WO2020088760A1

WO2020088760A1 - Turnchip circularly polarized antenna

Info

Publication number: WO2020088760A1
Application number: PCT/EP2018/079875
Authority: WO
Inventors: Peter De Maagt; Yiannis John VARDAXOGLOU
Original assignee: European Space Agency; Antrum Ltd.
Priority date: 2018-10-31
Filing date: 2018-10-31
Publication date: 2020-05-07

Abstract

This application relates to a circularly polarized, CP, antenna. The CP antenna comprises a first dipole antenna and a second dipole antenna arranged to cross at their respective centers, each dipole antenna comprising a pair of arms of equal electrical length. One arm of the first dipole antenna is linked to one arm of the second dipole antenna for being coupled to one conductor of a feed line for feeding the CP antenna, and the other arm of the first dipole antenna is linked to the other arm of the second dipole antenna for being coupled to the other conductor of the feed line. Each arm comprises a respective wound wire that is wound into a coil-like structure. The first dipole antenna and the second dipole antenna have different electrical lengths, with the difference in electrical length (possibly in conjunction with an angle between the two dipole antennas) being chosen such that a phase difference between radiated fields from the first and second dipole antennas is substantially 90 degrees.

Description

TURNCHIP CIRCULARLY POLARIZED ANTENNA

Technical Field

This application relates to circularly polarized (CP) antennas. In particular, the application relates to circularly polarized antennas with very small form factor in the VHF/UHF or L/S bands.

Background

Antennas at the UHF/VHF and/or L/S bands are generally large, normally between 0.5 to 0.25 wavelengths in size. Although some smaller antennas have been proposed, most antennas at these frequencies are only capable of providing linear polarization (LP). An example of such small LP antenna is a folded Spherical Helix antenna.

Thus, there is a need for antennas usable for the UHF/VHF and/or L/S bands that provide circular polarization and that have a small form factor.

Summary

In view of some or all of these needs, the present disclosure proposes CP antennas having the features of the respective independent claims.

In an aspect of the disclosure, a CP antenna includes a first dipole antenna and a second dipole antenna arranged to cross at their respective centers. Each dipole antenna may include a pair of arms of equal electrical length. The four arms may be arranged in the same two-dimensional plane. For each dipole antenna, its arms may be joined at the center of the dipole antenna to extend along a (substantially) straight line, with the two arms facing away from each other. One arm of the first dipole antenna may be linked to one arm of the second dipole antenna for being coupled to one conductor of a feed line for feeding the CP antenna. The other arm of the first dipole antenna may be linked to the other arm of the second dipole antenna for being coupled to the other conductor of the feed line. Each arm may include a respective wound wire that is wound into a coil-like structure. The coil-like structure may extend in two or three dimensions. The wound wires may be referred to as coils or springs, for example. The first dipole antenna and the second dipole antenna may have different electrical lengths, with the difference in electrical length chosen such that a phase difference between radiated fields from the first and second dipole antennas is substantially 90 degrees. In other words, the difference in electrical length may be chosen such that the resulting radiated fields result in a CP wave that is transmitted by the antenna.

The above configuration uses dipole antennas with arms comprising wound wires, wherein the dipole antennas have different electrical length and are shunt connected to the same port. Thereby, an antenna with a very small form factor can be provided. CP can be readily achieved by appropriate choice of the electrical length of the dipole antennas. The reduced sized of such an antenna, in conjunction with a gain in the link budget of about 3dB due to CP, allows employing the proposed antenna for various purposes, including machine-to-machine (M2M) and Internet of Things (loT) applications, to name only two examples of possible applications. At the same time, the proposed antenna can be manufactured at reduced cost, using a large share of elements that can be cheaply obtained off-the-shelf.

In some embodiments, the first and second dipole antennas may be arranged (e.g., mounted) at substantially right angles to each other.

In another aspect, a CP antenna includes a first dipole antenna and a second dipole antenna arranged to cross at their respective centers. Each dipole antenna may include a pair of arms of equal electrical length. The four arms may be arranged in the same two-dimensional plane. For each dipole antenna, its arms may be joined at the center of the dipole antenna to extend along a (substantially) straight line, with the two arms facing away from each other. An angle between the first dipole antenna and the second dipole antenna may be different from 90 degrees by a difference angle. The difference angle may be a small difference angle (e.g., small compared to 90 degrees) One arm of the first dipole antenna may be linked to one arm of the second dipole antenna for being coupled to one conductor of a feed line for feeding the CP antenna. The other arm of the first dipole antenna may be linked to the other arm of the second dipole antenna for being coupled to the other conductor of the feed line. Each arm may include a respective wound wire that is wound into a coil-like structure. The coil-like structure may extend in two or three dimensions. The first dipole antenna and the second dipole antenna may have different electrical lengths. The difference angle and the difference in electrical length may be jointly chosen such that the phase difference between the radiated fields from the first and second dipole antennas is substantially 90 degrees.

The above configuration uses dipole antennas with arms comprising wound wires, wherein the dipole antennas have different electrical length and are shunt connected to the same port. Thereby, an antenna with a very small form factor can be provided. CP can be readily achieved by appropriate choice of the electrical length of the dipole antennas and of the (small) difference angle from orthogonality of the two dipole antennas. The reduced sized of such an antenna, in conjunction with a gain in the link budget of about 3dB due to CP, allows employing the proposed antenna for various purposes, including M2M and loT applications, to name only two examples of possible applications. At the same time, the proposed antenna can be manufactured at reduced cost, using a large share of elements that can be cheaply obtained off-the- shelf.

In some embodiments, the difference in electrical length may be further chosen such that an axial ratio of the radiated fields is substantially unity.

In some embodiments, the difference in electrical length may be further chosen to match an impedance at the input port of the CP antenna. Accordingly, additional elements for impedance matching a re not necessary, thereby further reducing complexity and footprint of the antenna.

In some embodiments, at least one of the arms may include a patch coupled to its wound wire, for tuning the electrical length of that arm. Thereby, the electrical length of the arms involved can be tuned in a simple and efficient manner. Moreover, off-the-shelf elements can be used for the arms.

In some embodiments, the respective wound wire of at least one arm may be wound in a two-dimensional plane and may have a meandering structure. The four arms may be of equal configuration. That is, the respective wound wire of each arm may be wound in a two-dimensional plane and may have a meandering structure. Thereby, arms that enable the size reduction of the antenna can be implemented in a particularly simple and efficient manner.

In some embodiments, the respective wound wire of at least one arm may be wound in three dimensions and may have a helical structure. The four arms may be of equal configuration. That is, the respective wound wire of each arm may be wound in three dimensions and may have a helical structure. Thereby, arms that enable the size reduction of the antenna can be implemented in a particularly simple and efficient manner.

In some embodiments, the respective wound wire of at least one arm may have a cross section in a plane orthogonal to a winding direction of that arm that is of circular, elliptic, quadratic, or rectangular shape. The four arms may be of equal configuration. That is, the respective wound wire of each arm may have such cross-section. This would amount to three-dimensional shapes of the wound wires of a right cylinder, elliptic cylinder, or rectangular cuboid (with square or rectangular basis).

In some embodiments, the respective wound wire of at least one arm may be supported by a ceramic or dielectric material. The four arms may be of equal configuration. That is, the respective wound wire of each arm may be supported by a ceramic or dielectric material. Thereby, the size (e.g., diameter) of the antenna can be reduced even further.

In some embodiments, at least one arm may include a ceramic chip that includes the respective wound wire. The four arms may be of equal configuration. That is, each arm may comprise a respective ceramic chip. Thereby, arms that enable the size reduction of the antenna can be implemented in a particularly simple and efficient manner. In some embodiments, the respective wound wire of at least one arm may be mechanically self-standing. The four arms may be of equal configuration. That is, the respective wound wire of each arm may be mechanically self-standing (i.e., air-filled).

In some embodiments, the CP antenna may be configured for transmission and/or reception at a given wavelength. The electrical length of one of the dipole antennas may be larger than half the given wavelength by a small fraction of the given wavelength and the electrical length of the other one of the dipole antennas may be smaller than half the given wavelength by the small fraction.

In some embodiments, the CP antenna may be configured for transmission and/or reception in the UHF or VHF band. Since antennas in these bands are typically rather large, the proposed antenna allows for a significant size improvement in these bands, which are especially relevant for M2M or loT applications.

In some embodiments, the winding density of the respective wound wires of the arms may be such that the mechanical length of the first and second dipole antennas for the aforementioned bands is less than 0.2 meters.

In some embodiments, the CP antenna may further include a matching circuit for being coupled to either conductor of the feed line. The matching circuit may be a balun, for example. The matching circuit may be capacitive or inductive.

Brief Description of the Figures

Example embodiments of the disclosure are explained below with reference to the accompanying drawings, wherein

Fig- 1 and Fig. 2 schematically illustrate an example of a turnstile CP antenna,

Fig. 3A and Fig. 3B schematically illustrate an example of a turnchip CP antenna according to embodiments of the present disclosure, and

Fig. 4A and Fig. 4B schematically illustrate another example of a turnchip CP antenna according to embodiments of the present disclosure.

Detailed Description

In the following, example embodiments of the disclosure will be described with reference to the appended figures. Identical elements in the figures may be indicated by identical reference numbers, and repeated description thereof may be omitted. Broadly speaking, the present disclosure relates to a CP antenna with very small form factor especially in the VHF/UHF or L/S bands. The use of chip antennas, air-loaded helices or meandered printed structures in a turnstile-like configuration allows this very small form factor to be achieved. The consequence of using these structures in a turnstile-like configuration is a very low profile and low cost antenna with a typical size of about 0.1 wavelengths in diameter and 0.025 wavelengths in height. As a comparison, conventional antennas in this frequency range typically offer only linear polarization with dimensions in the range of 0.5 to 0.25 wavelengths in diameter and height. By virtue of its small form factor, the proposed CP antenna is suitable for use as terminal antenna and/or in mobile applications.

An important property of the proposed antenna is that it offers circular polarization. In embodiments of the present disclosure, this is achieved by using a turnstile-type configuration. A turnstile antenna, or crossed- dipole antenna, is a radio antenna consisting of a set of two dipole antennas mounted at right angles to each other and fed in phase quadrature, i.e., the two currents applied to the dipoles are 90° (degrees) out of phase. Their name reflects the notion the antenna looks like a turnstile when mounted horizontally.

An example of a turnstile antenna 100 is schematically illustrated in Fig. 1. The turnstile antenna comprises two orthogonal dipole antennas that are arranged to cross at their respective centers. Each dipole antenna comprises a pair of arms, i.e., a first dipole antenna comprises a pair of arms 110, 120 and a second dipole antenna comprises a pair of arms 130, 140. Within each dipole antenna, the arms have equal electrical length. The arms 110, 120 of the first dipole antenna are coupled to a main feed line 150. The arms 130, 140 of the second dipole antenna are coupled to the main feed line 150 through a 90 degree phasing line 160, so that the first and second dipole antennas are fed 90 degrees out of phase.

Fig. 2 shows a schematic top view of the first and second dipole antennas of the turnstile antenna 100. The length of each dipole antenna is substantially equal to half the wavelength for which the antenna is designed. The two dipole antennas are fed 90 degrees out of phase.

In summary, the turnstile configuration allows to provide a circularly polarized antenna by using two orthogonal dipoles (dipole antennas) of equal length and feedingthem with equal amplitudes and quadrature phase.

Embodiments of the disclosure use a helix-like or meandering structure for the arms of the dipole antennas to reduce the size of the CP antenna. Each arm comprises a respective wound wire that is wound into a coil like structure (in two or three dimensions). Due to the winding, the actual, physical length of the arm becomes significantly smaller than the electrical length of that arm. The amount of length reduction is determined by a winding density of the wound wires. The addition of high-constant dielectric materials to the wound wires allows the dimension of the CP antenna to be even further reduced. In the following, the proposed CP antenna design may be referred to as turnchip antenna. This design is based on the turnstile antenna described above. Accordingly, the proposed (turnchip) CP antenna comprises a first dipole antenna and a second dipole antenna arranged to cross at their respective centers. Each dipole antenna comprises a pair of arms of equal electrical length. Therein, one arm of the first dipole antenna is linked to one arm of the second dipole antenna. These linked arms are coupled to a first terminal that is intended to be coupled to one conductor of a feed line for feeding the CP antenna. The remaining arms are linked to each other as well, i.e., the other arm of the first dipole antenna is linked to the other arm of the second dipole antenna. These linked arms are coupled to a second terminal that is intended to be coupled to the other conductor of the feed line. The first terminal and the second terminal may be said to form an input port of the CP antenna.

As follows from the above, the two dipole antennas are shunt connected to the same port. However, the CP antenna can further comprise a matching circuit (impedance matching circuit, such as, e.g., a balun) for being coupled to either conductor of the feed line. For example, the matching circuit can be coupled to either terminal of the CP antenna.

Each of the four arms comprises a respective wound wire that is wound into a coil-like structure. Several implementations of such wound wire are feasible in the context of the present disclosure. For example, the respective wound wire of at least one arm can be wound in a two-dimensional plane (e.g., so as to have a meandering structure). As another example, at least one arm can comprise a ceramic chip that includes the respective wound wire. A CP antenna that comprises four ceramic chips as the arms of its dipole antennas is described below with reference to Fig. 3A and Fig. 3B. Also, the respective wound wire of at least one arm can be wound in three dimensions (e.g., so as to have a helical structure). For a wound wire that is wound in three dimensions, the wound wire can have a cross section in a plane orthogonal to a winding direction (i.e., direction of main extension) of the respective arm that is of circular, elliptic, quadratic, or rectangular shape, for example. A CP antenna that comprises four helical wound wires (i.e., coils or springs) as the arms of its dipole antennas is described below with reference to Fig. 4A and Fig. 4B. Also, for a wound wire that is wound in three dimensions, the wound wire can be supported by a ceramic or dielectric material. Alternatively, for a wound wire that is wound in three dimensions, the wound wire can be mechanically self-standing (i.e., air filled). Needless to say, the four arms of a CP antenna can be of the same implementation. For example, all four arms can be implemented by respective ceramic chips, by respective helices, etc.

In the proposed CP antenna, the first and second dipole antennas have (slightly) different electrical lengths. The length difference can be a small fraction of the total length of the dipole antennas. If the CP antenna is configured for transmission and/or reception at a given wavelength l, the length of the dipole antennas at which they are in resonance would be half that wavelength, l/2. In the proposed CP antenna, the electrical lengths of the two dipole antennas will differ from half of the given wavelength, l/2, by some small amount (e.g., small compared to the wavelength). When a dipole antenna is shortened below resonance, its impedance becomes capacitive and its current has positive phase relative to the resonant-length dipole antenna. On the other hand, if a dipole antenna is lengthened beyond resonance, it has an inductive impedance (i.e., reactance) and a negatively phased current. As has been found, the electrical lengths of the two dipole antennas can be adjusted until the phase difference of the radiated fields is 90 ° (degrees) and the susceptances from the two dipoles cancel at center frequency where the ideal circular polarization occurs. In other words, the difference in electrical length can be chosen such that a phase difference between radiated fields from the first and second dipole antennas is substantially 90 degrees. For example, the electrical length 1₊ of one of the dipole antennas can be larger than half the given wavelength l/2 by a small fraction d of the given wavelength, 1₊ = l/2 + d, and the electrical length I- of the other one of the dipole antennas can be smaller than half the given wavelength l/2 by the small fraction d, I- = l/2 - d. Then, the small fraction d can be chosen such that the phase difference between the radiated fields from the first and second dipole antennas is substantially 90 degrees.

The difference in electrical length (e.g., the small fraction d) can be further chosen such that an axial ratio of the radiated fields is substantially unity. Likewise, the small fraction d can be chosen to match a certain impedance at the input port of the CP antenna.

One way to achieve the tuning of electrical length of the first and second dipole antennas is to add a patch (e.g., copper patch) coupled to the respective wound wire for at least one of the arms of the CP antenna. In the example of Fig. 3, two of the four arms comprise such patches for tuning the electrical length of the respective arm.

In some embodiments, the two dipole antennas are orthogonal to each other, i.e., the two dipole antennas are arranged at substantially right angles to each other. However, also small deviations from orthogonality are feasible in the context of the present disclosure, i.e., an angle between the first dipole antenna and the second dipole antenna can be different from 90 degrees by a small difference angle. If there is a small difference angle from 90 degrees, this difference angle can be used as another parameter for optimization, in addition to the difference in electrical length. In other words, the small difference angle and the difference in electrical length (e.g., the small fraction d) can be jointly chosen such that the phase difference between the radiated fields from the first and second dipole antennas is substantially 90 degrees. Likewise, the small difference angle and the difference in electrical length (e.g., the small fraction d) can be further chosen such that an axial ratio of the radiated fields is substantially unity and/or to match an impedance at the input port of the CP antenna.

CP antennas according to any of the above implementations can be configured for transmission and/or reception in the UHF or VHF band. For such UHF or VHF CP antennas, the winding density of the respective wound wires of the four arms can be chosen such that the mechanical length of the first and second dipole antennas is less than 0.2 meters.

Fig. 3A and Fig. 3B schematically shows a design of a CP antenna 200 that is based on using small size ceramic chip antennas as the four arms. The use of chip antennas offers small, compact solutions for wireless products that can operate even in the absence of a large ground. Moreover, the proposed CP antenna 200 can be implemented in a particularly inexpensive manner. Fig. 3A shows a front view of the CP antenna 200 and Fig. 3B shows a back view.

The CP antenna 200 comprises first to fourth chip antennas (ceramic chips including a wound wire) 210, 220, 230, 240 implementing the four arms of the CP antenna. The first chip antenna 210 and the second chip antenna 220 are part of a first dipole antenna. The third chip antenna 230 and the fourth chip antenna 240 are part of a second dipole antenna. The first and second dipole antennas can be (substantially) orthogonal. The first chip antenna 210 and the fourth chip antenna 240 are linked by a first link conductor 280. The second chip antenna 220 and third chip antenna 230 are linked by a second link conductor 285. At least one of the dipole antennas can further comprise one or more patches that are coupled to respective ends of the arms (e.g., chip antennas). In the example of Fig. 3A, the second dipole antenna with the third and fourth chip antennas 230, 240 comprises two patches (e.g., copper patches). A first patch 270 is coupled to an end portion (the distal end when seen from the center of the CP antenna 200) of the third chip antenna 230 and a second patch 275 is coupled to an end portion (distal end when seen from the center of the CP antenna 200) of the fourth chip antenna 240. These patches 270, 275 can be used for tuning the electrical length of (the arms of) the second dipole antenna. The first to fourth chip antennas 210, 220, 230, 240, the first and second patches 270, 275, and the first and second link conductors 280, 285 can be mounted on a support structure 290, such as a breadboard, for example. On the back side of the support structure 290 an input port 250 of the CP antenna 200 can be provided, opposite the center of the CP antenna 200.

In a specific example, the turnchip CP antenna 200 can be implemented as follows. The initial design consists of four chip antennas (commercially available) of a given frequency (e.g., 433 MHz), mounted on a Taconic TLC32 substrate (8r = 3.2). The chip antennas are placed orthogonally and are fed in-phase acting as two cross-dipoles (orthogonal dipole antennas). At the two ends of one dipole (e.g., the second dipole antenna) copper patches are added in order to increase its electrical length in relation to the other dipole (e.g., the first dipole antenna). In accordance with the above theoretical considerations, this design generates two resonances (e.g., at 391.7 MHz and 420.8 MHz) with a local maximum between them. By properly trimming the length of the dipoles, a desired center frequency (at the local maximum) can be achieved (e.g., 406 MHz).

Fig. 4 schematically shows a design of a CP antenna 300 that is based on using small air filled helices as the four arms. Fig. 4A shows a front view of the CP antenna 300 and Fig. 4B shows a back view. The CP antenna 300 comprises first to fourth air filled helices (wound wires that are would in a coil-like or helical structure) 310, 320, 330, 340 implementing the four arms of the CP antenna. The first helix 310 and the second helix 320 are part of a first dipole antenna. The third helix 330 and the fourth helix 340 are part of a second dipole antenna. The first and second dipole antennas can be (substantially) orthogonal. The first helix 310 and the fourth helix 340 are linked by a first link conductor 380. The second helix 320 and third helix 330 are linked by a second link conductor 385. At least one of the dipole antennas can further comprise one or more patches (e.g., copper patches) that are coupled to respective ends of the arms (not shown in the figure). The first to fourth helices 310, 320, 330, 340, and the first and second link conductors 380, 385 can be mounted on a support structure 390, such as a breadboard, for example. On the back side of the support structure 390 an input port 350 of the CP antenna 300 can be provided, opposite the center of the CP antenna 300.

Several breadboards have been implemented for the proposed turnchip CP antenna design, either based on chip antennas or air-loaded helices. Test results show that the gain patterns of these antennas are sufficient for the use cases considered with reasonable safety margin.

In summary, the present disclosure provides CP antennas with a significantly reduced form factor. The simple fact that these antennas provides CP immediately implies a gain of 3dB on the link budget. This is a market enabler for M2M and loT applications. The proposed turnchip CP antenna can be easily constructed by using off-the-shelf chip antennas, small spirals or meanderline (printed) structures.

It should be noted that the features of the antenna described above may correspond to respective method (e.g., manufacturing method) features that may not be explicitly described, for reasons of conciseness, and vice versa. The disclosure of the present document is considered to extend also to such method and vice versa.

It should further be noted that the description and drawings merely illustrate the principles of the proposed antenna. Those skilled in the art will be able to implement various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and embodiment outlined in the present document are principally intended expressly to be only for explanatory purposes to help the reader in understanding the principles of the proposed method and system. Furthermore, all statements herein providing principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof.

Claims

1. A circularly polarized, CP, antenna, comprising:

a first dipole antenna and a second dipole antenna arranged to cross at their respective centers, each dipole antenna comprising a pair of arms of equal electrical length,

wherein one arm of the first dipole antenna is linked to one arm of the second dipole antenna for being coupled to one conductor of a feed line for feeding the CP antenna, and the other arm of the first dipole antenna is linked to the other arm of the second dipole antenna for being coupled to the other conductor of the feed line;

wherein each arm comprises a respective wound wire that is wound into a coil-like structure; and wherein the first dipole antenna and the second dipole antenna have different electrical lengths, with the difference in electrical length chosen such that a phase difference between radiated fields from the first and second dipole antennas is substantially 90 degrees.

2. The CP antenna according to claim 1, wherein the first and second dipole antennas are arranged at substantially right angles to each other.

3. A circularly polarized, CP, antenna, comprising:

wherein an angle between the first dipole antenna and the second dipole antenna is different from 90 degrees by a difference angle;

wherein each arm comprises a respective wound wire that is wound into a coil-like structure; wherein the first dipole antenna and the second dipole antenna have different electrical lengths; and

wherein the difference angle and the difference in electrical length are jointly chosen such that the phase difference between the radiated fields from the first and second dipole antennas is substantially 90 degrees.

4. The CP antenna according to any one of claims 1 to 3, wherein the difference in electrical length is further chosen such that an axial ratio of the radiated fields is substantially unity.

5. The CP antenna according to any one of claims 1 to 4, wherein the difference in electrical length is further chosen to match an impedance at the input port of the CP antenna.

6. The CP antenna according to any one of claims 1 to 5, wherein at least one of the arms comprises a patch coupled to its wound wire, for tuning the electrical length of that arm.

7. The CP antenna according to any one of claims 1 to 6, wherein the respective wound wire of at least one arm is wound in a two-dimensional plane and has a meandering structure.

8. The CP antenna according to any one of claim 1 to 7, wherein the respective wound wire of at least one arm is wound in three dimensions and has a helical structure.

9. The CP antenna according to any one of claims 1 to 8, wherein the respective wound wire of at least one arm has a cross section in a plane orthogonal to a winding direction of that arm that is of circular, elliptic, quadratic, or rectangular shape.

10. The CP antenna according to any one of claims 1 to 9, wherein the respective wound wire of at least one arm is supported by a ceramic or dielectric material.

11. The CP antenna according to any one of claims 1 to 10, wherein at least one arm comprises a ceramic chip that includes the respective wound wire.

12. The CP antenna according to any one of claims 1 to 11, wherein the respective wound wire of at least one arm is mechanically self-standing.

13. The CP antenna according to any one of claims 1 to 12,

wherein the CP antenna is configured for transmission and/or reception at a given wavelength; and wherein the electrical length of one of the dipole antennas is larger than half the given wavelength by a small fraction of the given wavelength and the electrical length of the other one of the dipole antennas is smaller than half the given wavelength by the small fraction.

14. The CP antenna according to any one of claims 1 to 13, wherein the CP antenna is configured for transmission and/or reception in the UHF or VHF band.

15. The CP antenna according to claim 14, wherein the winding density of the respective wound wires of the arms is such that the mechanical length of the first and second dipole antennas is less than 0.2 meters.

16. The CP antenna according to any one of claims 1 to 15, further comprising a matching circuit for being coupled to either conductor of the feed line.