Automotive gesture detection system with improved sensitivity

Abstract

The gesture detection device (1) intended to be mounted on a motor vehicle comprises: - a radar module (2) having at least one radio frequency antenna (21) for transmitting a transmitted radio frequency signal and receiving a return radio frequency signal resulting from the reflection of the transmitted signal on a target (50); - a signal processing module (3) for receiving data related to the return signal and deriving information related to the detection of gestures from the return signal. The radio frequency antenna (21) is configured to transmit a linearly polarized signal along a polarization axis (201) oriented substantially vertically during use. The system (1) comprises at least one housing (4) accommodating the radar module (2), and the distance (d1) between the antenna (21) and the outer surface (41) of the housing (4) forming part of the outer surface of the vehicle in use is: d d lim = 2*D 2 / λ + d0 where: D is the diameter of the antenna (21); λ is the central wavelength of the transmitted signal; d0 is a positive or zero constant. ​

Description

[Technical Field]

[0001] The present invention relates to the field of automobiles, and more specifically to a gesture detection system intended to be installed in an automobile for detecting predetermined gestures made by a user outside the vehicle.

[0002] Upon detecting a predetermined gesture, the gesture detection system generates a command that allows for direct or indirect control of opening elements, such as the side doors or trunk doors of a vehicle, and / or unlocking them.

[0003] Background technology Such gesture detection systems based on radio frequency detection are known in the prior art. In particular, a radar module is configured to transmit a radio frequency signal called a transmit signal and to receive a radio frequency signal called a return signal, which arises from the reflection of the transmit signal on a target.

[0004] By analyzing the return signal, information related to the target's movements can be obtained, and therefore, a given gesture can be identified, particularly by comparison with a threshold.

[0005] In practice, the return signal is favorably mixed with the transmission signal's frequency signal to obtain data on the in-phase component l(t) and quadrature-phase component Q(t) of the return signal. By analyzing these components, it becomes possible to extract phase values ​​that describe the movement performed by the target, in particular.

[0006] When in use, the target is formed by a part of the user's body, generally the hands or feet. For example, it is known to detect foot movements made by a user under the rear bumper of a vehicle to control the opening of the trunk or tailgate. Another application involves detecting hand movements made by a user near a lateral intermediate pillar (e.g., pillar B) of a vehicle to control the opening of a side door (e.g., a sliding door).

[0007] However, false movements in the external environment, particularly those generated by raindrops, can potentially trigger the detection of a given gesture.

[0008] In fact, when it rains, the raindrops falling around the vehicle become moving targets for the gesture detection system. These raindrops then generate significant detection noise. Furthermore, because the falling speed of the raindrops is roughly the same as the moving speed of the target in a given gesture, this noise lies in the same frequency band as the payload signal associated with the gesture performed by the user.

[0009] Solutions exist to overcome this noise. These solutions are based on complex processing of digital signals and have limitations in heavy rainfall and extreme weather conditions.

[0010] The object of the present invention is to propose a gesture detection system intended to be installed in a vehicle to detect predetermined gestures made by a user outside the vehicle, and which is reliable even in the event of heavy rainfall and encounters noise generated by rainfall.

[0011] A further objective of the present invention is to propose a solution that does not involve extremely complex signal processing operations.

[0012] Summary of the Invention This objective is achieved by a gesture detection system intended to be installed in a vehicle to detect predetermined gestures made by a user outside the vehicle, and this system is -A radar module comprising a printed circuit board equipped with at least one radio frequency antenna, wherein the at least one radio frequency antenna is configured to transmit a radio frequency signal called a transmit signal and to receive a radio frequency signal called a return signal resulting from the reflection of the transmit signal on a target, - A signal processing module connected to a radar module and configured to receive data about the return signal and to estimate information about the detection of a given gesture from the return signal. It is equipped with.

[0013] According to the present invention, - At least one radio frequency antenna is configured to transmit a linearly polarized transmission signal along a predetermined polarization axis, the predetermined polarization axis is intended to be oriented substantially vertically during use. - The system comprises at least one housing that accommodates a radar module, and the distance d1 between at least one radio frequency antenna and the outer surface of the housing is d lim =2*D 2 / λ+d0 That concludes the explanation. D is the diameter of the radio frequency antenna. λ is the center wavelength of the transmitted signal. d0 is a positive or zero constant, i.e., greater than or equal to 0. The aforementioned outer surface of the housing is intended to form part of the vehicle's exterior when in use.

[0014] Therefore, the present invention is based on an advantageous combination of two features.

[0015] On the other hand, linearly polarized transmitted radar signals are used for gesture detection. The gesture detection system is configured so that, when used in a vehicle, the polarization axis is approximately vertical when the vehicle's four wheels are in contact with the horizontal ground. The term "approximately vertical" is understood to mean "parallel to the axis of gravity at an angle of approximately 10°, or further 5°, or 1° (180° is equal to π radians)." Therefore, a given polarization axis is approximately parallel to the axis of raindrop movement when in use.

[0016] On the other hand, between at least one radio frequency antenna and the outer surface of the housing which forms part of the outer surface of the vehicle when in use, d lim =2*D 2A distance d1 greater than or equal to / λ + d0 is guaranteed. If there are several radio frequency antennas for transmitting the transmission signal and / or receiving the return signal, each of them complies with this condition. d1 is the minimum distance between the antenna under consideration and the outer surface of the housing that forms part of the outer surface of the vehicle during use.

[0017] D corresponds to the diameter of the radio frequency antenna, is defined within the plane of the printed circuit board, and corresponds to the maximum linear dimension of the antenna within this plane. Preferably, D is the diameter of the smallest square or rectangle within which the radio frequency antenna under consideration is inscribed. The ground plane is not considered part of the antenna.

[0018] λ corresponds to the center wavelength of the transmission signal, and λ = C / f, where C is the speed of light in a vacuum and f is the center frequency of the transmission signal.

[0019] 2*D 2 / λ defines the furthest distance between the far field, near field and Fresnel region of the antenna under consideration.

[0020] The constant d0 defines a safety margin for ensuring operation in the far field, particularly taking into account the mechanical tolerance values of the system according to the present invention.

[0021] The constant d0 can take the value of 0, but is preferably in the range of 2 mm to 7 mm, for example equal to 5 mm.

[0022] This distance d1 being lim greater than or equal to d ensures that the external environment that may generate noise, more specifically any raindrops, are guaranteed to be in the far field of at least one radio frequency antenna.

[0023] By combining only far field detection and vertical polarization during transmission, it is guaranteed that the return signal reaching the radar module is not affected, or is hardly affected, by the movement of raindrops.

[0024] In particular, the configuration according to the present invention avoids any near-field detection related to raindrops. In the far field, it may be possible to detect raindrops with a circularly polarized transmitted signal. By using a vertically linearly polarized transmitted signal that is substantially parallel to the axis of movement of the raindrop, the detection of far-field raindrops is also minimized.

[0025] Therefore, the present invention proposes a clever solution for minimizing the impact of rain on the return signal in a gesture detection system intended to be mounted in a vehicle to detect a predetermined gesture performed by a user outside the vehicle. Instead of optimizing the received signal processing operations that those skilled in the art would certainly perform, the present invention proposes optimizing the physical characteristics of the detection system. This solution eliminates complex processing operations that are costly in terms of computational resources and energy. Furthermore, this solution exhibits optimal reliability even under heavy rainfall and extreme weather conditions.

[0026] Reducing the impact of rain on the return signal increases the signal-to-noise ratio between the payload signal associated with user gestures and noise, particularly that associated with rain. This can reduce the false detection rate of a given gesture (false detection of a given gesture, or conversely, failure to detect a given gesture despite proper user operation).

[0027] In practice, the distance between at least one radio frequency antenna and the outer surface of the housing is advantageously 5 mm or more, or 8 mm or more, or 10 mm or more.

[0028] In an advantageous embodiment, a single antenna is configured to transmit a transmission signal and receive a return signal.

[0029] As a variation, one antenna is configured to transmit a transmission signal, and another separate antenna is configured to receive a return signal. The two antennas are advantageously integrated on the same printed circuit board. According to this invention, the distance to the outer surface of the housing is 2*Di for both. 2The condition that it is greater than or equal to / λ is met, where Di is the diameter of the antenna under consideration and λ is the center wavelength of the signal transmitted by the transmitting antenna.

[0030] In one advantageous embodiment, the radar module comprises at least two radio frequency antennas, one having a linearly polarized antenna for transmitting a transmission signal and the other a circularly polarized antenna for receiving a return signal.

[0031] The transmitted signal is, advantageously, a radio frequency signal with a center frequency in the range of 5 GHz to 10 GHz.

[0032] Furthermore, or as a variation, the transmitted signal is, advantageously, a radio frequency signal with a spectral width greater than 500 MHz.

[0033] The housing may, advantageously, comprise a trim or structural component for a vehicle having a recess for housing a radar module, and a decorative panel that covers the recess and forms the outer surface of the housing.

[0034] The radar module can be mounted on the bottom of the housing, on the opposite side of the aforementioned outer surface of the housing.

[0035] Additionally or as a modification, the radar module includes at least one shim on the side of the aforementioned outer surface of the housing.

[0036] The present invention also relates to an automobile equipped with the system according to the present invention.

[0037] The system is advantageously positioned near the structural columns of the vehicle, and the polarization axis of the linearly polarized radio frequency antenna is approximately parallel to the extension axis of the column at an angle of about 10°.

[0038] Further features and advantages of the present invention will become clearer upon reading the following description. This description is purely illustrative and should be read with reference to the accompanying drawings. [Brief explanation of the drawing]

[0039] [Figure 1] A schematic diagram of the gesture detection system according to the present invention, used in a rainy environment, is shown. [Figure 2] Figure 1 schematically shows a top view of the printed circuit board of the radar module in the system shown. [Figure 3] A gesture detection system according to a first modification of the present invention is schematically shown. [Figure 4] A gesture detection system according to a second modification of the present invention is schematically shown.

[0040] Modes for carrying out the invention First, we will describe the first embodiment of the gesture detection system 1 according to the present invention, shown in Figure 1, in use in a rainy environment.

[0041] A gesture detection system 1, which detects predetermined gestures made by a user outside the vehicle, is installed in a vehicle not shown.

[0042] Advantageously, the detection of a predetermined gesture directly or indirectly generates a command to open and / or unlock an opening element of the vehicle, such as a side door or trunk door.

[0043] For example, when in use, the gesture detection system 1 is incorporated into the rear bumper of the vehicle to detect predetermined gestures made by the user's foot for the purpose of controlling the opening of the trunk or tailgate door.

[0044] As a variation, the gesture detection system 1 is incorporated into a lateral intermediate column of the vehicle, preferably column B, to detect a predetermined gesture made by the user's hand for the purpose of controlling the opening of a side door. It is clear that the present invention is not limited to these two embodiments.

[0045] System 1 comprises a radar module 2, a signal processing module 3, and a housing 4 that accommodates at least the radar module 2.

[0046] Radar module 2 is shown in more detail and as a top view in Figure 2.

[0047] The radar module 2 includes a printed circuit board 20, and Figure 2 is a plan view of the printed circuit board.

[0048] The printed circuit board 20 is provided with at least one radio frequency antenna 21, which is schematically shown in Figures 1 and 2.

[0049] Throughout this specification, the term “radio frequency” refers to a signal having a carrier frequency in the range of 3 kHz to 300 GHz. Preferably, in the present invention, the carrier frequency is in the range of 5 GHz to 30 GHz. For example, the carrier frequency is in the range of 5 GHz to 10 GHz, more preferably 7 GHz to 9 GHz, more preferably 6.5 GHz to 8 GHz, and including the limit, equal to, for example, 8 GHz. As a variation, the carrier frequency is in the range of 20 GHz to 30 GHz, for example, 24 GHz.

[0050] At least one radio frequency antenna 21 is configured to transmit a radio frequency signal called a transmit signal and to receive a radio frequency signal called a return signal resulting from the reflection of the transmit signal at the target.

[0051] It is advantageous for the transmitted signal to be a pulsed signal, and the pulse is carried by a carrier at a radio frequency as defined above. The carrier frequency can vary as a function of time within each pulse.

[0052] Preferably, the transmitted signal has a spectral width-to-center frequency ratio of 20% or more, and / or its spectral width is greater than 500 MHz. In other words, the transmitted signal is an ultra-wideband (UWB) signal.

[0053] As a variation, the transmitted signal is a frequency-modulated continuous wave (FMCW) signal. This is a continuous signal whose frequency changes as a function of time in order to sweep a wide frequency range.

[0054] In either case, it is advantageous for the transmitted signal to have a broad spectral bandwidth for performing gesture detection. However, the present invention is not limited to this feature and also applies to continuous transmitted signals.

[0055] During use, the transmitted signal is reflected by at least one target present in the environment of radar module 2 and returned to the radar module in the form of a return signal. The return signal has physical characteristics, particularly regarding phase, beat frequency, and amplitude, which make it possible to describe the target's movement. The return signal is received by at least one radio frequency antenna 21.

[0056] In the example shown in Figure 2, the same radio frequency antenna 21 performs both the transmission of the transmit signal and the reception of the return signal. In a modified version, two separate antennas transmit the transmit signal and receive the return signal, respectively. According to yet another modification, several antennas transmit the transmit signal and / or several antennas receive the return signal. Each antenna can then be dedicated to a determined detection area.

[0057] In this case, and advantageously, the printed circuit board 20 also includes a pre-processing unit 22 connected to at least one antenna 21.

[0058] The pre-processing unit 22 is, -In order to transmit a transmission signal, an appropriate electrical signal is generated and this signal is transmitted to the input of at least one antenna 21. - An electrical signal originating from at least one antenna 21, which receives an electrical signal corresponding to the return signal, - At least one mixing operation is performed between an electrical signal corresponding to the return signal and an electrical signal of the frequency of the transmit signal, preferably a common-mode signal and a quadrature-phase signal, to generate signals l(t) and Q(t), respectively. It is structured in this way.

[0059] For this purpose, the preprocessing unit 22 advantageously comprises elements such as an electrical oscillator for generating an electrical signal that can be converted into a radio frequency signal, at least one mixer, and at least one analog-to-digital converter for performing time sampling and converting the analog signal into a digital signal.

[0060] The radar module 2 is connected to the signal processing module 3. The signal processing module comprises at least one processor and at least one memory for storing a data analysis computer program.

[0061] The signal processing module 3 is configured to receive data relating to the return signal, such as the signals l(t) and Q(t) described above, as input (for example, in the form of a matrix called a CIR matrix, which groups these signals into packets transmitted at regular intervals). It is configured to analyze the data relating to this return signal so as to recognize when the target performed a predetermined gesture toward antenna 21.

[0062] Gesture recognition is based on extracting features such as phase shifts between the transmitted and returned signals, and / or Doppler frequencies, and / or amplitude variations between the transmitted and returned signals. The extracted data is analyzed, for example, by comparing it to predetermined thresholds that define a given gesture. Advantageously, the transmitted signal is a broadband signal as described above, and the signal processing module 3 pre-extracts data regarding a desired distance range between the target and the antenna 21.

[0063] The signal processing module 3 is configured to output information regarding a predetermined gesture when it detects the gesture. This information is sent to the input of a control unit, such as a unit for controlling the unlocking and / or opening of the vehicle's opening elements, and an unlocking and / or opening command is generated. In a modified configuration, the control unit forms an integral part of the signal processing module 3.

[0064] The purpose of the gesture detection system 1 is to recognize that a predetermined gesture has been performed by a target 50 located outside the vehicle and formed by a part of the user's body (typically a hand or foot). However, it is understood that any moving object within the field of view of the radio frequency antenna 21 may reflect at least a portion of the transmitted signal, thereby generating a false signal. This false signal interferes with the detection of the signal reflected by the target 50. Limiting this interference is even more difficult if it is generated by an object moving at a speed comparable to that of the target 50. This is especially true if the object is formed by a raindrop 60.

[0065] According to the present invention, at least one radio frequency antenna 21 is configured to transmit a linearly polarized transmission signal along a predetermined polarization axis. In other words, -When the same radio frequency antenna 21 transmits a transmission signal and receives a return signal, the return signal is linearly polarized. -When two separate antennas each transmit a transmission signal and receive a return signal, at least the transmitting antenna is linearly polarized. The antenna for receiving the return signal may have nonlinear polarization, such as circular polarization.

[0066] In Figure 1, arrow 201 is used to schematically show the polarization axis of the linearly polarized transmitted signal.

[0067] When the system 1 according to the present invention is mounted on a vehicle with its wheels on a horizontal and flat surface, the polarization axis 201 is oriented substantially vertically, that is, substantially parallel to the gravity axis.

[0068] The term "substantially parallel" is understood to mean an angular deviation of less than or equal to 10° in absolute value, or less than or equal to 5° in absolute value, or less than or equal to 1° in absolute value (an angle of 180° is equal to π radians).

[0069] In an advantageous but non-limiting example, the system 1 according to the invention is configured to be housed in the vicinity of a lateral intermediate pillar of a vehicle, for example pillar B, and the polarization axis 201 is substantially parallel to the longitudinal axis of said pillar. Also in this case, the term "substantially parallel" is understood to mean an angular deviation of less than or equal to 10° in absolute value, or less than or equal to 5° in absolute value, or less than or equal to 1° in absolute value (an angle of 180° is equal to π radians). Since the lateral intermediate pillar generally extends along a vertical axis, this arrangement of the polarization axis 201 with respect to the longitudinal axis of the pillar makes it possible to easily achieve the desired orientation with respect to the vertical axis.

[0070] A person skilled in the art will be able to easily find examples of linearly polarized radio frequency antennas that can preferably be directly incorporated into a printed circuit board from the literature and general knowledge. For example, a monopole antenna incorporated on a printed circuit can be cited.

[0071] Also according to the invention, the system 1 comprises a housing 4 that houses the radar module 2. In this case, and without being limited thereto, the signal processing module 3 is arranged remotely outside the housing 4. In fact, the housing 4 is configured such that an electrical connection line extending externally from the radar module 2 emerges.

[0072] The radar module 2 is fixedly arranged within the housing 4, and a distance d1 between at least one radio frequency antenna 21 and the outer surface 41 of the housing satisfies d1≧d lim 、d lim =2*D 2 / λ+d0 is guaranteed to be met.

[0073] D is the diameter of the radio frequency antenna 21 under consideration. The diameter D of the antenna 21 is its maximum transverse linear dimension within the plane of the printed circuit board 20. In practice, the diameter of the shortest rectangle (or square) inscribed in the antenna is advantageous as a top view of the plane of the printed circuit board 20. Figure 2 shows this smallest rectangle 29 as a dashed line. This defines the layout of the antenna 21 within the plane of the printed circuit board 20. In this case, the layout includes a rectangular transmitting area and an adapted area between the rectangular transmitting area and the track for circulating the electrical signal. However, the antenna layout excludes the grounding plate. D is, for example, in the range of 1 cm to 3 cm, and equal to, for example, 2 cm.

[0074] λ is the center wavelength of the transmitted signal (approximately equal to the wavelength of the return signal).

[0075] d0 is a positive or zero constant, favorably in the range of 2mm to 20mm, or even 2mm to 5mm, for example, equal to 5mm.

[0076] The outer surface 41 of the housing is an outer surface of the housing intended to form part of the outer surface of the vehicle when in use.

[0077] Therefore, the outer surface 41 of the housing faces the detection area of ​​the gesture detection system 1 when in use.

[0078] Value 2*D 2 / λ is the region R1(2*D) of the detection area corresponding to the far field of the radio frequency antenna 21. 2 (distance of / λ or greater), detection area corresponding to the near field and Fresnel region R2(2*D 2 It defines a distance less than / λ.

[0079] Therefore, the arrangement of the housing 4 and the radar module 2 within the housing 4 ensures that distant field objects are reliably detected.

[0080] However, in far-field detection, detection is sensitive to polarization. In other words, in far-field detection, the return signal is unaffected, or only slightly affected, by objects whose movement is parallel to the polarization axis of the transmitted signal. By designing the system so that the polarization axis of the transmitted signal is aligned with an axis parallel to the axis of movement of raindrops, the system can be highly reliable even when encountering noise generated by rain.

[0081] Therefore, the present invention minimizes false signals generated by the movement of raindrops relative to the radar module 2 by combining linear polarization signal transmission with far-field detection only.

[0082] According to a favorable embodiment, the transmitted signal has a center frequency f equal to 8 GHz (where λ = C / f, and C is the speed of light in a vacuum). This is 2*D 2 / λ ≈ 5.3 mm. Advantageously, next, d lim The range is selected as 8mm to 20mm, for example, d lim =8mm, or d lim = 10mm

[0083] Figure 3 schematically shows a gesture detection system 1' according to a first modification of the present invention. Regarding Figure 3, only the differences from the embodiment in Figure 1 will be explained.

[0084] In this modified form, the housing 4 is formed by a recess 43 in a trim part 44 or a structural part of the automobile, and at least a portion of a decorative plate 45 that covers the recess 43.

[0085] The recess 43 is formed, for example, in the vicinity of column B as defined above, either directly within the column or within the cover of the column.

[0086] The decorative panel 45 is formed of, for example, glass or plastic. The outer surface of the decorative panel 45 located opposite the recess 43 and the radar module 2 forms the outer surface 41 of the housing 4 as described above.

[0087] In the embodiment shown in Figure 3, the radar module 2 is fixed to the bottom of the housing 4 on the opposite side of the outer surface 41.

[0088] Figure 4 schematically shows a gesture detection system 1'' according to a second modification of the present invention. Regarding Figure 4, only the differences from the embodiment in Figure 3 will be explained.

[0089] In this case, the radar module 2 is provided with a shim 25 located between the printed circuit board 20 of the radar module 2 and the outer surface 41 of the housing 4, thereby ensuring a desired distance between these two elements.

[0090] In modified versions not shown, the wall of the housing 4 located on the detection area side of the radar module 2 is very thick, for example, 5 mm or more, which ensures a desired distance between the printed circuit board 20 of the radar module 2 and the outer surface 41 of the housing 4.

Claims

1. A gesture detection system (1, 1', 1'') intended to be mounted in a vehicle to detect a predetermined gesture performed by a user outside the vehicle, wherein the system - A radar module (2) comprising a printed circuit board (20) on which at least one radio frequency antenna (21) is provided, wherein the at least one radio frequency antenna (21) transmits a radio frequency signal called a transmission signal, A radar module (2) is configured to receive a radio frequency signal called a return signal that arises from the reflection of the transmitted signal on the target (50), - A signal processing module (3) connected to the radar module (2) is configured to receive data relating to the return signal and derive information relating to the detection of a predetermined gesture from the return signal. Equipped with, - The at least one radio frequency antenna (21) is configured to transmit a linearly polarized transmission signal along a predetermined polarization axis (201), and the predetermined polarization axis is intended to be oriented substantially vertically during use. - The system (1, 1', 1'') comprises at least one housing (4) that houses the radar module (2), and the distance (d1) between the at least one radio frequency antenna (21) and the outer surface (41) of the housing (4) is, d ｌｉｍ =2*D ２ / λ+d ０ That's all, here D is the diameter of the radio frequency antenna (21), λ is the center wavelength of the transmitted signal. d ０ is a constant greater than or equal to 0. And, A system (1, 1', 1'') characterized in that the outer surface (41) of the housing (4) is intended to form a part of the outer surface of the vehicle in use.

2. The system (1, 1', 1'') according to claim 1, characterized in that the distance (d1) between the at least one radio frequency antenna (21) and the outer surface (41) of the housing (4) is 8 mm or more.

3. The system (1, 1', 1'') according to claim 1 or 2, characterized in that the radar module (2) comprises at least two radio frequency antennas (21) having a linearly polarized antenna for transmitting the transmission signal and a circularly polarized antenna for receiving the return signal.

4. The system according to any one of claims 1 to 3 (1, 1', 1''), characterized in that the transmitted signal is a radio frequency signal having a center frequency in the range of 5 GHz to 10 GHz.

5. The system according to any one of claims 1 to 4 (1, 1', 1''), characterized in that the transmitted signal is a radio frequency signal having a spectral width greater than 500 MHz.

6. The system (1, 1', 1'') according to any one of claims 1 to 5, characterized in that the housing (4) comprises a vehicle trim part or structural part (44) provided with a recess (43) for housing the radar module (2), and a decorative panel (45) that covers the recess (43) and forms the outer surface (41) of the housing (4).

7. The system (1') according to any one of claims 1 to 6, characterized in that the radar module (2) is fixed to the bottom of the housing (4) on the opposite side of the outer surface (41) of the housing.

8. The system (1'') according to any one of claims 1 to 7, characterized in that the radar module comprises at least one shim (25) on the outer surface (41) side of the housing.

9. An automobile comprising the system (1, 1', 1'') according to any one of claims 1 to 8.

10. The automobile according to claim 9, wherein the system (1, 1', 1'') is positioned near a structural column of the vehicle, and the polarization axis (201) of the linearly polarized radio frequency antenna (21) is substantially parallel to the extension axis of the column at an angle of about 10°.

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