JP3218983B2 - Electromagnetic field strength measurement device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic field intensity measuring device, and more particularly to a measuring device capable of accurately measuring an electromagnetic field intensity in a narrow space such as an automobile or an electronic device.

[0002]

2. Description of the Related Art In recent years, in addition to a conventional radio, a large number of devices that require radio wave reception, such as a television, a telephone, and a navigation device, are mounted on a recent automobile. We are having difficulty securing. Therefore, it is conceivable to integrate a small receiving antenna into each device.However, the electromagnetic field strength in a metal structure of an automobile is not uniform. In order to determine, it is necessary to measure the electromagnetic field strength of each part in the vehicle. FIG. 7 shows an example of a measuring apparatus conventionally used for this type of electromagnetic field strength measurement. As shown in the figure, a small antenna 11 for radio wave reception is provided in the sensor unit 1, which is connected to an impedance converter 17 to achieve impedance matching with the coaxial cable 4 for signal transmission. The display unit 2 is placed outside the vehicle so as not to disturb the electromagnetic field distribution in the vehicle. The display unit 2 detects the received signal that has passed through the coaxial line 4 with the detector 23, and detects the detected signal according to the received radio wave intensity (electromagnetic field intensity). Is output to the display device 24 and displayed.

[0003]

However, when measuring the electromagnetic field strength in a small space such as a vehicle cabin with the above-mentioned conventional electromagnetic field strength measuring apparatus, there is a problem that the measured values often vary greatly. Was. As a result of inventors have variously examined the reason, it was found that that the impedance of the antenna 11 under the influence of an object, such as the surrounding metal or dielectric is varied is the main cause.

[0004] The present invention has been made based on this finding, and its object is to provide an electromagnetic field strength measuring device capable of accurately measuring the field strength in narrow space .

[0005]

[0006]

[0007]

[0008]

[0009]

[0010]

[MEANS FOR SOLVING THE PROBLEMS] To achieve the above object
Two in the first aspect of the invention, an antenna for receiving a radio wave (11)
And a detector (13) for detecting a signal received from the antenna (11), and an impedance converter (12) provided between the antenna (11) and the detector (13). The absolute value of the ratio (Zt / Za) of the impedance (Zt) of the output terminal of the impedance converter (12) to the input impedance (Za) of (13) is set so that the worst value of the mismatch loss is minimized. . The absolute value of this ratio (Zt / Za) is preferably set in the range of 0.05 to 0.2.

In the first aspect of the present invention, in the case of a small antenna in which the dimensions of the antenna are very small as compared with the wavelength, the real part of the impedance is very small at several Ω or less, and the imaginary part is minus several hundred to several thousand Ω. . Therefore, an impedance converter is provided between the receiving antenna and the receiving antenna in order to achieve impedance matching with the detector. Usually, the ratio of the impedance (Zt) at the output terminal of the impedance converter to the input impedance (Za) of the detector ( The impedance is perfectly matched by setting the absolute value of Zt / Za) to 1. However, according to the experiments of the inventor, when the impedance is perfectly matched, the sensitivity of the measurement is improved, but if the impedance of the antenna changes slightly even in a small space due to the influence of an object around the antenna. The measured value fluctuates sensitively according to the measurement, and the accuracy of the measurement is rather lowered. Therefore, the concept of “worst value of mismatch loss” is introduced as an index that comprehensively represents measurement sensitivity and measurement accuracy, and the absolute value of the ratio (Zt / Za) (hereinafter referred to as impedance ratio) The worst value of the loss was set to be a minimum. Particularly, the impedance ratio is 0.05 to
When it is set in the range of 0.2, the worst value of the mismatch loss is minimized, and the measurement accuracy can be significantly improved without greatly lowering the measurement sensitivity, and the electromagnetic field intensity can be measured accurately. Becomes

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) FIG. 1 shows the overall configuration of an electromagnetic field strength measuring apparatus. The electric field strength measuring device includes a sensor unit 1 provided inside a vehicle cabin and a display unit 2 provided outside the vehicle cabin. The sensor unit 1 includes an antenna 11, an impedance converter 12, a detector 13, a voltage / frequency (V / F) converter 14, and a voltage / optical (E / O) converter 15, and a display unit 2 Is the counter 21 and the display device 2
2 is comprised. Then, the sensor unit 1 and the display unit 2
Are connected by an optical fiber 3.

The high-frequency reception signal from the antenna 11 that has received the radio wave is transmitted via the impedance converter 12 to the detector 1.
Sent to 3. The detector 13 detects the intensity of the received signal and outputs it to the V / F converter 14 as a voltage detection signal.
The V / F converter 14 converts the voltage detection signal into a voltage pulse signal having a frequency proportional to the voltage value. The voltage pulse signal is converted into an optical pulse signal by the E / O converter 15 and becomes an output of the sensor unit 1. The optical pulse signal is sent to the display unit 2 via the optical fiber 3. Since the frequency of the light pulse signal is proportional to the electromagnetic field strength at the spatial position where the antenna 11 is installed, the number of pulses per unit time of the light pulse signal is integrated by the counter 21 and multiplied by an appropriate coefficient to multiply the display device 22. Displayed above, this is the electromagnetic field strength at the measurement point.

As described above, since the sensor section 1 and the display section 2 are connected by the optical fiber 3 to transmit an optical pulse signal, the electromagnetic field to be measured is different from the case where a conventional coaxial line is used. There is almost no disruption. Therefore, by using a sufficiently long optical fiber 3 and disposing the display unit 2 outside the vehicle compartment away from the sensor unit 1 so as not to affect the electromagnetic field to be measured, the electromagnetic field to be measured is almost completely eliminated. Its intensity can be accurately measured without disturbing.

Further, since the information on the electromagnetic field strength at the installation point of the antenna 11 is digitally transmitted as the frequency of the optical pulse, the ambient temperature changes or the transmission loss of the optical fiber 3 changes depending on the installation state. However, the measurement accuracy is not affected at all.

FIG. 2 shows details of the antenna 11 and the impedance converter 12 in the present embodiment. The antenna 11 is a dipole antenna having a length of 100 mm and a wire diameter of 2 mm, and 300 MHz when installed in a free space.
The impedance of the antenna 11 at (1.4−
j1223) Ω. As shown in the figure, the impedance converter 12 includes a pair of inductors 121 and 122 and a transformer 123. The impedance converter 12 performs the impedance conversion as well as the reception signal of the dipole antenna 11, which is a balanced signal. Has a function of converting the unbalanced signal into an unbalanced signal. Assuming that the inductance L of the impedance converter 12 is L = 617 nH and the turns ratio N of the transformer 123 is N = 2, the output terminal impedance Zt of the impedance converter 12 as viewed from the detector 13 is (5.6 + j0Ω). The input impedance Za of the detector 13 is set to 50Ω, and therefore, the absolute value (hereinafter referred to as impedance ratio) of the ratio (Zt / Za) between the output terminal impedance Zt and the input impedance Za is 0.11. When the impedance ratio is set to 0.11, the worst value of the mismatch loss (which is an index comprehensively representing the measurement sensitivity and the measurement accuracy) is minimized as shown in FIG.

The worst concept of the mismatch loss will be described below. FIG. 4 shows the relationship between the resonance frequency and the mismatch loss when the resonance frequency of the antenna 11 changes due to an object existing around. The solid line in the figure shows the case of the present invention in which the impedance ratio is set to 0.11, and the broken line in the figure shows the impedance Zt at the output terminal of the impedance converter 12 as in the prior art.
3 shows a case where the input impedance Za is matched to the input impedance Za of 3 (that is, a case where the impedance ratio is set to 1). In the conventional case where the impedance is matched, the center frequency fo
Does not cause a mismatch loss, but when the resonance frequency of the antenna 11 slightly shifts due to the influence of a peripheral object, the mismatch loss rapidly increases. That is, as shown in the figure, when there is a deviation of the resonance frequency from the center frequency fo by only 3%, the mismatch loss reaches as much as 22 dB. Therefore, even if the intensity of the electromagnetic field to be measured is constant, the measurement result fluctuates as much as 22 dB due to the installation state of the antenna and the influence of peripheral objects, and lacks measurement accuracy.

On the other hand, in the present invention (solid line in the figure), even if the resonance frequency of the antenna 11 deviates from the center frequency fo by about 3% due to the influence of the peripheral object, the mismatch loss at this time will be smaller at the center frequency fo. 4.7dB to 11.7d
It only increases by 7 dB to B. The change of the resonance frequency due to the influence of the surrounding objects is usually about 3% or less. Therefore, by applying the present invention, the variation of the mismatch loss can be reduced from 22 dB to 7 dB, and the measurement accuracy can be remarkably improved.

In the case of the present invention, since a mismatch loss of 4.7 dB occurs even at the center frequency fo, the lowest measurable electric field strength is obtained when impedance matching is attempted (that is, when the impedance ratio is 1). And the measurement sensitivity is relatively poor. As the performance of the electromagnetic field intensity measuring device, it is desirable that the measurable electric field intensity be as low as possible to increase the sensitivity and that the measurement accuracy be as high as possible. However, this requirement is conflicting and is illustrated in FIG. As shown by the broken line in the figure, the mismatch loss when there is no peripheral object is smallest when the impedance ratio is set to 1 (that is, the measurement sensitivity is the best). On the other hand, when there is a peripheral object, the variation of the mismatch loss due to the influence becomes maximum when the impedance ratio is 1 as shown by the one-dot chain line in the figure (that is, the measurement accuracy is worst). Become). Therefore, assuming that the sum of the mismatch loss and the variation thereof according to the impedance ratio is “the worst value of the mismatch loss”, the worst value has a minimum value with respect to the change of the impedance ratio as shown by the solid line in the figure. Have. According to the experiment of the inventor, the impedance ratio at which the worst value of the mismatch loss shows the minimum value is 0.11 as described above (see FIG. 3), and by setting the impedance ratio to this value, a high measurement is performed. Accuracy and high measurement sensitivity can be compatible. The effect is not limited to 0.11, and if the impedance ratio is set in the range of 0.05 to 0.2, the effect of significantly improving the measurement accuracy without significantly lowering the measurement sensitivity can be sufficiently obtained.

As described above, according to the present embodiment, the variation of the electromagnetic field to be measured can be suppressed by using the optical fiber for connecting the sensor unit and the display unit, and the impedance ratio can be reduced. By setting the worst value to be minimum, highly accurate and highly sensitive measurement of the electromagnetic field to be measured can be performed.

(Second Embodiment) FIG. 6 shows a second embodiment of the present invention. In the present embodiment, the sensor section 1 is further provided with a battery exhaustion detection circuit 16. Battery exhaustion detection circuit 16
Is provided between the V / F converter 14 and the E / O converter 15 and includes a resistor 161 as power supply voltage detecting means for dividing the power supply voltage, and an AND gate 16 as transmission stopping means.
2 is comprised. And the AND gate 1
The output signal of the V / F converter 14 and the divided signal from the resistor 161 are input to 62, and the output signal of the AND gate 162 is input to the E / O converter 15.

If the power supply voltage drops due to battery exhaustion,
The characteristics of the detector 13 and the V / F converter 14 change, and the measurement accuracy decreases. Here, in this embodiment, when the power supply voltage decreases, the level of the divided voltage from the resistor 161 decreases, and the AND gate 162 is closed. For this reason, E /
No voltage pulse signal is input to the O converter 15 and no optical pulse signal is output. If the power supply voltage is normal, the voltage signal output from the detector 13 is not zero due to the influence of noise even if the electromagnetic field to be measured is very weak.
The O converter 15 outputs some optical pulse signal. Therefore, whether or not the electromagnetic field intensity measuring device is operating normally can be easily determined based on the presence or absence of the output of the optical pulse signal from the optical fiber 3, and the device is used in a state where the measuring accuracy is lowered. Can be prevented beforehand.

(Other Embodiments) In the first embodiment, an optical fiber is used for signal transmission, and the impedance ratio is determined so that the worst value of the mismatch loss is minimized. A remarkable effect can be obtained by using only one of the configurations as compared with the related art. In addition, the second
In the embodiment, the power supply voltage is detected by the voltage-dividing resistor, and the transmission of the optical pulse signal is prohibited by the AND gate. However, the invention is not limited thereto. In this case, the drop in the power supply voltage is not limited to the battery power supply. In each of the above embodiments, a dipole antenna is used.
The antenna is not particularly limited. For example, a loop antenna or the like can be used.

[0024]

As described above, according to the electromagnetic field intensity measuring apparatus of the present invention, even when an object made of a metal, a dielectric, or the like exists around the antenna, the intensity of the electromagnetic field can be increased. It is possible to measure with accuracy. Therefore, the electromagnetic field strength measuring apparatus to which the present invention is applied can be used even in a narrow space such as in an automobile or an electronic device, and is very useful as an apparatus for determining the cause of radio interference which has recently become a problem. it is useful to.

[Brief description of the drawings]

FIG. 1 is an overall block configuration diagram of an electromagnetic field intensity measuring device according to a first embodiment of the present invention.

FIG. 2 is a detailed configuration diagram of a sensor unit.

FIG. 3 is a graph showing a change in a worst value of a mismatch loss.

FIG. 4 is a graph comparing the change in mismatch loss with that of the related art.

FIG. 5 is a graph illustrating a change in a worst value of a mismatch loss.

FIG. 6 is a detailed configuration diagram of a sensor unit according to a second embodiment of the present invention.

FIG. 7 is an overall block configuration diagram of a conventional electromagnetic field intensity measuring device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Sensor part, 11 ... Antenna, 12 ... Impedance converter, 13 ... Detector, 15 ... Voltage / optical converter, 16
... A battery exhaustion detection circuit, 161, a resistor, 162, an AND gate, 3 ... an optical fiber.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-57-192877 (JP, A) JP-A-3-72366 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01R 29/08 JICST file (JOIS)

Claims (2)

(57) [Claims] An antenna (11) for receiving a radio wave, a detector (13) for detecting a signal received from the antenna (11), an antenna (11) and the detector (1).
3) an impedance converter (12) provided between
The absolute value of the ratio (Zt / Za) of the impedance (Zt) of the output terminal of the impedance converter (12) to the input impedance (Za) of the detector (13) is determined by the worst case of the mismatch loss. An electromagnetic field strength measuring device characterized in that the value is set to a minimum. 2. An antenna (11) for receiving a radio wave, a detector (13) for detecting a signal received from the antenna (11), and the antenna (11) and the detector (1).
3) an impedance converter (12) provided between
The absolute value of the ratio (Zt / Za) of the impedance (Zt) of the output terminal of the impedance converter (12) to the input impedance (Za) of the detector (13) is 0.05 to 0. 2. An electromagnetic field intensity measuring device set within the range of 2.