comprising a circular ground plane of radius R=34 cm instead of the cylindrical cavity, which are likewise shown in FIG. 2.

(19) As can be seen from these curves, the gain of the antenna 10 is high and constant, in the order of 13.7 dB, over the band [800 MHz, 980 MHz] which is indeed wider than the frequency band desired for world passive RFID applications, or in practice for 865 MHz to 965 MHz. Similarly, the ISM bands around 2.45 GHz and 5.8 GHz require no more than 150 MHz of bandwidth. It is higher by at least 2.2 dB than that of the antenna with a circular ground plane.

(20) The axial ratio of the antenna 10 varies between 1.5 dB and 1.8 dB over the desired frequency band. By comparison, the axial ratio of the antenna with a circular ground plane varies between 2 dB and 5 dB. The antenna 10 thus has very good circular polarization.

(21) FIG. 3 shows the performance in terms of directivity of the antenna 10 with a cylindrical cavity and of the antenna with a circular ground plane at the frequency of 865 MHz. As can be seen in this figure, the mid-power angle of aperture of the antenna 10 with a cylindrical cavity (=34) is smaller than that of the antenna with a circular ground plane (=55), which allows better directivity to be obtained.

(22) All of the performance data for the antenna 10 with a cylindrical cavity and for the antenna with a circular ground plane at the frequency of 865 MHz are recapitulated in the table below:

(23) TABLE-US-00001 Short antenna Short antenna with a circular with a cylindrical ground plane cavity Gain 11 dB 13.7 dB Axial ratio 2.2 dB 1.5 dB Mid-power 55 34 aperture Bandwidth >500 MHz >500 MHz

(24) The antenna 10 is thus particularly advantageous in terms of gain (>13.7 dB), polarization (axial ratio <2 dB), directivity (mid-power aperture angle in the order of 30) and bandwidth (>500 MHz). Moreover, the gain is substantially constant over a wide frequency band.

(25) It should be noted that the dimensions of the cavity may vary without any great adverse effect on the performance mentioned above. It has been stated that, in order to obtain a maximum aperture of 36, it is advisable to observe the following dimension ranges for the cavity:

(26) TABLE-US-00002 Dimension range Cavity height H 0.4 < H < 0.88 Cavity radius R 0.92 < R < 1.05

(27) It is possible to use other shapes of cavities, for example a frustoconical or substantially frustoconical cavity (truncated cone made from a plurality of substantially identical polygons).

Second Embodiment

(28) Such a variant with a frustoconical cavity is illustrated by FIGS. 4 to 6. The antenna, referenced 20, comprises a helicoidal radiating element 21 that is identical to the radiating element 11 and a frustoconical cavity 22, the axis of symmetry of which coincides with the axis A of the element. The frustoconical cavity 22 has a bottom in the lower part and is open at the top. The lower end of the radiating element 21 is electrically connected to the bottom of the frustoconical cavity.

(29) The dimensions of the frustoconical cavity are: height H=21 cm=0.60, radius R.sub.top=40 cm=1.15, radius R.sub.base=19 cm=0.54.

(30) The gain and axial ratio curves for the antenna 20 are shown in FIG. 5 and can be compared with those of the identical antenna comprising a circular ground plane which have already been shown in FIG. 2 and which are reprised in FIG. 5.

(31) As can be seen in this figure, the gain of the antenna 20 is relatively constant over the frequency band [850 MHz, 950 MHz]. It is moreover very high, beyond 16 dB, and is higher by at least 4 dB in relation to that of the antenna with a circular ground plane.

(32) The axial ratio is in the order of 1.5 dB over the frequency band [850 MHz-950 MHz]. It is lower by at least 1 dB than that of the antenna with a circular ground plane.

(33) FIG. 6 shows the directivity performance of the antenna 20 at the frequency of 865 MHz. As can be seen in this figure, the mid-power aperture angle of the antenna 20 with a frustoconical cavity is smaller than that of the antenna with a circular ground plane, which allows better directivity to be obtained.

(34) All of the performance data for the antenna 20 with a frustoconical cavity and for the antenna with a circular ground plane at 865 MHz are recapitulated in the table below:

(35) TABLE-US-00003 Short antenna Short antenna with a circular with a cylindrical ground plane cavity Gain 11 dB 16.1 dB Axial ratio 2.2 dB 1.3 dB Mid-power aperture 55 30 Bandwidth >500 MHz >500 MHz

(36) The antenna 20 with a frustoconical cavity is therefore even more advantageous than the antenna 10 with a cylindrical cavity in terms of gain (16.1 dB), polarization (axial ratio <1.5 dB) and directivity (mid-power aperture angle in the order of 30).

(37) The dimensions of the frustoconical cavity may vary without any great adverse effect on the performance mentioned above. It has been stated that, in order to obtain a maximum aperture of 30, it is advisable to observe the following dimension ranges for the cavity:

(38) TABLE-US-00004 Dimension range Cavity height H 0.4 < H < 0.88 Base radius R.sub.base 0.54 < R < 0.65 Top radius R.sub.top 1.15 < R < 1.35

Third Embodiment

(39) It is possible to further reduce the height of the radiating element and the height of the cavity without adversely affecting the performance of the antenna. To this end, the cavity is advantageously equipped, at its open end, with a periodic metal structure forming a frequency-selective surface. In the description which follows, this periodic structure is denoted by the acronym FSS (Frequency Selective Surface). In this embodiment, the whole of the radiating element is placed inside the cavity.

(40) Such an embodiment with a cylindrical cavity and FSS is shown by FIGS. 7 to 10.

(41) With reference to FIGS. 7 and 8, the antenna, referenced 30, comprises a helicoidal radiating element 31 arranged inside a cylindrical cavity 32. The open end of the cavity is equipped with a periodic metal structure or FSS 33.

(42) The radiating element 31 has the following features: height H=10 cm=0.288, turn diameter D=11 cm=0.32, element width L=2 cm=0.057, and winding angle of 12.5.

(43) The dimensions of the cylindrical cavity 32 are: height H=15.5 cm=0.45, radius D=34 cm=0.98.

(44) The periodic metal structure 33 is in the form of wire netting comprising a plurality of square meshes. The length a of the mesh and the thickness e of the metal wires forming the netting are equal to 0.288 and 0.008, respectively. These values correspond to a reflectivity of 21%, the value from which the energy leaving the cavity can be directed and thus good directivity can be obtained.

(45) The gain and axial-ratio curves for the antenna 30 with and without an FSS structure are shown in FIG. 9.

(46) As can be seen in this figure, the gain of the antenna 30 with an FSS structure reaches 14.9 dB around 900 MHz and is relatively constant over the band [840 MHz, 915 MHz]. In the absence of FSS, the gain varies only between 11 dB and 12 dB. The axial ratio of the antenna 30 with an FSS structure varies between 2 dB and 3.3 dB whereas it is higher than 3 dB in the absence of FSS.

(47) In terms of directivity, FIG. 10 shows that the directivity of the antenna 30 with FSS and that of the antenna without FSS are substantially identical. The mid-power aperture angle is between 32 and 36.

(48) The performance data for the antenna 30 with and without FSS are recapitulated in the table below:

(49) TABLE-US-00005 Short antenna Short antenna with cylindrical with cylindrical cavity of low cavity of low height height and FSS Gain 12 dB 14.6 dB Axial ratio 4.7 dB 3.3 dB Mid-power 36 32 aperture Bandwidth 200 MHz 185 MHz

(50) In relation to the antennas 10 and 20, the antenna 30 is particularly advantageous in terms of compactness, since its axial length is almost divided by two, that is to say 15.5 cm instead of 30 cm. This reduction in size is obtained without adversely affecting the gain and directivity of the antenna. By contrast, the circular polarization is slightly adversely affected (axial ratio in the order of 3 dB) as is the bandwidth.

(51) The length a of the mesh and the thickness e of the wires forming the mesh may vary without adversely affecting the performance mentioned above. It has been stated that, in order to preserve a maximum aperture of 36, it is advisable to observe the following dimension ranges for the mesh:
0.27<a<0.3 and 0.003<e<0.012.

(52) Equally, the shape of the mesh may vary. According to one variant embodiment, shown by FIG. 11, the mesh is a hexagonal shape so that the FSS has a honeycomb structure.

(53) The FSS structure may be implemented in one or more layers of material so as to form a 2D or 3D structure.

(54) According to another embodiment, which is not shown by the figures, it is likewise possible to further reduce the height of the cavity by depositing a meta-material layer onto the internal surface of the cavity and more particularly onto the bottom of the cavity. This meta-material layer makes it possible both to reduce the volume of the cavity and to increase the directivity of the antenna.

(55) It goes without saying that the invention can be applied to frequency bands other than the band [865 MHz, 960 MHz].

(56) By way of example, the invention can be applied to frequency bands around the frequencies 2.45 GHz and 5.8 GHz for remote monitoring or remote payment applications. An ISM band around 2.45 GHz, for example the 2400-2500 MHz band, can be used. Equally, for remote payment applications, it is possible to use the 5725-5875 MHz band around 5.8 GHz.

(57) Although the invention has been described in connection with various particular embodiments, it is quite evident that it is in no way limited thereto and that it comprises all of the technical equivalents of the means described as well as combinations thereof if these are within the scope of the invention.