2-5 Antenna Calibration 2-5-1 Calibration of Loop Antennas for EMI Measurements in the Frequency Range Below 30 Mhz
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Title:J2016E-02-05-01.indd p71 2017/03/01/ 水 10:26:55 2 Research and Development of Calibration Technology 2-5 Antenna Calibration 2-5-1 Calibration of Loop Antennas for EMI Measurements in the Frequency Range Below 30 MHz Katsumi FUJII, Kojiro SAKAI, Tsutomu SUGIYAMA, Kouichi SEBATA, and Iwao NISHIYAMA This paper describes the calibration method of loop antennas for EMI measurements in the frequency range below 30 MHz. NICT developed the method by ourselves and started to provide the calibration service that is certified by ASNITE accreditation program. Because the ASNITE is compliant with the international standard ISO/IEC 17025, the loop antennas calibrated by NICT can be used for all tests such as validations of radio equipment and EMI measurements of electronic devices. 1 Introduction from a transmitting loop antenna that interlinks with an antenna under calibration (receiving antenna). The strength New ways of using 30 MHz and lower radio frequencies of the magnetic field emitted from a transmitting antenna have appeared in recent years: IH induction rice cookers, is determined by correctly measuring the RF current flow- contactless chargers, contactless IC cards, etc., that differ ing through the element of the transmitting antenna. The from conventional radio broadcasts and commercial wire- RF current flowing through the element is measured by less communications. Both new and conventional uses using the conversion component attached to the element, share the same frequency band, so actually one must and converting the RF current flowing through the element measure the electromagnetic fields, and check that there is into DC voltage. A resistor that converts RF current into no interference. Also, electric and electronic devices such heat, or a thermocouple in a vacuum tube that uses heat as AC power supply adapters and LED lights that contain to generate DC voltage, are used for conversion compo- switching regulators are often used in our daily lives, so nents. In this case, correctness and traceability to national there are increasing needs to take measurements to regulate standards of the strength of the magnetic field emitted from unintentionally emitted electromagnetic noise. In this en- the transmitting loop antenna can be maintained by cor- vironment, NICT obtained ASNITE certification to fulfill rectly calibrating the conversion coefficient of the conversion the ISO/IEC 17025 standard for loop antenna calibration, component. However, there were limits to improving its and began providing calibration services in July 2015. precision, partly because this calibration is complex [2][3]. NICT developed a system that can issue internationally To solve this problem, NICT developed a new calibra- recognized calibration certificates which can be used for all measurements, without distinguishing between use for wireless communications or for electromagnetic noise Plane wave Loop antenna measurements. E FaH Electromagnetic field measurements of 30 MHz or lower have conventionally used loop antennas that have sufficiently smaller dimensions than the wavelengths. NICT H Receiver has provided calibration services and performed research and development on calibration of loop antennas since the V time of its Radio Research Laboratory [1]. The “standard magnetic field method” is used to calibrate loop antennas; this method theoretically obtains a magnetic field emitted FFig. 1 Magnetic field measurement and definition of magnetic field antenna factor 71 Title:J2016E-02-05-01.indd p72 2017/03/01/ 水 10:26:55 2 Research and Development of Calibration Technology tion method in recent years [4][5], and started providing 3 Calibration method calibration services. This method is classified the same as a conventional “standard magnetic field method”, but in- Several methods of calibrating loop antennas have al- stead of conventional calibration methods that calibrate ready been proposed [6]–[8]. Some of these methods have from different physical quantities, this method uses an been made international standards by the International already calibrated loop antenna to calibrate another loop Special Committee of Radio Interference (CISPR) [8]. antenna. The advantages of this calibration method are that There are also several classification methods; Table 1 clas- it measures RF power instead of RF current, and that it sifies them into absolute calibration methods which obtain uses a loop antenna to obtain traceability to the national the loop antenna’s magnetic field antenna factor from standard. This paper shows the result of calibrating an other physical amounts (RF current or RF power), and actual commercially sold loop antenna, and explains a relative calibration methods that use a loop antenna with method to evaluate uncertainty associated with the calibra- an already known magnetic field antenna factor to obtain tion result. the magnetic field antenna factor of the antenna under calibration (hereinafter written as “AUC”). Table 1 writes 2 Definition of magnetic field antenna the types of devices needed for calibration, number of factor and how to use it times to measure, features, etc. This calibration method developed at NICT [4][5] is A loop antenna operates by electricity generated by a classified as a “relative calibration” method. It requires a magnetic field that interlinks with the loop plane. Therefore, loop antenna with an already known magnetic field an- as shown in Fig. 1, the loop antenna’s characteristic is de- tenna factor, but this calibration method has the advan- fined by using the ratio of strength H of the magnetic field tages that it can ensure direct traceability to the national that interlinks with the loop plane, and voltage V gener- standard, and that it only needs one measurement. ated in the receiver connected to the loop antenna. Requiring few measurements is an essential condition for H reducing uncertainty of calibration. FaH [S/m] (1) V Here, we consider using a “loop antenna with already This FaH is called the “magnetic field antenna factor”. known magnetic field antenna factor” (hereinafter, “Standard”) as the transmitting antenna. The magnetic field antenna fac- Usually, it is expressed using dB. tor, as defined in Equation (1), is a parameter that expresses dB characteristics of the receiving antenna. If an antenna has FaH log20 FaH10 [dB(S/m)] (2) reciprocity, then it can be used as the transmitting antenna. The unit is dB(S/m). By calibrating the loop antenna, if the As shown in Fig. 2, a circular loop antenna with radius magnetic field antenna factor was already obtained, then size rRx for which we want to obtain the magnetic field one can read the output voltage V of the receiver, and use antenna factor (AUC) is placed at distance d from the the following equation to know the strength of the mag- Standard (radius rTx), with its loop plane parallel to the netic field that interlinks with the antenna. Standard, with their centers aligned. Then, if we use a dB dB dB vector network analyzer (hereinafter, “VNA”) to measure H A/m)][dB( V V)][dB( FaH [dB(S/m)] S21 between the Standard and AUC, the magnetic field (3) antenna factor of the AUC can be determined from the If the transmitting source is sufficiently far, and plane waves following equation. are being received, then in addition to wave impedance in 2K 1 F (AUC) (5) aH )( SSTDFZ free space (0 0.377120 ) , one can use aH00 21 dB dB dB Here, E V/m)][dB( V V)][dB( FaH [dB(S/m)] 51.53 )][dB( FaH (STD) ：Magnetic field antenna factor of Standard (4) [S/m] and convert into electric field strength. ：Angular frequency (2πf [rad/s]) -7 （ 53.510.377log20 ） 0 ： 10 Permeability of vacuum (4π × 10 H/m) Z 0 ：Characteristic impedance of measurement system (50 Ω) 72 Journal of the National Institute of Information and Communications Technology Vol. 63 No. 1 (2016) Title:J2016E-02-05-01.indd p73 2017/03/01/ 水 10:26:55 Calibr ation of L oop A nteenns for EMI Measurements in the F requency R ange B elow 30 MHz 2-5-1 Also, On the other hand, as shown in Fig. 3 (b), the Standard 2 2 4 at NICT is a shielded loop antenna with ϕSTD＝3.7 mm and 1 R 15 r r 315 r r 0 Tx Rx Tx Rx K 3 1 2 2 (6) radius rRx＝ 5.0 cm (REPIC, M201A-100R). It is calibrated 2R0 8 R0 64 R0 at the National Physical Laboratory (NPL) which maintains 2 2f the UK’s national standards. Figure 4 shows the traceabil- (λ is wavelength, c is speed of light) c ity chart. 2 2 2 Table 2 shows the calibration points where NICT pro- R0 d rTx rRx vides calibration values. NPL does not provide all the d Distance between planes of Standard and AUC [m] calibration points that NICT provides, so we use the inter- rTx Loop radius of Standard [m] polated values of NPL’s calibration certificate. When inter- rRx Loop radius of AUC [m] polating, two regression curves are prepared with an 8 MHz frequency boundary, and used as the calibration Refer to [9]. If dB is used to calculate, then it is values. The next chapter describes the size of uncertainty dB dB dB due to interpolation. FaH AUC 45.9 20log10 fMHz S 21 20log10 K FaH STD As shown in Fig. 2, measurements used a VNA (Rohde [dB(S/m)] (7) & Schwarz ZNB4) near the center in a semi-anechoic (See its derivation in the Appendix). chamber, with antennas placed with distance between an- tennas d = 20 cm, height from ground plane until loop 4 Calibration results element’s center h = 1.6 m, and S21 was measured. The VNA was calibrated by a procedure called “Unknown Thru” This paper shows results of calibrating the loop an- before measurement. tenna for EMI measurements shown in Fig.