Cable-free chip antenna impedance and gain measurement method

Abstract

The invention discloses a cable-free chip antenna impedance and gain measurement method, which comprises the following steps of: firstly establishing an electromagnetic model of a probe and a measured antenna, then establishing a free space propagation model based on a one-way transmission equation and a model of a measurement system, and then establishing an electromagnetic model of the measurement system comprising the probe, the free space propagation model and the measured antenna; the method comprises the following steps: firstly, deducing a complete electromagnetic model of a measurement system comprising any probe, free space propagation and any measured antenna, then determining scattering matrixes of the probe, the free space propagation and the measured antenna, and extracting antenna impedance and gain from near-field distance measurement; according to the invention, the influence of leakage current, scattering effect and the like of a traditional probe and a feed cable on a chip antenna is overcome, the method can be well applied to a scene in which a feed line has great influence on a measured antenna, and the measurement precision of the antenna is greatly improved.

Description

Technical Field

[0001] The invention relates to an antenna testing method in the communication field, and in particular to an impedance and gain measurement method for a cable-free chip antenna. Background Art

[0002] In recent years, antenna miniaturization technology has been widely used in hearing aids, mobile phones, portable wireless devices, sensors and radio frequency identification, chip packaging and other fields. Although products with miniaturized antennas are easy to carry and transport, which greatly facilitates people's lives, antenna miniaturization can lead to problems such as narrow antenna impedance bandwidth and low radiation efficiency. If you need to accurately characterize the performance of small antennas, the most important parameters are input impedance and gain. However, for the several measurement technologies currently available, measuring impedance and gain is still a challenging task.

[0003] The radar cross-section method is used to measure antenna impedance and gain. RJGarbacz described it in his article. Later, J.Appel-Hansen, W.Wiesbeck and E.Heidrich found that this method can be well applied to antennas with multiple wavelengths, but when it is used for ESA with low radiation efficiency, the dynamic range of the measured signal becomes quite low, which greatly reduces the measurement accuracy. Then Hsin-Chia Lu and Tah-Hsiung Chu proposed another improved measurement method, which is a combination of the RCS method and the three-antenna method. However, it is only partially cable-free because the feed cable is used to measure the reflection coefficient of the AUT. Then, the antenna gain is determined from the RCS method by considering the above reflection coefficient, where the influence of the cable still exists. B.Monsalve and MHCapstick, JO et al. reported an indirect non-contact small antenna impedance measurement technique in their article, but it does not provide gain and radiation efficiency.

[0004] For chip antennas, the traditional test method uses a probe station system. The feeding of the chip antenna includes a probe head and a feeding cable. The physical size of the probe head is larger than that of the chip antenna. The structural scattering of the probe head and the leakage current on the feeder affect the test results. A non-contact chip antenna impedance and gain measurement technology is needed. Summary of the invention

[0005] In order to solve the above problems, the present invention provides a new cable-free chip antenna impedance and gain measurement method, which successfully overcomes the influence of leakage current and scattering effect of traditional probes and feeding cables on chip antennas, thereby significantly improving the accuracy of antenna measurement.

[0006] The technical solution adopted by the present invention to solve the technical problem is: a method for measuring impedance and gain of a cable-free chip antenna, comprising the following steps:

[0007] Step 1: Establish the electromagnetic model of the probe;

[0008] Step 2: Establish the electromagnetic model of the antenna under test (AUT);

[0009] Step 3: Establish a free space propagation model based on the one-way transmission equation;

[0010] Step 4: Establish a model of the measurement system;

[0011] Step 5: Establish an electromagnetic model of the measurement system including the probe, free space propagation and the antenna under test (chip antenna);

[0012] Step 6: Derive a complete electromagnetic model of the measurement system including arbitrary probes, free space propagation, and arbitrary antenna under test (AUT);

[0013] Step 7: Determine the scattering matrices of the probe, free space propagation, and the antenna under test (AUT);

[0014] Step 8: Extract the fixed antenna impedance and gain from the near-field distance measurement:

[0015] (1) The reflection coefficient of the probe is measured three times when the antenna under test (AUT) is short-circuited, open-circuited, and loaded with a known impedance.

[0016] (2) Measure the reflection coefficient of the load connected to the antenna under test (AUT) in sequence, including short circuit, open circuit and known impedance;

[0017] (3) Extract parameters from the corresponding scattering matrix based on the probe reflection coefficient and the measured antenna reflection coefficient And determine along Direction Partial gain;

[0018] (4) Perform another measurement to determine the edge of the orthogonal direction. The partial gain in the direction, i.e.

[0019] (5) and Add together to obtain the total gain of the antenna under test (AUT).

[0020] This method proposes a complete electromagnetic model of the measurement system based on the spherical wave expansion technique (SWE), and derives a complete electromagnetic model of the measurement system including an arbitrary probe, free space propagation, and an arbitrary chip antenna (AUT);

[0021] The test probe and chip antenna (AUT) are modeled. The chip antenna (AUT) proposed in this paper can be of any mode. The entire transmission system includes the scattering and multiple reflection effects of the probe and chip antenna (AUT), and is modeled using a scattering matrix that considers all spherical modes. Based on this model, the impedance and far-field characteristics of the antenna can also be extracted from the near-field distance measurement; determine the total scattering matrix When the chip antenna (AUT) is measured in the short circuit, open circuit and known load states, the reflection coefficient of the transmitting probe is measured, and then the scattering parameters of the total scattering matrix are extracted. And finally determine the impedance and gain of the AUT. The model takes into account the influence of structural scattering and multiple reflections between the probe and the AUT on the measured impedance and gain. The measurement is accurate. This method can be well applied to scenarios where the feed line has a great influence on the antenna under test, such as small antennas, chip antennas, etc.

[0022] As a further improvement of the present invention, the probe is any probe, and the antenna to be tested is any chip antenna.

[0023] As a further improvement of the present invention, for any antenna under test (AUT), a complete spherical wave mode is considered here, and the impedance and far-field characteristics of the antenna based on this model can also be extracted from the near-field distance measurement. The model uses spherical wave expansion (SWE) to model the probe and the antenna under test (AUT), and the measurement system uses three coupled multi-port network cascade models of the spherical wave expansion (SWE). When establishing the electromagnetic model of the measurement system, the measurement device is modeled by three coupled multi-port network cascades, namely the measurement probe, free space propagation and the chip antenna (AUT).

[0024] As a further improvement of the present invention, for any chip antenna (AUT), a complete spherical wave mode is considered here, and the impedance and far-field characteristics of the antenna based on this model can also be extracted from the near-field distance measurement. The model uses spherical wave expansion (SWE) to model the probe and AUT. The modeling equations for the probe and the antenna under test (AUT) are as follows:

[0025] Electric field around the antenna It is surrounded by the smallest sphere with radius r0, that is:

[0026]

[0027] In the above formula, is the power normalized spherical vector wave function, where the superscripts c=3,4 represent the outward propagating wave and the inward propagating wave, respectively, K is the wave number, η is the intrinsic admittance of the medium, is the position vector of the point in the (r, θ, φ) spherical coordinates.

[0028] As a further improvement of the present invention, the antenna can be modeled as a multi-port network, where one excitation port is connected to the emission source and each radiation port represents an outward and inward spherical wave mode outside the smallest sphere of the antenna. The equivalent network is as follows Figure 2 and Figure 3 As shown, the total scattering matrix of the antenna under test is expressed as:

[0029]

[0030] In the above formula, v and w are the complex amplitudes of the input and output waves at the antenna excitation port, respectively. and denote the complex amplitudes of the incoming and outgoing spherical modes, respectively, is a row vector containing the antenna reception coefficients, is a column vector containing the antenna transmission coefficients, is a square matrix containing the scattering coefficients of the antenna structure. The mode coefficients smn and s'm'n' represent the output and input spherical waves in the probe coordinate system, respectively. is the total scattering matrix;

[0031] The probe and the antenna under test (AUT) are modeled by the above scattering matrix.

[0032] As a further improvement of the present invention, when establishing a free space propagation model based on a one-way transmission equation in step 3, the probe coordinates (x, y, z) and the coordinates (x′, y′, z′) of the antenna under test (AUT) are first defined respectively, wherein the coordinate system of the antenna under test (AUT) is obtained by rotating the probe coordinate system by three angles ν, θ, χ and translating the probe coordinate system by a distance A;

[0033] The transmission formula of the free space propagation model describes the complex signal received by the antenna under test (AUT) based on the transmission coefficient of the probe, the reception coefficient of the antenna under test (AUT), three rotation angles and the translation distance A, that is:

[0034]

[0035] and

[0036] s=1, 2; m=-n, -n+1,...,n; n=1, 2... (4);

[0037] σ'=1, 2; μ'=-v', -v'+1,..., v'; v'=1, 2... (5);

[0038] In the above formula, smn and 'σ′μ′v′ are the spherical wave index (SWEs) of the probe and the antenna under test (AUT), respectively. is the emission coefficient of the probe, is the reception coefficient of the antenna under test (AUT), e imφ 、e iμ′x and is the rotation coefficient, is the translation coefficient.

[0039] As a further improvement of the present invention, before determining each scattering matrix, the equation of the total scattering matrix is ​​derived in the complete electromagnetic model of the measurement system:

[0040] When establishing the electromagnetic model of the measurement system, since the electromagnetic models of the probe and the antenna under test (AUT) and the scattering matrix are derived in step 6 and model, in order to obtain the amplitude of the scattered field pattern, rather than just the outgoing field, it is also necessary to subtract the contribution of the incident field from the scattering matrix of the probe antenna under test (AUT);

[0041] (1) The amplitude equation of the scattered field pattern is:

[0042]

[0043] In the above formula and is the identity matrix;

[0044] (2) The relationship between the complex amplitude of the incident spherical wave in the coordinate system of the antenna under test (AUT) and the complex amplitude of the outgoing spherical wave in the coordinate system of the probe is expressed by the following equation:

[0045]

[0046] in is the combined rotation and translation coefficient;

[0047]

[0048] (3) The relationship between the amplitude of the incident spherical wave of the probe and the amplitude of the outgoing spherical wave of the antenna under test (AUT) is expressed by the following equation:

[0049]

[0050] In the formula It is used to convert the output wave in the coordinate system of the antenna under test (AUT) into the input wave in the probe coordinate system, that is:

[0051]

[0052] (4) The scattering matrix model equation for free space propagation is derived from the above method as follows:

[0053]

[0054] In the formula and For the above and

[0055] The above equations give the network model of the antenna under test (AUT), the probe, and free space propagation. Then, the total scattering matrix is ​​defined. The entire measurement system can also be modeled as having a total scattering matrix Two-port network.

[0056] (5) Establish the total scattering matrix with the probe and the input end of the antenna under test (AUT) as the reference plane The parameters of the scattering matrix are:

[0057]

[0058] When the antenna under test (AUT) is loaded, the reflection coefficient measured at the probe input is expressed as It is expressed that its relationship with the two-port scattering parameters is:

[0059]

[0060] (6) By associating the above cascaded free space scattering matrix model with the total scattering matrix, the parameters of the total scattering matrix are derived as follows:

[0061]

[0062]

[0063] The above formula is applicable at any distance between the probe and the antenna under test (AUT).

[0064] As a further improvement of the present invention, as the distance between the probe and the AUT increases, the multiple scattering term decreases due to the spatial attenuation of free space propagation in the forward and backward directions. The use of a minimum scattering probe will further reduce the multiple scattering term. By ignoring the multiple scattering term and simplifying equations (17) to (19), the simplified equation of the total scattering matrix without the multiple reflection effect is derived as follows:

[0065]

[0066] As a further improvement of the present invention, the total scattering matrix is ​​determined

[0067]

[0068] in is the measured reflection coefficient of each load, The probe reflection coefficients measured when the antenna under test (AUT) is loaded in three cases;

[0069] According to the measured extract Right now:

[0070]

[0071] When i is equal to 1, 2, and 3. It is worth noting that there are three sets of data available for this equation, which are obtained from three measurements. Averaging the results of different sets can improve accuracy.

[0072] As a further improvement of the present invention, in step eight, the probe structure scattering and multiple reflections are set to be extremely small. When determining the antenna impedance and gain without considering the structure scattering and multiple reflections, the impedance of the antenna under test (AUT) is first determined, and then the gain of the antenna under test (AUT) is determined. The input impedance of the antenna under test (AUT) can be obtained by Determine, that is:

[0073]

[0074] Z0 is the reference impedance.

[0075] When determining the total gain of the antenna under test (AUT), two gain measurements are made at two orthogonal directions of the probe, and the relationship equation is:

[0076]

[0077] Among them, G PRB For known data, Extract according to the above equation; when determining the gain of the antenna under test (AUT) parameter Extracted from equation (7) and equation (8).

[0078] The beneficial effects of the present invention are as follows: the chip antenna under test of the present invention does not need to be connected to a feeder cable. When the chip antenna under test is loaded with three known loads, the scattered signal of the antenna under test is measured by a probe antenna to extract antenna characteristics. This technology is different from the known radar cross section method in that a complete electromagnetic model of a measurement system including an arbitrary probe, free space propagation and an arbitrary chip antenna under test is derived. The model uses spherical wave expansion technology (SWE) to model the probe and AUT, and the entire measurement system is modeled by three coupled multi-port network cascades of SWE. The model considers the effects of structural scattering and multiple reflections between the probe and the AUT on the measured impedance and gain. The results show that the present invention overcomes the effects of leakage current and scattering effects of traditional probes and feeder cables on chip antennas. The present invention can be well applied to scenarios where the feeder has a greater impact on the antenna under test, such as small antennas, chip antennas, etc., greatly improving the measurement accuracy of such antennas. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Figure 1 The electromagnetic model diagram of the impedance and gain measurement method of the cable-free chip antenna;

[0080] Figure 2 is a network representation diagram of the antenna;

[0081] Figure 3 This is a diagram showing the spherical wave pattern of the antenna;

[0082] Figure 4 is the coordinate diagram of the probe and the antenna under test;

[0083] Figure 5 It is the scattering matrix model diagram of the measurement system;

[0084] Figure 6 The two-port model diagram of the measurement system for the impedance and gain of the antenna under test;

[0085] Figure 7 Configuration diagram for the measurement setup used in the HFSS simulation;

[0086] Figure 8 Comparison plots (comparing dipole impedance vs. frequency) using the cable-free technique (with and without smoothing) and direct simulation in free space;

[0087] Fig. 9 Comparison plots (comparing dipole gain vs. frequency) using the cable-free technique (with and without smoothing) and direct simulation in free space;

[0088] Fig.10 An electrical small loop antenna diagram designed for the antenna under test at 910 MHz;

[0089] Fig.11Diagram (not to scale) of the configuration setup for the measurements used in the HFSS simulation.

[0090] Fig.12 Comparison diagrams (comparing loop impedance vs. frequency) using the cable-free technique (with and without smoothing) and direct simulation in free space;

[0091] Fig.13 Comparison plots (comparing loop gain vs. frequency) are shown using the cable-free technique (with and without smoothing) and direct simulation in free space. DETAILED DESCRIPTION

[0092] Embodiment: To make the advantages, technical solutions and innovations of the present invention clearer, the present invention will be further described in detail below in conjunction with the above drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the protection scope of the present invention.

[0093] When using the traditional technology of feeder cable for measurement, two effects are likely to occur: one is the leakage current outside the feeder cable, and the other is the scattering effect.

[0094] Based on the above problems, the present invention derives a complete electromagnetic model including an arbitrary probe, free space propagation and an arbitrary antenna under test, and designs a cable-free measurement technology. A half-wavelength dipole and an electric small loop antenna are used as simulation examples of the antenna under test.

[0095] Embodiment 1:

[0096] Firstly, the input impedance and gain of the antenna under test (AUT) are directly obtained by simulating a half-wavelength dipole itself (AUT) in free space using lumped port excitation, and reference results are presented to be compared with the impedance and gain extracted by the proposed cable-free technique;

[0097] Secondly, the measurement setup of the proposed cable-free measurement method is simulated, e.g. Figure 7 When the antenna under test (AUT) is loaded with open circuit, short circuit and 50 ohm impedance respectively, the probe is excited by lumped port and the input reflection coefficient of the probe is recorded;

[0098] In order to study the effect of noise in a real measurement environment, the probe A random noise of -60 dB is added to the measured reflection coefficient, denoted as ST′, and the noise is

[0099]

[0100] Then, by solving equations (23), (24), (26), and (27), we can get The three simulations focus on extracting the antenna's reflection coefficient, input impedance, and the gain product of two half-wavelength dipoles.

[0101] Figure 8 and Fig. 9 The following are simulation diagrams comparing the relationship between dipole impedance and frequency and dipole gain and frequency between the technology of the present invention and the previous technology. This example shows that the proposed cable-free technology works well in an actual measurement environment considering noise. Accurate input impedance and gain can be obtained.

[0102] Embodiment 2:

[0103] like Fig.10 The figure shows a self-resonant balanced electric small loop antenna using distributed capacitive loading elements. Its working mechanism is based on the capacitive load and inductive coupling between two small loops.

[0104] like Fig.11 As shown in Figure 1, again, a half-wavelength dipole is used as the probe and a small loop antenna is modeled as the antenna under test. The simulation process and noise model are similar to the previous example.

[0105] Fig.12 A comparison of the reference and extracted antenna input impedance versus frequency is given. The reference and extracted antenna gain versus frequency are shown in Fig.13 As shown in the figure, the loop gain deviation is less than 0.05dB.

[0106] The diagrams obtained through a series of simulations show that in an actual measurement environment considering noise, the proposed cable-free antenna impedance and gain measurement technology works well and can be well applied to the measurement of antenna impedance and gain.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution, which should be included in the scope of the claims of the present invention.

Claims

1. A method for measuring impedance and gain of a cable-free chip antenna, characterized in that: The following steps are involved: Step 1: Establish the electromagnetic model of the probe; Step 2: Establish the electromagnetic model of the antenna under test; Step 3: Establish a free space propagation model based on the one-way transmission equation; Step 4: Establish a model of the measurement system; Step 5: Establish an electromagnetic model of the measurement system including the probe, free space propagation and the antenna under test; Step 6: Derive a complete electromagnetic model of the measurement system including any probe, free space propagation, and any antenna under test; Step 7: Determine the scattering matrices of the probe, free space propagation and the antenna under test; Step 8: Extract the fixed antenna impedance and gain from the near-field distance measurement: (1) When the antenna under test is short-circuited, open-circuited, and loaded with a known impedance, the reflection coefficient of the probe is measured three times respectively; (2) Measure the reflection coefficient of the load connected to the antenna under test in sequence, including short circuit, open circuit and known impedance; (3) Extract parameters from the corresponding scattering matrix based on the probe reflection coefficient and the measured antenna reflection coefficient And determine along Direction Partial gain; (4) Perform another measurement to determine the edge of the orthogonal direction. The partial gain in the direction, i.e. (5) and Add together to obtain the total gain of the antenna under test.

2. The method for measuring impedance and gain of a cable-free chip antenna according to claim 1, characterized in that: The probe is any probe, and the antenna to be tested is any chip antenna.

3. The method for measuring impedance and gain of a cable-free chip antenna according to claim 1, characterized in that: The model uses spherical wave expansion (SWE) to model the probe and the antenna under test (AUT), and the measurement system is modeled by cascading three coupled multi-port networks of the spherical wave expansion (SWE).

4. The method for measuring impedance and gain of a cable-free chip antenna according to claim 3, characterized in that: The equations for modeling the probe and antenna under test (AUT) are as follows: Electric field around the antenna It is surrounded by the smallest sphere with radius r0, that is: In the above formula, is the power normalized spherical vector wave function, where the superscripts c=3,4 represent the outward propagating wave and the inward propagating wave, respectively, K is the wave number, η is the intrinsic admittance of the medium, is the position vector of the point in the (r, θ, φ) spherical coordinates.

5. The method for measuring impedance and gain of a cable-free chip antenna according to claim 3, characterized in that: The total scattering matrix of the antenna under test is expressed as: In the above formula, v and w are the complex amplitudes of the input and output waves at the antenna excitation port, respectively. and denote the complex amplitudes of the incoming and outgoing spherical modes, respectively, is a row vector containing the antenna reception coefficients, is a column vector containing the antenna transmission coefficients, is a square matrix containing the scattering coefficients of the antenna structure. The mode coefficients smn and s'm'n' represent the output and input spherical waves in the probe coordinate system, respectively. is the total scattering matrix; The probe and the antenna under test (AUT) are modeled by the above scattering matrix.

6. The method for measuring impedance and gain of a cable-free chip antenna according to claim 3, characterized in that: When establishing a free space propagation model based on the one-way transmission equation in step 3, first define the probe coordinates (x, y, z) and the coordinates of the antenna under test (AUT) (x′, y′, z′), respectively, where the coordinate system of the antenna under test (AUT) is obtained by rotating the probe coordinate system by three angles ν, θ, χ and translating the distance A. The transmission formula of the free space propagation model describes the complex signal received by the antenna under test (AUT) based on the transmission coefficient of the probe, the reception coefficient of the antenna under test (AUT), three rotation angles and the translation distance A, that is: and s=1, 2; m=-n, -n+1,...,n; n=1, 2... (4); σ'=1, 2; μ'=-v', -v'+1,..., v'; v'=1, 2... (5); In the above formula, smn and 'σ′μ′v′ are the spherical wave index (SWEs) of the probe and the antenna under test (AUT), respectively. is the emission coefficient of the probe, is the reception coefficient of the antenna under test (AUT), e imφ 、e iμ′χ and is the rotation coefficient, is the translation coefficient.

7. The method for measuring impedance and gain of a cable-free chip antenna according to claim 3, characterized in that: Before determining the individual scattering matrices, the equation for the total scattering matrix is ​​derived in the complete electromagnetic model of the measurement system: (1) The amplitude equation of the scattered field pattern is: In the above formula and is the identity matrix; (2) The relationship between the complex amplitude of the incident spherical wave in the coordinate system of the antenna under test (AUT) and the complex amplitude of the outgoing spherical wave in the coordinate system of the probe is expressed by the following equation: in is the combined rotation and translation coefficient; (3) The relationship between the amplitude of the incident spherical wave of the probe and the amplitude of the outgoing spherical wave of the antenna under test (AUT) is expressed by the following equation: In the formula It is used to convert the output wave in the coordinate system of the antenna under test (AUT) into the input wave in the probe coordinate system, that is: (4) The scattering matrix model equation for free space propagation is derived from the above method as follows: In the formula and For the above and The above equations give the network model of the antenna under test (AUT), the probe, and free space propagation. Then, the total scattering matrix is ​​defined. The entire measurement system can also be modeled as having a total scattering matrix Two-port network. (5) Establish the total scattering matrix with the probe and the input end of the antenna under test (AUT) as the reference plane The parameters of the scattering matrix are: When the antenna under test (AUT) is loaded, the reflection coefficient measured at the probe input is expressed as It is expressed that its relationship with the two-port scattering parameter is: (6) By associating the above cascaded free space scattering matrix model with the total scattering matrix, the parameters of the total scattering matrix are derived as follows:

8. The method for measuring impedance and gain of a cable-free chip antenna according to claim 7, characterized in that: Ignoring the multiple scattering terms, equations (17) to (19) are simplified, and the simplified equation of the total scattering matrix without multiple reflection effects is derived as:

9. The method for measuring impedance and gain of a cable-free chip antenna according to claim 8, characterized in that: Determine the total scattering matrix in is the measured reflection coefficient of each load, The probe reflection coefficients measured when the antenna under test (AUT) is loaded in three cases; According to the measured extract Right now:

10. The method for measuring impedance and gain of a cable-free chip antenna according to claim 9, characterized in that: In step eight, the probe structure scattering and multiple reflections are set to be minimal. When determining the antenna impedance and gain without considering the structure scattering and multiple reflections, the impedance of the antenna under test (AUT) is first determined, and then the gain of the antenna under test (AUT) is determined. The input impedance of the antenna under test (AUT) can be obtained by Determine, that is: Z0 is the reference impedance. When determining the total gain of the antenna under test (AUT), two gain measurements are made at two orthogonal directions of the probe, and the relationship equation is: Among them, G PRB For known data, The extraction was performed according to the above equation; Parameters when determining the gain of the antenna under test (AUT) Extracted from equation (7) and equation (8).

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