A wireless charging positioning calibration device and a calibration method

Abstract

The application discloses a wireless charging positioning calibration device and a calibration method. The wireless charging positioning calibration device comprises a calibration coil arranged above a transmitting end coil and used for calibrating the transmitting end coil. The wireless charging positioning calibration device further comprises a first controller connected with the calibration coil and capable of providing an excitation signal for the calibration coil so that the calibration coil is turned on, and a second controller connected with the transmitting end coil and used for collecting a voltage parameter on the transmitting end coil and calculating a calibration coefficient of each coil frame in the transmitting end coil according to the voltage parameter so as to calibrate the transmitting end coil. Compared with the prior art, the application can make the alignment of the transmitting end and the receiving end more accurate, and improve the use experience and charging efficiency of the vehicle owner.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of vehicle-mounted system wireless charging, and particularly relates to a wireless charging positioning calibration device and a calibration method. BACKGROUND

[0002] With the needs of energy saving and emission reduction and atmospheric pollution control, new energy vehicles (pure electric vehicles and hybrid vehicles) are becoming the new main force of the automobile industry and developing rapidly. At present, many developed countries and all famous vehicle enterprises and research institutions are committed to promoting and applying new energy vehicle technology. As an important part of the new energy vehicle development industry chain, the demand for corresponding matching charging piles is also growing rapidly.

[0003] Wireless transmission refers to wireless charging. At present, well-known vehicle enterprises, research institutions and colleges and universities at home and abroad are investing heavily in research and development. Wireless charging is divided into three parts: pile end, transmitting end and receiving end. The pile end part first rectifies the power frequency alternating current into high-voltage direct current, and then converts it into high-frequency alternating current through inversion, and finally transmits it to the transmitting end through a cable. The pile end is similar to a charging pile. The transmitting end, which is actually composed of a coil and a magnetic core, generates a magnetic field through high-frequency alternating current and transmits energy to the vehicle end through magnetic coupling. The receiving end, which is actually composed of a coil and a magnetic core, converts the energy received from the transmitting end into high-voltage direct current through a current conversion device, and charges the high-voltage battery of the new energy vehicle, which is similar to a vehicle-mounted charger.

[0004] In the wireless charging process, the alignment of the transmitting end installed on the ground or underground and the receiving end installed on the vehicle directly affects the power transmission efficiency of wireless charging. Therefore, the wireless charging system must have the ability to accurately align the receiving end with the transmitting end. At present, positioning coils are usually installed on the transmitting end and the receiving end to achieve this through the principle of resonance. However, due to differences in actual position and resonance parameters, the current technology cannot achieve accurate positioning function, and cannot accurately guide the driver to park and align.

[0005] Therefore, how to design a wireless charging positioning calibration device and calibration method to improve the alignment accuracy of the transmitting end and the receiving end is a technical problem that needs to be solved in the industry. SUMMARY

[0006] In view of the problem in the prior art that the transmitting end and the receiving end in the wireless charging system cannot achieve good positioning function, the present application provides a wireless charging positioning calibration device and a calibration method.

[0007] The technical scheme of the present application is to provide a wireless charging positioning calibration device, which comprises a calibration coil arranged above a transmitting end coil and used for calibrating the transmitting end coil, and further comprises:

[0008] a first controller connected with the calibration coil and capable of providing an excitation signal to the calibration coil to turn on the calibration coil;

[0009] a second controller connected with the transmitting coil and capable of collecting voltage parameters on the transmitting coil and calculating calibration coefficients of each coil frame in the transmitting coil according to the voltage parameters to calibrate the transmitting coil.

[0010] Further, the calibration coil is a multi-coil array, and each coil frame in the calibration coil is matched with a coil frame in the transmitting coil, and the shape of each coil frame in the calibration coil is one of a circle, a triangle and a square.

[0011] Further, the calibration coil is installed above the transmitting coil by 20-300 mm.

[0012] The application further provides a wireless charging positioning calibration method, comprising:

[0013] The calibration coil is placed above the transmitting coil, and the first controller is triggered to turn on the calibration coil at a specific frequency;

[0014] The voltage parameters of each coil frame on the transmitting coil are detected.

[0015] The transmitting coil is calibrated according to the voltage parameters, so that the product of the voltage parameter of each coil frame and its calibration coefficient is the same;

[0016] The calibration coefficients of each coil frame in the transmitting coil are recorded by the second controller, and the calibration of the transmitting coil is ended.

[0017] Further, the calibration of the transmitting coil according to the voltage parameters comprises:

[0018] The average voltage parameters of all coil frames on the transmitting coil are calculated.

[0019] The calibration coefficients of each coil frame in the transmitting coil are calculated according to the average voltage parameters, and the calibration coefficient is the ratio of the average voltage parameter to the voltage parameter of each coil frame.

[0020] Further, before the second controller records the calibration coefficients, the method further comprises: labeling each coil frame on the transmitting coil, and matching a plurality of calibration coefficients with the coil frames one by one, so that each coil frame has a unique label and a calibration coefficient corresponding thereto.

[0021] Further, the detection of the voltage parameters on the transmitting coil comprises:

[0022] calibrating a coil frame in a transmitting coil, and moving the calibration coil above the calibrated coil frame to turn on the calibration coil at a specific frequency;

[0023] detecting and recording a voltage parameter on the calibrated coil frame;

[0024] calibrating the coil frames in the transmitting coil in sequence, and repeating the detection action until all the coil frames in the transmitting coil are detected.

[0025] Further, before calculating the calibration coefficient, it further comprises: converting the voltage parameter and the average voltage parameter into a detection value by the second controller, and the calibration coefficient is calculated by the detection value.

[0026] Further, when calibrating the transmitting coil and the receiving coil, the wireless charging positioning calibration method further comprises:

[0027] According to the calibration coefficient recorded by the second controller, the calibration correction is performed on each coil frame of the transmitting coil, and the receiving coil is placed above the transmitting coil to turn on the receiving coil at a specific frequency.

[0028] The receiving coil is moved above the coil frame of the transmitting coil in a preset order in sequence, the voltage parameter on the transmitting coil is detected, and the position of the receiving coil relative to the transmitting coil is determined according to the voltage parameter.

[0029] If the error is within the preset error range, it is determined that the calibration is passed.

[0030] Compared with the prior art, the present application has at least the following beneficial effects:

[0031] The present application provides a wireless charging positioning calibration method, which can realize more accurate alignment of the transmitting end and the receiving end, and improve the use experience and charging efficiency of the vehicle owner. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0033] Figure 1 Equivalent circuit diagram for the transmitting coil close to metal;

[0034] Figure 2 Structure block diagram of the wireless charging system of the present application;

[0035] Figure 3 Fig. 1 is a schematic diagram of a calibration coil according to the present application;

[0036] Figure 4 Fig. 2 is a schematic diagram of a structure of the calibration coil;

[0037] Figure 5 Fig. 3 is a schematic diagram of a structure of the calibration coil according to another embodiment of the present application;

[0038] Figure 6 Fig. 4 is a schematic diagram of a structure of the calibration coil according to another preferred embodiment of the present application;

[0039] Figure 7 Fig. 5 is a flow chart of a working process of the calibration process according to the present application. DETAILED DESCRIPTION

[0040] In order to make the technical problems to be solved by the present application, technical solutions and beneficial effects more clear, the present application will be further described in detail below in combination with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application.

[0041] Therefore, one feature indicated in the specification will be used to explain one feature of one embodiment of the present application, and it is not implied that each embodiment of the present application must have the explained feature. In addition, it should be noted that the specification describes many features. Although some features can be combined together to show possible system designs, these features can also be used in other combinations which are not explicitly explained. Therefore, unless otherwise specified, the explained combinations are not intended to be limited.

[0042] The principles and structures of the present application will be described in detail below in combination with the drawings and embodiments.

[0043] In the wireless charging process, the alignment of the transmitting end installed on the ground or underground and the receiving end installed on the vehicle directly affects the power transmission efficiency of the wireless charging, so the wireless charging system must have the ability to accurately align the receiving end and the transmitting end. The idea of the present application is to propose a wireless charging positioning and calibration device and a calibration method, by setting a calibration coefficient for each coil frame in the transmitting end coil, when aligning the transmitting end and the receiving end, aligning through the calibrated voltage parameter, avoiding the problem of deviation of the calculated coordinates caused by the inconsistent impedance change range of each coil frame.

[0044] The wireless charging system comprises a pile end, a transmitting end, and a receiving end, the pile end supplies power to the transmitting end, the transmitting end comprises a transmitting end aluminum plate, a transmitting end magnetic core, and a transmitting end coil, and the receiving end comprises a receiving end aluminum plate, a receiving end magnetic core, and a receiving end coil. After the transmitting end is powered on, the transmitting end coil is magnetically coupled with the receiving end coil, so that an induced current is generated in the receiving end coil and power is supplied to the load. The receiving end is installed on a vehicle and is also called a vehicle end, and the induced current generated on the receiving end can supply power to the vehicle-mounted system, so as to realize the wireless charging function of the vehicle-mounted system.

[0045] To ensure the energy transmission efficiency between the transmitting end and the receiving end, the alignment accuracy of the transmitting end and the receiving end needs to be ensured. When the transmitting end and the receiving end are aligned, the impedance of the transmitting end coil changes due to the proximity of the receiving end coil. At this time, the transmitting end coil can be equivalent to a coupled transmitting end coil inductance L MO and an equivalent resistance R MO .

[0046] Please refer to Figure 1 , the transmitting end coil can be represented as:

[0047]

[0048]

[0049] that is

[0050]

[0051]

[0052] wherein w is the angular frequency of the current signal in the receiving end coil, and M is the mutual inductance. As can be seen from the above formula, when the receiving end coil is close to the transmitting end coil, the equivalent inductance decreases and the equivalent resistance increases. It is assumed that

[0053] L coilMO = aL coil (a < 1)

[0054] R coilMO = bR coil (b > 1)

[0055] Then the coil impedance changes to

[0056]

[0057] Under the influence of high frequency, the influence of inductance on impedance is greater than that of resistance. Therefore, the impedance change can be simplified as:

[0058]

[0059] General foreign matter, which has little effect on the inductance of the detection coil, is difficult to detect, and therefore, a resonance circuit must be introduced. For example, in series resonance, the impedance of the coil on the LOOP board changes to

[0060]

[0061] where Qcoil is the quality factor of the detection coil. As can be seen, the amount of impedance change is amplified, improving the detection sensitivity.

[0062] Because the amount of impedance change produced by each coil frame in the transmitting coil is different, a large error will occur during alignment, affecting the positioning accuracy.

[0063] In the traditional alignment method, when aligning between the transmitting end and the receiving end, the receiving coil is positioned directly above each individual coil frame in the transmitting end, causing the impedance of the corresponding coil frame to change by the same amount. Then, the voltage change of each coil frame in the transmitting coil is collected, and each individual transmitting coil corresponds to different X-axis and Y-axis coordinates. Because the voltage change of the transmitting coil that is closer to the receiving coil is larger, the coil frame with the largest change is selected, and it is considered to be the closest to the receiving coil. As shown in FIG. 1, N0 is the coil frame with the largest voltage change, Figure 3 Figure 3 (a) The coordinates of the five frames including N0 are weighted and calculated. Assuming that the coordinates of N0-N4 are (0, 0), (0, 10), (10, 0), (0, -10), and (-10, 0), when the receiving coil is positioned directly above N0, the coordinates of N1-N4 cancel each other out, and the final coordinate of the receiving coil is (0, 0).

[0064] When the receiving coil deviates from the origin (0, 0) in the y-axis direction, the coordinates of N4 and N2 in the x-axis direction still cancel each other out, and the voltage change of N1 in the y-axis direction is greater than that of N3. Therefore, the coordinate of N1 has a greater weight than that of N3, and the coordinate of the receiving coil detected will deviate towards N1 in the y-axis direction.

[0065] When the receiving coil deviates in other directions, the principle is the same, and the change in each frame is used for weighted calculation.

[0066] Further, Figure 3 (b) The coordinates of the nine frames including N0 are weighted and calculated. The calculation is more complex, but the result is more accurate.

[0067] ​In practical application, the resonant frequency, quality factor and other parameters of each coil frame in the transmitting end coil are different, which results in different impedance change amounts caused by the same receiving end coil located directly above the transmitting end coil, that is, when the receiving end positioning coil is located directly above N0, the voltage change amount of N0 is the largest under ideal conditions, the voltage change amounts of the surrounding frames are smaller than that of N0 and are equal, and in practice, there is a large difference in the impedance change amounts of the frames, which produces errors in the weighted calculation.

[0068] Thus, the alignment of the transmitting end coil and the receiving end coil is completed.

[0069] To this end, the application provides a wireless charging positioning calibration device, which comprises a calibration coil arranged above the transmitting end coil and used for calibrating the transmitting end coil, and the following:

[0070] A first controller connected with the calibration coil and capable of providing an excitation signal for the calibration coil to turn on the calibration coil;

[0071] A second controller connected with the transmitting end coil and used for collecting voltage parameters on the transmitting end coil and calculating calibration coefficients of each coil frame in the transmitting end coil according to the voltage parameters to calibrate the transmitting end coil.

[0072] The voltage parameter is a voltage change amount (voltage change amount before and after the excitation signal is sent), and the design idea of the application is that the impedance change on the transmitting end coil is simulated by the calibration coil to make the receiving end coil, the voltage change amount (which can be used to reflect the impedance change amount) of each coil frame on the transmitting end coil is obtained, and the calibration coefficients of each coil frame are calculated according to the voltage change amount, so that the product of the voltage change amount on each coil frame and the calibration coefficient is the same, and then the calibration coefficients are applied to each coil frame of the transmitting end coil when the transmitting end coil and the receiving end coil are aligned, so as to eliminate the errors caused by different impedance change amounts and improve the accuracy of alignment.

[0073] Please refer to Figure 4 which is a plan view of the calibration coil array in an embodiment of the application, the coil is square-wound thereon, the calibration coil array is the same as the transmitting end coil array, and each coil frame corresponds to a coil frame in the transmitting end coil one by one, the resonant frequency is adjusted by adjusting the resonant capacitance size, the detection value change caused by the calibration coil array in the transmitting end is the same as that in the receiving end, and the calibration coil array plays a role of simulating the receiving end coil. In the positioning process, the controller selects the individual coil frames in the positioning coil through the internal address line.

[0074] Please refer to Figure 5 The calibration coil comprises a plurality of coil frames, and the coil frames in the calibration coil are arranged one by one with the coil frames in the transmitting end coil, and the shape of the coil frames in the calibration coil is one of a circle, a triangle and a square.

[0075] Please refer to Figure 6 In another embodiment of the present application, the area of the calibration coil can also be increased so that the calibration coil covers four independent coil frames of the transmitting end coil with a single coil frame. In this way, the coefficients of the four coil frames can be calibrated at the same time, and the calibration time is shortened. The actual number of covered frames can be adjusted, and is usually between 1 and 8. The calibration coil and the receiving end coil can be wound on a PCB or copper wire to save costs.

[0076] Furthermore, the calibration coil in the present application is installed between 20 mm and 300 mm above the transmitting end coil.

[0077] Please refer to Figure 7 Based on the wireless charging positioning calibration device proposed in the present application, the present application also proposes a wireless charging positioning calibration method, which comprises the following steps:

[0078] The calibration coil is placed above the transmitting end coil, and the first controller is triggered to turn on the calibration coil.

[0079] The voltage parameters of each coil frame of the transmitting end coil are detected.

[0080] The transmitting end coil is calibrated according to the voltage parameters so that the product of the voltage parameter and the calibration coefficient of each coil frame is the same.

[0081] The calibration coefficients of each coil frame of the transmitting end coil are recorded by the second controller, and the calibration of the transmitting end coil is ended.

[0082] In the positioning calibration method, the calibration coil can simulate the receiving end coil. After the calibration coefficients of each coil frame are recorded by the second controller, the calibration coefficients can be directly used to calibrate each coil frame when the receiving end coil is positioned, so that the error caused by different impedance changes is avoided.

[0083] If the transmitting end coil includes four coil frames, the voltage parameter changes caused by the impedance changes of each coil frame are 100 mV, 125 mV, 175 mV, and 200 mV, respectively, during positioning calibration. Four matching calibration coefficients, 1.5, 1.2, 0.857, and 0.75, can be obtained during calibration. The calibration coefficients correct the weight of each frame in the coordinate weighting calculation during positioning, that is, the voltage parameters of the four frames participating in the calculation are multiplied by the respective calibration coefficients in the actual change. After each calibration coefficient is recorded by the second controller, the calibration coefficient can be applied in the positioning process by the second controller when the transmitting end coil and the receiving end coil are positioned, so that the error is reduced.

[0084] The calibration of the transmitting coil according to the voltage parameter comprises:

[0085] The average voltage parameter of all coil frames of the transmitting coil is calculated.

[0086] The calibration coefficient of each coil frame of the transmitting coil is calculated according to the average voltage parameter, and the calibration coefficient is the ratio of the average voltage parameter to the voltage parameter of each coil frame.

[0087] The calibration coefficient is calculated by the average voltage parameter, so that the calculated calibration coefficient can be directly applied to the calibration of each coil frame, and the weight of each frame in coordinate calculation is changed.

[0088] Further, before the second controller records the calibration coefficient, the method further comprises: labeling each coil frame of the transmitting coil, and setting a plurality of calibration coefficients one by one to match the coil frame, so that each coil frame has a unique label and a calibration coefficient corresponding thereto.

[0089] After each coil frame is labeled, the coil frame can be matched with the corresponding calibration coefficient by the user, and the recording of the second controller is facilitated, and when the transmitting coil and the receiving coil are aligned, the calibration coefficient and the coil frame can be directly matched, and the execution of the positioning action is facilitated.

[0090] Further, the detection of the voltage parameter of the transmitting coil comprises:

[0091] A coil frame in the transmitting coil is demarcated, and the calibration coil is moved above the demarcated coil frame;

[0092] The voltage parameter on the demarcated coil frame is detected and recorded;

[0093] The coil frame in the transmitting coil is demarcated again, and the detection action is repeated until all coil frames of the transmitting coil are detected.

[0094] Further, the calibration coil uses a coil array corresponding one by one to the transmitting coil, and the calibration coil can be turned on in a preset order, and the second controller records the voltage parameter of the corresponding coil frame and calculates the calibration coefficient, thereby avoiding repeated movement of the calibration coil.

[0095] Further, before the calibration coefficient is calculated, the method further comprises: converting the voltage parameter and the average voltage parameter into a detection value by the second controller, and the calibration coefficient is calculated by the detection value.

[0096] By converting the voltage parameter and the average voltage parameter into a detection value convenient for calculation, the second controller can more conveniently process the data of a plurality of voltage parameters, and the calculation amount of the second controller is reduced.

[0097] Specifically, the detection step of the present application is:

[0098] 1. The first controller and the second controller are powered on and in normal working state;

[0099] 2. A calibration instruction is issued, the first controller sends an excitation signal and an enable signal to the calibration coil, and each coil frame is sequentially turned on, and the second controller records the number of the coil frame turned on at this time, 1, 2, 3…n;

[0100] 3. When each coil frame is turned on, the voltage of the corresponding coil frame on the transmitting end changes, the second controller collects the voltage change and converts it into a detection value easy to compare, and records the detection value M1, M2…Mn at this time, which is corresponding to the number of the turned-on frame;

[0101] 4. After all the coil frames are turned on in turn, the first controller stops sending the excitation signal, the second controller calculates the average value Mave of all the detection values, and calculates the calibration coefficient km (m=1, 2, 3…n) according to the proportional relationship between each detection value and the average value and saves it, and the final detection value is the original detection value multiplied by the calibration coefficient, and the calibration is completed.

[0102] Wherein, the voltage parameter is the voltage change of the coil frame, and the detection value can be the absolute value of the voltage change, which is easy to compare, the average value of the detection value, and the calibration coefficient satisfy:

[0103]

[0104]

[0105] Wherein, k is the calibration coefficient.

[0106] For this, the wireless charging positioning calibration method proposed by the application further includes the step of detecting the calibration coefficient using the receiving end coil, which includes:

[0107] According to the calibration coefficient recorded by the second controller, each transmitting end coil frame is calibrated and corrected, and the receiving end coil is placed above the transmitting end coil, and the receiving end coil is turned on at a specific frequency;

[0108] In a predetermined order, the receiving end coil is moved above the coil frame of the transmitting end in turn, the voltage parameter on the transmitting end coil is detected, and the position of the receiving end coil relative to the transmitting end coil is determined according to the voltage parameter, if the coordinate error of the receiving end coil above each transmitting end coil meets the requirement, it is judged that the calibration is passed; if the error does not meet the requirement, it is considered that a fault occurs, and the positioning calibration device needs to be checked.

[0109] The wireless charging positioning calibration method can eliminate errors caused by different coil frame impedance change amounts in the alignment process, can realize more accurate alignment of the transmitting end and the receiving end, and improves the use experience and charging efficiency of the vehicle owner.

[0110] The above merely describes preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A wireless charging positioning calibration method of a wireless charging positioning calibration device, characterized in that, the wireless charging positioning calibration device comprises a calibration coil arranged above a transmitting coil and used for calibrating the transmitting coil, and further comprises: a first controller connected with the calibration coil and capable of providing an excitation signal for the calibration coil to turn on the calibration coil; a second controller connected with the transmitting coil and used for collecting voltage parameters on the transmitting coil and calculating calibration coefficients of each coil frame in the transmitting coil according to the voltage parameters to calibrate the transmitting coil; the wireless charging positioning calibration method comprises: placing the calibration coil above the transmitting coil and triggering the first controller to turn on the calibration coil at a specific frequency; detecting voltage parameters of each coil frame on the transmitting coil; calibrating the transmitting coil according to the voltage parameters so that the product of the voltage parameters on each coil frame and the calibration coefficient thereof is the same; recording the calibration coefficients of each coil frame in the transmitting coil by the second controller and ending the calibration of the transmitting coil. 2.The wireless charging positioning calibration method of claim 1, wherein, The calibration of the transmitting coil according to the voltage parameters comprises: calculating average voltage parameters of all coil frames on the transmitting coil; calculating the calibration coefficients of each coil frame in the transmitting coil according to the average voltage parameters, wherein the calibration coefficient is the ratio of the average voltage parameters to the voltage parameters on each coil frame. 3.The wireless charging positioning calibration method of claim 2, wherein, Before the second controller records the calibration coefficients, it further comprises: labeling each coil frame on the transmitting coil and setting a plurality of calibration coefficients one by one to match the coil frames, so that each coil frame has a unique label and a calibration coefficient corresponding thereto. 4.The wireless charging positioning calibration method of claim 1, wherein, The detection of the voltage parameters on the transmitting coil comprises: labeling a coil frame in the transmitting coil and moving the calibration coil above the labeled coil frame to turn on the calibration coil at a specific frequency; detecting and recording the voltage parameters on the labeled coil frame; labeling the coil frames in the transmitting coil one by one and repeating the detection action until all coil frames of the transmitting coil are detected.

5. The wireless charging positioning calibration method of claim 2, wherein, Before calculating the calibration coefficients, it further comprises: converting the voltage parameters and the average voltage parameters into detection values by the second controller, and the calibration coefficients are calculated by the detection values.

6. The wireless charging positioning calibration method of claim 1, wherein, When calibrating the transmitting coil and the receiving coil, the wireless charging positioning calibration method further comprises: calibrating and correcting each transmitting coil frame according to the calibration coefficients recorded by the second controller and placing the receiving coil above the transmitting coil to turn on the receiving coil at a specific frequency; moving the receiving coil above the coil frames of the transmitting coil in a predetermined order one by one, detecting the voltage parameters on the transmitting coil, and determining the position of the receiving coil relative to the transmitting coil according to the voltage parameters; if the error is within the preset error range, it is determined that the calibration is passed.

7. A wireless charging positioning calibration device using the wireless charging positioning calibration method according to any one of claims 1 to 6, characterized by, The calibration coil is a multi-coil array, and the coil frames in the calibration coil are arranged one by one in matching with the coil frames in the transmitting end coil, and the shape of the coil frame in the calibration coil is one of a circle, a triangle and a square.

8. The wireless charging positioning calibration device of claim 7, wherein, The calibration coil is installed between 20 mm and 300 mm above the transmitting end coil.

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