Patent Application
20150155619
The present invention relates to a circularly polarized compact helical antenna that is capable of being used in RFID devices and more particularly in RFID readers. Said antenna is intended to transmit or receive signals in the UHF band and more particularly in the ISM band.
Helical antennas are well known in the field of wireless communications because, in axial mode, they are able to provide a high gain over a relatively wide frequency band with good circular polarization.
Conventionally, they have a helicoidal radiating element made of conductive material extending along a longitudinal axis and a ground conductor connected to one of the ends of said element. The axial length of the radiating element is generally equal to several times the wavelength of the signals transmitted or received and the ground conductor is in the form of a plate or a hollow element such as a cylindrical or frustoconical cavity.
The performance of such antennas is described in the document entitled “Enhancing the gain of helical antennas by shaping the ground conductor” by A. R Djordjevic and A. G. Zajic, IEEE Antennas and wireless propagation letters, vol. 5, 2006. II
This document notably describes the performance of three antennas designed to operate in the frequency band [1250 MHz, 2150 MHz]:
In the three cases, the antenna has a helicoidal radiating element having an axial length L=684 mm, a turn diameter D=56 mm and a helical winding angle a (or pitch angle) of 13.5°. The radiating element is of circular cross-section and its diameter d is equal to 2 mm. If λc denotes the wavelength associated with the minimum frequency (1250 MHz) of the frequency band [1250 MHz, 2150 MHz], the radiating element has the following dimensions: L=3.87 λc and D=0.31 λc.
In the case of the single-wire helical antenna with a ground plane of circular or square shape, the diameter or the side length recommended for the ground plane is between 0.5 λc and 0.75 λc. Over this range, the gain is very low band but it can attain 14.4 dB. In the case of a helical antenna with a square ground plane, a ground plane having a side of length equal to 1.5 λc makes it possible to maximize the average gain over the frequency band. The maximum gain (or peak gain) of the antenna is thus 14.3 dB.
In the case of the single-wire helical antenna with a cylindrical cavity, it has been determined that the optimum dimensions for the cavity are as follows: diameter D′=1 λc and height H′=0.25 λc. The presence of the cylindrical cavity makes it possible to increase the gain by 1 dB compared to the antenna with a square ground plane.
Finally, in the case of the single-wire helical antenna with a frustoconical cavity, the optimum dimensions are as follows: small diameter (in lower part of the cavity) D′1=0.75 λc, large diameter (in upper part of the cavity) D′2=2.5 λc, and height H′=0.5 λc. The presence of the frustoconical cavity has made it possible to increase the gain by 3.4 dB compared to the antenna with a square ground plane. It has likewise been stated that the presence of the frustoconical cavity makes it possible to obtain a lower axial ratio and weaker secondary lobes.
Although this document shows that the antennas with a cylindrical or frustoconical cavity have good performance in terms of axial gain and directivity, it is nevertheless the case that the antennas proposed in this document are not compact, since the helicoidal radiating element forming the helix has an axial length corresponding to several wavelengths.
It is an object of the invention to propose a circularly polarized helical antenna which is compact, that is to say having a helicoidal radiating element of relatively small axial length, in order to be able to be placed in a relatively small space, for example in the false ceiling of a room.
It is another object of the invention to propose a helical antenna having a high gain over a relatively wide bandwidth with good circular polarization.
It is another object of the invention to propose a circularly polarized helical antenna having a constant high gain over an extended frequency band.
It is another object of the invention to propose a circularly polarized helical antenna having good directivity.
The invention relates to a circularly polarized directional helical antenna capable of transmitting or receiving radio-frequency signals in a predetermined frequency band, λ being the wavelength associated with the minimum frequency of said predetermined frequency band, comprising a helicoidal radiating element made of conductive material extending along a longitudinal axis, and a cavity made of conductive material having an open end and a closed end and having an axis of symmetry that substantially coincides with the longitudinal axis of the radiating element, at least one lower portion of said 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, characterized in that the axial length of the radiating element is less than the wavelength λ.
The relatively small axial length of the radiating element makes it possible to obtain a compact antenna without any adverse effect on the performance of the antenna.
According to a first embodiment, the axial length of the radiating element is substantially equal to 0.865 λ.
If the antenna has a cylindrical cavity, the height of said cavity is thus advantageously between 0.4 λ and 0.88 λ and the radius of the cavity is between 0.92 λ and 1.05 λ. Preferably, the height of said cavity is equal to 0.60 λ and the radius of the cavity is equal to 0.98 λ.
If the antenna has a frustoconical cavity, the height of said cavity is advantageously between 0.4 λ and 0.88 λ, the base radius of the cavity is thus between 0.54 λ and 0.65 λ and the top radius of the cavity is between 1.15 λ and 1.35 λ. Preferably, the height of said cavity is equal to 0.60 λ, the base radius of the cavity is equal to 0.54 λ and the top radius of the cavity is equal to 1.15 λ.
According to another embodiment that is even more compact, the axial length of the radiating element is substantially equal to 0.288 λ 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 is a wire mesh network.
According to an embodiment with a cylindrical cavity, the height of the cavity may be reduced to 0.45 λ, the radius of the cavity remaining equal to 0.98 λ and the mesh width being between 0.27 λ and 0.30 λ.
According to an even more refined embodiment, the internal surface of the cavity is covered with a meta-material layer so as to reduce the height of the cavity even more.
Other advantages will emerge for a person skilled in the art upon reading the examples below, which are illustrated by the attached figures and given by way of illustration.
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.
In the description which follows, λ 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.
According to a first embodiment that is illustrated by
The radiating element 11 has the following features:
The length of each winding of the element has a length substantially equal to the wavelength λ.
The dimensions of the cylindrical cavity are:
The gain and axial ratio curves for the antenna 10 are shown in
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.
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.
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:
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.
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:
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).
Such a variant with a frustoconical cavity is illustrated by
The dimensions of the frustoconical cavity are:
The gain and axial ratio curves for the antenna 20 are shown in
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.
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.
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:
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°).
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:
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.
Such an embodiment with a cylindrical cavity and FSS is shown by
With reference to
The radiating element 31 has the following features:
The dimensions of the cylindrical cavity 32 are:
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 λ and 0.008 λ, 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.
The gain and axial-ratio curves for the antenna 30 with and without an FSS structure are shown in
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.
In terms of directivity,
The performance data for the antenna 30 with and without FSS are recapitulated in the table below:
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.
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:
0.27λ<a<0.3λ and 0.003λ<e<0.012λ.
Equally, the shape of the mesh may vary. According to one variant embodiment, shown by
The FSS structure may be implemented in one or more layers of material so as to form a 2D or 3D structure.
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.
It goes without saying that the invention can be applied to frequency bands other than the band [865 MHz, 960 MHz].
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1352446 | Mar 2013 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2013/055182 | 6/24/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61663324 | Jun 2012 | US |
