MEASUREMENT OF MAGNETIC COMPLEX ANTENNA FACTOR OF A DOUBLE-LOADED SHIELDED LOOP ANTENNA BY 3-ANNTENA METHODS Takashi Iwasaki and Makoto Takashima The University of Electro-Communications 1-5-1 Chofugaoka, Chofu, Tokyo, 182-8585 Japan E-mail: [email protected] Abstract - This paper presents two methods for determining The S-parameters of an actual component is somewhat the complex antenna factor (CAF) of a shielded loop antenna different from (1) due to reflection and loss. for HF magnetic field measurements. A circuit model is introduced to apply the field-transferred 3-antenna method Matched Load to double-loaded shielded loop antennas. In addition, a new Gap 3-antenna method is proposed, where theoretical calculation Conductor Port 2 L and circuit measurement are combined. Experiments are made in an anechoic chamber in the frequency range of 50 Port 1 0° MHz to 1000 MHz. The magnetic CAF of various shielded loop antennas can be determined by the proposed methods. 180° Keywords - EMC/EMI, Electromagnetic sensor, Shielded Port 3 loop antenna, Antenna factor, 3-anntena method Coaxial Line 180°Hybrid Circuit 1. INTRODUCTION Fig.1 Structure of double-loaded shielded loop antenna It is important to measure electromagnetic fields in the problems related to electromagnetic compatibility (EMC) The complex antenna factor (CAF) is usually used in electric such as electromagnetic interference (EMI) measurements, field measurements. Here, following magnetic CAF is radiation from electrostatic discharges (ESD), and defined for small loop antennas in magnetic field electromagnetic biological effects. In the electromagnetic measurements, field measurements, the characteristics of a detecting antenna have to be determined. As a characteristic of the antenna, H (ω) complex antenna factor (CAF) has been proposed to measure F (ω) = (2) M I(ω) transient electromagnetic fields [1]. This paper presents two methods for determining the magnetic CAF of a double- ω ω loaded shielded loop antenna. The double-loaded shielded , where H( ) and I( ) are the magnetic field of an incident loop antenna is useful for measuring HF magnetic fields, plane wave and the current of the load, respectively, as because it has the advantage that the effect of electric field shown in Fig.2. The load shall be matched to the can be reduced [2]. characteristic impedance of the feeder for the use of waveform measurements of transient magnetic fields. 2. ANTENNA AND COMPLEX ANTENNA FACTOR π Characteristic Impedance Z0 We consider a double-loaded shielded loop with a /2 Antenna Element hybrid circuit as shown in Fig.1. The ideal S-matrix Current = of the hybrid circuit is expressed as, X ij (i, j 1,2,3) Coupling Z0 I(ω) Circuit − H(ω) X 11 X 12 X13 0 1 1 1 X X X = 1 0 0 . (1) 21 22 23 Fig.2 Incident magnetic field and the output current 2 − X 31 X 32 X 33 1 0 0 characteristic impedance of the transmission line in Fig.2. λ, 3. FIELD-TRANSFERRED 3-ANTENNA METHOD η0 and k are the wavelength for ω, the wave impedance, and the wave number, respectively, in free space. The CAF may be measured by the 3-antenna method in The required far field condition can be hardly satisfied which the transmission S-parameter, S21, between in the measurements for small loop antennas, because of too transmitting and receiving antennas is needed. Since CAF is small receiving power and interference of undesired waves. defined for a plane wave incidence, the S21 has to be To cope of this problem, the field-transferred 3-antenna measured under the far field condition. method has been developed for monopole antennas and If the far field condition is satisfied, the magnetic dipole antennas in HF electric fields measurements [3]. In the complex antenna factors FM i (i=1,2,3) of three antennas can field-transferred 3-antenna method, the transmission S- be calculated from the following equations, parameters are measured in near field and the corresponding far field values are estimated with field transfer factor (FTF). − jkR FTF can be obtained by a theoretical calculation using the Z0 T 32 e method of moment (MoM). F = (3) M1 λη In order to calculate FTF for the double-loaded shielded j0 T 21 T 13 R loop antennas, a new circuit model is required, because of the Z T e − jkR two exciting points as shown in Fig.2. We introduce a circuit F = 0 13 (4) M 2 λη model as shown in Fig.3, where [A] and [B] are S-matrices of j0 T 21 T 32 R 3-port hybrid circuits of the transmitting and receiving Z T e − jkR antennas, respectively, and [Se] is a 4-port S-matrix of the F = 0 21 (5) shielded loop elements. M 3 λη j0 T13 T 32 R Using this circuit mode, the transmission S-parameter S50 =[a5/b0](b5=0) between the reference planes 0 and 5 can be derived as the following equation (6): ,where Tmn=(S21)mn (m,n=1,2,3) are transmission S-parameters between antenna m to antenna n. R is the distance between a transmitting antenna to a receiving antenna. Z0 is the −1 A A =[ ][][][][][][]{} − −1 []10 + [][][][]−1 10 SBBMSAMSBS50 53 54 4 3 1 1 2 1 3 SAMS3 1 1 1 A21 A20 (6) 1 3 0 5 a1 b1 b3 a3 b0 a0 a5 b5 [A] [Se] [B] a4 a2 b 2 b4 2 4 0° 0° 180° 180° r R Fig.3 Circuit model of the transmission , where Since this equation is expressed only by the S-parameters of [Se] of the loop elements at r and R, not including the S- S S S S parameters of the hybrid circuits, the FTF q(r,R) can be 11 12 13 14 obtained numerically by the MoM. Once the FTF is S S S S S S []= 21 22 23 24 = 1 2 calculated, the transmission S-parameters between antenna m Se , (7) to antenna n, Tmn(R)=(S50(R))mn satisfying the far field S31 S32 S33 S34 S3 S4 condition, is estimated from Tmn(r) =(S50(r))mn measured in a S41 S42 S43 S44 near field as, = (16) A A A Tmn (R) q(r, R)Tmn (r) 00 01 02 A00 A01 A02 []A = A A A = A10 , (8) 10 11 12 The procedure is shown in Fig.4. A1 A A20 A21 A22 20 Measurement of S50 (r) in near field B33 B34 B35 B B 35 []B = B B B = 1 , (9) 43 44 45 B45 Field transfer factor B B B B53 B54 B55 53 54 55 (15) [][][][]M = 1 − S A , (10) 1 1 1 [][][][]= − Evaluation of Tmn (R)= M 4 1 S4 B1 . (11) (S50(R))mn The FTF is defined by － S (R) 3-antenna method (3) (5) q(r,R) = 50 (12) S50 (r) , where S50(R) and S50(r) are the transmission S-parameters at Magnetic complex the far field distance R and at the near field distance r, antenna factor respectively. To calculate the FTF by using (12), the S- matrices [A] and [B] of the hybrid circuits have to be known. Fig.4 Procedure of the field-transferred 3-antenna method In order to estimate the FTF without the measurements of [A] and [B], the S-parameters of the hybrid circuits are approximated as follows: 4. MEASURING HYBRID CIRCUIT METHOD If the hybrid circuits are detachable from the antenna A A − A 00 01 01 elements, the S-parameters of [A] and of [B] can be measured []= by using a vector network analyser. In this case, the far field A A01 0 0 (13) transmission S-parameters between antenna m to antenna n, − A 0 0 01 Tmn(R)=(S50(R))mn satisfying the far field condition are calculated from (6) without the measurements of Tmn(r) B B − B =(S50(r))mn in the near field. The procedure is shown in Fig.5 33 34 34 []= B B34 0 0 . (14) 5. MEASUREMENT AND COMPARISON − B 0 0 34 Double-loaded shielded loop antennas in which L = 10.0 cm and a = 1.0 cm in Fig.2 were constructed using the hybrid Then, (12) can be expressed as circuit with the specifications shown in Table I. The near field measurements were carried out in an anechoic chamber. S (R) − S (R) − S (R) + S (R) The frequency range is 50 MHz to 1000 MHz. q(r, R) = 31 32 41 42 (15) S (r) − S (r) − S (r) + S (r) Fig.6 and Fig.7 show the magnitude in dB1/m and the 31 32 41 42 phase in deg of the magnetic CAF determined by the two methods. Here, the near field distance r and the far field distance R are 2 cm and 100 m, respectively. The two results agree well. Measuring Hybrid Circuit Mehod 100 Field Transferred 3-antenna Method 50 Calculation of Measurement of [A] g] De [ far field [Se] and [B] 0 Phase -50 Calculation of -100 Tmn(R)=(S50 (R))mn by (6) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Frequency [GHz] 3-antenna method (3) －(5) Fig.7 Phase of magnetic complex antenna factor 6. CONCLUSION Magnetic complex antenna factor The field transferred 3-antenna method can measure the magnetic CAF of the shielded loop antennas without the Fig.5 Procedure of the measuring hybrid circuit method measurements of the hybrid circuits. On the other hands, the measuring hybrid circuit method can determine the magnetic CAF of the shielded loop antennas without the measurements Table I – Specifications of the hybrid circuits.