Radiation Noise Reduction using Spread Spectrum
for Inductive Power Transfer Systems considering
Misalignment of Coils
Keisuke Kusaka, Kent Inoue, Jun-ichi Itoh
Department of Electrical, Electronics and Information Engineering
Nagaoka University of Technology
Nagaoka, Niigata, Japan
kusaka@vos.nagaokaut.ac.jp, k_inoue@stn.nagaokaut.ac.jp, itoh@vos.nagaokaut.ac.jp
Abstract—This paper proposes spread spectrum methods toreduce radiation noise caused by an inductive power transfer(IPT) system. A noise reduction method with spread spectrum
techniques has been proposed. The proposed noise reduction
method randomly changes the output frequency of an inverter
within a range of 79-to-90 kHz. Changing the output frequency,
the radiation noise is spread in a frequency domain. A previous
study has proposed the spread spectrum method with the biased
probability distribution, which improves the noise reduction
performance. However, the suitable probability distribution has
not been clear. This paper proposes two spread spectrum
methods with biased probability distributions, which areproportional and inverse proportional to an input impedance of
the IPT system. The proposed methods are assessed andcompared with the conventional noise reduction method.
Considering the input impedance of the IPT system, the radiationnoise from the IPT system with or without the coil misalignmentis reduced by 13.4 dBμA and 7.0 dBμA in comparison with theconstant frequency operation. Moreover, the reduction
performance is improved compared to the conventional spread
spectrum method.
Keywords—inductive power transfer; wireless power transfer;radiation noise; EMI; spread spectrum; random carrier

I. INTRODUCTION
Inductive power transfer (IPT) systems for electricalvehicles (EVs) are actively studied [1-7] in recent years. The
IPT is expected to improve the convenience of users of EVs
when the users charge an onboard battery. Especially, adevelopment of a suppression method of radiation noise froman IPT system for EVs is highly required [8-14] in order to put
it to practical use. The radiative emission has to be suppressed
because the radiation noise may cause malfunctions of wirelesscommunication systems or electronic equipment placed nearby
IPT systems. From a practical application perspective, radiationnoise has to be suppressed to respect the regulations, which
will be legislated in each area or nations. The regulations ineach area or nations will be legislated to be conformed to theguidelines published by CISPR [15]. Ref. [8-11] reduce theradiation noise using magnetic shields and metal shields, which
are put nearby the coils. These shields effectively suppress
radiation noise because a reluctance of a leakage path ofmagnetic flux is increased. However, eddy current loss occurs

in these shields. As another approach, a noise reduction method
using a low-pass filter is used. The low-pass filter is inserted
between a primary inverter and a transmitting coil. A cut-off
frequency of the low-pass filter has to be set at the frequency
between the fundamental frequency and three times thefundamental frequency to reduce the low-order harmonics
components of the current. It decreases the efficiency of the
IPT system because it will degrade a power factor from theviewpoint of the inverter at the fundamental frequency. In [12],the EMI reduction technique using two-channel IPT systemwith currents of opposite phase has been proposed. The two setof the coils are placed to cancel out the radiation noise. It iseffective to cancel out the radiation noise. However, the twochannel IPT system has poor freedom on the coil placement
because it needs four coils.
In [13-14], spread spectrum methods have been proposed.
The spread spectrum achieves noise reduction by randomlychanging an output frequency of the inverter. Thus, the spread
spectrum method does not require additional components suchas metal plates, magnetic shields, or additional coils. By
selecting the output frequency of the inverter from a biased
probability distribution, the effect of spread spectrum isimproved in comparison with the spread spectrum with an
uniform probability distribution because the IPT system has afrequency dependence at around a resonance frequency.
However, the optimum probability distribution is not discussed
in [13-14]. In addition, the reduction effect has been tested
under the condition that a misalignment of the transmission
coils is unconsidered. In order to put it into a practical use, thereduction methods have to be effective even if themisalignment of the coils occurs.
In this paper, novel spread spectrum methods are proposed
and assessed. The probability distribution is determined byconsidering the input impedance of the IPT system at anominal position in the proposed method. Considering afrequency dependence of the input impedance of the IPT
system, the radiation noise is reduced regardless of themisalignment. This paper consists of following sections; first,
the regulations for the radiation noise from the IPT system inJapan is explained. Then, two reduction methods using spread
spectrum are proposed. Third, the proposed method is

compared to the conventional method considering themisalignment of coils with a 3-kW prototype.
II. INDUCTIVE POWER TRANSFER SYSTEMS FOR EVS
A. Regulations of Radiation Noise
Figure 1 shows the guideline of the radiation noise in Japanfor the IPT systems toward EVs with a power of 7.7 kW or less[16]. The regulation was legislated in Japan in 2015. Basically,
the standard is conformed to the CISPR 11/Group 2/Class B[15]. Note that the allowable limits are converted to the limits
with measuring at 10 m according to CISPR/B/587A/INF
because CISPR 11/Group 2/Class B requires measuring thenoise at 3 m. Moreover, CISPR11 requires measuring theradiation noise with a quasi-peak measuring method.
In the regulation, the frequency band from 79 to 90 kHz has
been assigned as the transmission frequency of the IPT systems
for EVs. The allowable limits of the radiation noise on thetransmission frequency is relaxed from 23.1 dBμA/m to 68.4
dBμA/m at 10 m. In addition, the guideline on the following
frequency bands is relaxed by 10 dB from CISPR 11/Group
2/Class B.

Fig. 1. Guideline of radiation noise for 7.7-kW or less IPT system for
EVs in Japan.

• 158 – 180 kHz• 237 – 270 kHz• 316 – 360 kHz

Fig. 2. System configuration example of IPT system for EVs.

• 395 – 450 kHz
In contrast, the regulation within 526.5 to 1606.5 kHz is set
at -2.0 dBμA/m because these frequency bands have been
assigned to the amplitude modulation (AM) broadcasting. Thisfrequency overlaps with the seventh harmonics. Thus thefrequency components and the low-order harmonics have to be
suppressed.
B. System configuration
Figure 2 shows the system configuration of the typical IPT
system for EVs. A primary coil is buried in the parking,whereas a secondary coil is beneath the bottom of the vehicles.
Thus the magnetic coupling between these coils is weak. The
compensation circuits such as a series-parallel compensation(S/P), series-series compensation (S/S), parallel-parallel
compensation (P/P), or parallel-series compensation (P/S) havebeen proposed [17] in order to overcome the problems. In
addition to the above methods, there are several combinationsof connections of capacitors and inductors as the compensation
circuits. However, the common concept of the compensation
circuit is the improvement of the power factor from the inverter
and the induced voltage on the secondary coils.
Besides, a buck or boost converter is typically connected inorder to maintain the transmission efficiency regardless of thechange of input voltage [18].
In this paper, the series–series compensation (S/S) is usedin order to cancel out the reactance caused by the leakage
inductance. The current on the primary coil and the secondary
coil are expressed as (1) and (2) using the equivalent load
resistance Req.

1 
r2 + Req + jωL2 −ωC2 

I1 =
V1



1 
1 
2 2
r2 + Req + j ωL2 − + ω Lm
r1 + j ωL1 −
ωC1 
ωC2 




I2 =

jωLm
V1






1 
1 
2 2
r + Req + j  ωL2 − + ω Lm
r1 + j  ωL1 −ωC1  2
ωC 2 




(1)

(2)

Note L1 is the primary inductance, L2 is the secondary
inductance, Lm is the mutual inductance, r1 is the series
resistance of the primary winding, r2 is the series resistance ofthe secondary winding, C1 is the primary compensation
capacitor, C2 is the secondary compensation capacitor, ω is theangular frequency of the power supply, and V1 is thefundamental component of the output voltage of the inverter.
The relationship between the primary voltage V1 andprimary DC-voltage when the inverter is operated with asquare wave output voltage is expressed by

V1 =

2 2

π

V1 DC

(3).

The equivalent AC load resistance is calculated by (4)using the analysis given in [19].

Req =8 V22DC8= 2 R
π 2 P2π(4)where V2DC is the secondary DC voltage and P2 is the output
power.
The compensation capacitors are designed to cancel out thereactance at the transmitting frequency. Thus, the capacitancevalue of the compensation capacitors should be designed
according to

IZ = IZ − p ⊕ IZ −q
Fig. 3. 9-bit pseudo random number generation based on a M-sequence.

C1 =

1
L1ω 2

(5),

C2 =

1
L2ω 2

21] on a DSP (Digital signal processor). The M-sequence based
generation algorithm of the pseudo random numbers is suitable
to implement to the DSP because it does not require muchcalculation.

(6).

A random number based on the M-sequence is generated by

Substituting (5) and (6) to (1), the primary current with thecompensation is expressed by

I1 =

(

r2 + Req)r1 r2 + Req + ω 2 L2m

V1

(7)

under the resonance conditions.
The power factor from the output of the power supply isunity owing to the compensation capacitors.
III. NOISE REDUCTION METHODS
Radiation noise is caused by the currents, which flow in thetransmission coils. In [13-14], a noise reduction method using
the spread spectrum technique has been proposed. The output
frequency of the primary inverter is changed within a range of
79-to-90 kHz at every period, which is allowable frequency
band for IPT system shown in Fig. 1. The output frequency is
randomly selected. Due to this operation, the radiation noise
will be spread in a frequency domain. Moreover, Ref. [14]proposed to select the switching frequency with the probability
distribution, which has a bias but not the uniform discrete
probability distribution. The probability distribution proposedin [14] is proportional to the combined impedance of the
primary inductance and the primary capacitance. Ref. [14]experimentally assessed the effect of the biased probability
distribution. However, the adequate probability distribution to
suppress the noise is not discussed. In this paper, two spread
spectrum methods are proposed and experimentally comparedto the conventional spread spectrum method.
Figure 3 explains the generation method of pseudo-random
numbers. For all of the spread spectrum methods tested in thispaper, pseudo-random numbers are used in order to select theoutput frequency in random. The pseudo random numbers are
generated using a maximal length sequence (M-sequence) [20-I z = I z − p ⊕ I z−q(8),where Iz-p and Iz-q are the present value Iz delayed by p and q
periods, respectively (p > q). The present value is an ExclusiveOr of the IZ-p and IZ-q. In this experiments, p = 9, q = 1 are used.
Moreover, a bit number of the pseudo-random number is nine.
A. Conventional Method (Probability distribution
proportional to combined impedance of LC)
Figure 4(a) shows the probability distribution, which isused as the conventional method. The output frequency is
selected according to the Table I (a). Table I indicates therandom numbers, which are generated by M-sequence. The
conventional method uses the biased discrete probability
distribution for the spread spectrum technique. The probability
distribution is determined proportionally to a combined
impedance of the transmission coil and the primary
compensation capacitor. The frequency span among eachfrequency is limited to 650 Hz owing to a clock frequency of
the FPGA (Field-programmable gate array), which is used togenerate the carrier for the inverter.
The combined impedance is expressed as (9) when an
equivalent series resistance of the primary coils can be ignored.
1
Z LC = jωL − j
ωC(9)

B. Proposed Method I (Probability distribution proportional
to ZinIn this paper, two spread spectrum methods are proposed.
Figure 4(b) shows the first proposed probability distribution.
The first proposed method uses biased probability distribution,
which is proportional to an absolute value of an input
impedance |Zin|. The output frequency is selected according toTable I (b) using the pseudo-random numbers, which are
generated by M-sequence.
From (1), the absolute value of the primary current iscalculated by (10) using (9).

(22
− Req2
Req2 ω 4 L4m + Z LC
ω 2 L2m − Z LC

I1 =(ω L − Z ) + R Z
2

2
m

2
LC

2

2
eq

2
LC

) 
2

(10)

V1

From (10), the input impedance |Zin| is derived as (11) when
the optimal load for the maximum efficiency is connected to
the load.

0.15
0.12

650 HzTABLE I.

0.09
0.06

Psuedo random
numbers

0.03
0
78

80

82

84

86

88

90

92

Selected frequency [kHz](a) In proportion to combined impedance of inductor and capacitor ZLC(conventional)

0.15

Probability

0.12
0.09
0.06
0.03
0
78

80

82

84

86

88

90

0.15
0.12
0.09
0.06
0.03
0
78

80

82

84

86

88

90

(c) In inverse proportion to input impedance Zin
Fig. 4. Probability distributions.

92

Psuedo random
numbers

Frequency [kHz]0000000000⁞
45⁞
000101101

46

000101110

80.00
80.65

⁞
82

⁞

80.65

001010010

83
⁞
111

Frequency [kHz]

80.00

168

010101000

85.47

80.00

⁞
189

⁞
010111101

85.47
85.47

190

010111110

86.21

80.65

⁞
220

⁞
011011100

86.21

001010011

81.30

221

011011101

86.96

⁞

81.30

001101111

81.30

⁞
260

⁞
100000100

86.96

112

001110000

81.97

261

100000101

87.72

⁞
132

⁞

81.97

010000100

81.97

⁞
309

⁞
100110101

87.72

133

010000101

82.64

310

100110110

88.50

⁞
144

⁞

82.64

010010000

82.64

⁞
367

⁞
101101111

88.50

145

010010001

83.33

368

101110000

89.29

⁞
148

⁞

83.33

010010100

83.33

⁞
434

⁞
110110010

89.29

149

010010101

84.03

435

110110011

90.09

⁞
153

⁞

84.03
84.03
84.75

⁞
111111110

90.09

154

010011001
010011010

⁞
510
511

111111111

-

⁞
167

⁞
010100111

84.75

92

(b) In proportion to input impedance Zin

Probability

ASSIGNMENT OF OUTPUT FREQUENCY.

(a) In proportion to combined impedance of inductor and capacitor ZLC(conventional)

86.21

86.96

87.72

88.50

89.29

90.09

84.75

(b) In proportion to input impedance Zin
Psuedo random
numberFrequency [kHz]Psuedo random
numbers

0000000000

80.00

Frequency [kHz]266

100001010

⁞
26

⁞

80.00

85.47

⁞
303

⁞
100101111

000011010

27

000011011

80.00
80.65

85.47

⁞
55

⁞

80.65

304

100110000

86.21

80.65

⁞
340

⁞
101010100

000110111

86.21

56
⁞
86

000111000

81.30

341

101010101

86.96

⁞

81.30

001010110

81.30

⁞
377

⁞
101111001

86.96

87

001010111

81.97

378

101111010

87.72

⁞
119

⁞

81.97

001110111

81.97

⁞
412

⁞
110011100

87.72

120

001111000

82.64

413

110011101

88.50

⁞
154

⁞

82.64

010011010

82.64

⁞
446

⁞
110111110

88.50

155

010011011

83.33

447

110111111

89.29

⁞
190

⁞

83.33

010111110

83.33

⁞
479

⁞
111011111

89.29

191

010111111

84.03

480

111100000

90.09

⁞
227

⁞

84.03
84.03
84.75

⁞
111111110

90.09

228

011100011
011100100

⁞
510
511

111111111

-

⁞
265

⁞
100001001

84.75
84.75

85.47

86.21

86.96

87.72

88.50

89.29

90.09

P suedo random
numbers

Psuedo random
numbers

Frequency [kHz]0000000000⁞
41⁞
000101001

42000101010

80.00
80.65

⁞
80

⁞

80.65

001010000

81

001010001

⁞
117
118
⁞
152

Frequency [kHz]

80.00

279

100010111

85.47

80.00

⁞
308

⁞
100110100

85.47

309

100110101

86.21⁞
101010011

86.21

80.65

⁞
339

81.30

340

101010100

86.96

⁞

81.30
81.30

⁞
101110011

86.96

001110101⁞
371

001110110

81.97

372

101110100

87.72

⁞

81.97
81.97

⁞
110010011

87.72

010011000

⁞
403

153

010011001

82.64

404

110010100

88.50

⁞
185

⁞

82.64
82.64

⁞
110110101

88.50

010111001

⁞
437

186

010111010

83.33

438

110110110

89.29

⁞
217

⁞

83.33
83.33

⁞
472

⁞
111011000

89.29

011011001

218

011011010

84.03

473

111011001

90.09

⁞
509

⁞
111111101

90.09

⁞

84.03

249

011111000
011111001

84.03
84.75

⁞
278

⁞
100010110

84.75

⁞
248

Input impedance |Zin| [ ](c) In inverse proportion to input impedance Zin

85.47

86.21

86.96

160
140
120
100
80
60
40
20
0
60

90.09

510

111111110-

111111111

-

4

4
m

80

2
LC

2
LC

2

2

2
eq

2
m

2
LC

2
LC

2
eq

2

100

110

Input DC voltage
Transmission distance

SPECIFICATION OF IPT SYSTEM.
Symbol

Value

Note

VDC1

400 V
150
mm
0.2

@150 mm (nominal position)

l

84.75

2
m

90

Frequency f [kHz]TABLE II.

89.29

511

(ω L − Z ) + R ZR ω L + Z (ω L − Z − R )
2
eq

70

Fig. 5. Input Impedance of the IPT system.

88.50

Primary inductance2

Calculation

87.72

Coupling coefficientZ in =

Frequency band Measurement
for IPT(11)

Figure 5 shows the measurement result and the calculation
result of the input impedance at the nominal position. It
confirms that the calculation result shows good agreement withthe measurement result.

k
L1

0.3
392 uH(nominal position)@100 mm@150 mm (nominal position)

422 uH@100 mm@150 mm (nominal position)@100 mm

Secondary inductanceL2

Primary capacitanceC1

401 uH428 uH
8.9 nF

Secondary capacitanceC2

8.8 nF

Ferrite plates

PC40 (TDK Corporation)

MOSFETs

SCH2080KEC (ROHM Co., Ltd.)

DiodesSCS220AE (ROHM Co., Ltd.)

Secondarycoil

C. Proposed Method II (Probability distribution inverse
proportional to Zin)
Figure 4(c) shows the probability distribution for the second
proposed method. The probability distribution is inverse
proportional to the input impedance Zin. The input impedance
can be calculated by (11) using the parameters at nominal
position. The output frequency is selected according to Table I(c) using the pseudo-random numbers.

FerriteplatesPrimarycoil

IV. EXPERIMENTS
A. Experimental Setup
Table II shows the specifications of the experimental setup.
In these experiments, a 400-V DC voltage source is directly
connected to the primary inverter for simplicity. Moreover, aresistance load is directly connected to the output of the fullbridge rectifier.
Figure 6 shows the transmission coils of the prototype. A
solenoid type is used because the solenoid type typically has
higher magnetic coupling in comparison with a circular coil in
return for large radiation noise.

Fig. 6. Primary and Secondary coils of prototype.

When the coils are placed in the nominal position,
magnetic coupling is k = 0.2. Assuming the change oftransmission distance from 150 to 100 mm, a magnetic
coupling is increased from 0.2 to 0.3. The change of thetransmission distance also causes a change of the primary andthe secondary inductance. It will cause the difference between
the resonance frequency and the transmitting frequency
because the resonance capacitor is designed to resonate with
the inductance at the nominal transmission distance.

Inverter output voltage 400 [V/div]00Inverter output current 10 [A/div]0

Output voltage 400 [V/div]

10 [ s/div](a) In proportion to ZLC (conventional)(b) In proportion to Zin (proposed method I)(c) In inverse proportion to Zin (proposed method II)(d) Constant frequency

Fig. 7. Operation waveforms without coil misalignment (3 kW) where Zin is the input impedance and ZLC is the combined impedance of the inductor
and capacitor.

(a) In proportion to ZLC (conv.) (b) In proportion to Zin (proposed) (c) In inverse proportion to Zin (proposed) (d) Constant frequency
Fig. 8. Radiation noise without misalignment when output power is 3 kW where Zin is the input impedance and ZLC is the combined impedance of theinductor and capacitor.

B. Operation without coil misalignment
Figure 7 shows the waveforms with a 3.3-kW IPT system.
Fig. 7 (a) is operation waveforms with the conventional spread
spectrum method. The probability distribution is biased inproportion to the combined impedance ZLC. Figs. 7 (b) and (c)are the waveforms with the proposed probability distributions.
The probability distributions are in proportion and inverse
proportion to the input impedance Zin. Fig. 7 (d) is the
waveforms when the inverter is operated at a constantfrequency (84.75 kHz). In Figs. 7 (a), (b) and (c), the output
frequency is changed within the range or 79-to-90 kHz at every
period according to the probability distribution. Due to thechange of an inverter output frequency, the amplitude of theinverter output current i1 changes.
Figure 8 shows the spectrum of the radiation emission
observed at 0.5 m from the edge of coils. The transmission

10095(c) Inverse proportion to Zin(d) Constant frequency

90(a) Proportion to ZLC(conventional)

85(b) Proportion to Zin

80
500

1000

1500

2000

Output power P [W]2500

Fig. 9. DC-to-DC efficiency withtout coil misalignment.

3000

(a) In proportion to ZLC (conventional) (b) Proportion to Zin (proposed method I)(c) Constant frequency

Fig. 10. Operation waveforms with a coil misalignment (1.9 kW) where Zin is the input impedance and ZLC is the combined impedance of theinductor and capacitor.

(a) In proportion to ZLC (conv.)(b) In proportion to Zin (proposed)(c) Constant frequency

Fig. 11. Radiation noise with a coil misalignment (1.9 kW) where Zin is the input impedance and ZLC is the combined impedance of the inductor andcapacitor.

coils are placed in the nominal position in this experiment.
When the output frequency of the inverter is constant (Fig. 7(d)), the sharp fundamental and low-order harmoniccomponents are observed. Using the biased distribution, whichis in proportion to ZLC, a maximum value at the fundamental
frequency is suppressed by 10.8 dBμA in comparison with theconstant frequency operation. The first proposed method using
proportional distribution to Zin suppresses the noise by 13.4
dBμA. In contrast, the second proposed method using inverse
proportional distribution suppresses the noise by 1.0 dBμA. It
can be understood from these results, the first proposed method
using the probability distribution, which is proportional to the
input impedance is the most effective to suppress the noise.
Fig. 9 shows the experimental results of the DC-to-DC
efficiencies without coil misalignment. In this paper, theefficiency from DC input to DC output is evaluated. The
maximum efficiency is 94.9% at an output power of 3.0 kW
when the inverter is operated at a constant frequency. The
efficiency of the first proposed method, which uses a
proportional distribution to Zin, is decreased from 94.9% to94.4% with at the same output power of 3.0 kW. It is
confirmed that the effect of the proposed noise reduction
method on the efficiency is small enough to be negligible atthe rated power. Furthermore, the efficiency of the first
proposed method is higher than the conventional spread
spectrum method in the light-load region.

100
95
90
85
80

0

500

1000

1500

2000

Fig. 12. DC-to-DC efficiency with a coil misalignment.

C. Operation with coil misalignment
Figure 10 shows the operation waveforms of the IPT
systems with a coil misalignment. The transmission distance ischanged from the nominal distance of 150 mm to 100 mm. Dueto the change of the transmission distance, the magnetic
coupling changes from 0.2 to 0.3. The output power is limited
to 1.9 kW owing to a mismatch of resonance parameters. The
proposed method I, which uses the probability distribution
being in proportion to |Zin|, is compared to the constantoperation and conventional spread spectrum method.
Regardless to the coil misalignment, the power can still be
supplied from the primary to the secondary side.

Figure 11 shows the radiation noise with the coil
misalignment. The peak of the radiation noise on thefundamental frequency is suppressed from 49.0 dBμA
(constant frequency) to 42.3 dBμA (the conventional method)
and 42.0 dBμA (the proposed method). The difference in theperformance of the proposed method and the conventional
method is small. Besides, when the coil misalignment occurs,
the reduction performance degrades in comparison with theoperation without the coil misalignment. It is confirmed that,
however, the spread spectrum is also effective to reduce theradiation noise even if the coil misalignment occurs.
Figure 12 shows the efficiency characteristics of the IPT
system with the coil misalignment. The maximum efficiencies
are 96.3%, 96.0%, and 96.0% at the rated power when the
inverter is operated with the constant frequency operation andtwo noise reduction methods. The use of the spread spectrum
decreases the DC-to-DC efficiency, however, the decrease in
the efficiency is only 0.3%. It is confirmed that the effect of the
proposed noise reduction method on the efficiency is smallenough to be negligible at the rated power.
V. CONCLUSION
This paper experimentally assessed spread spectrum
methods to reduce the radiation noise from the IPT system
considering the misalignment of the coils. By randomlychanging the transmission frequency of the inverter at random,
the radiation emission was spread in the frequency domain.
The output frequency was selected from the biased probability
distribution considering the frequency dependence of the
impedance. In this paper, two new biased distribution based on
the input impedance were proposed in addition to theconventional method, which determined the biased distribution
based on the combined impedance of the inductor and thecompensation capacitor. The noise reduction effect was
experimentally assessed. One of the proposed methods with thebiased probability distribution, which was in proportion to the
input impedance Zin, provide the highest effect on the noise
suppression on the fundamental and low-order harmoniccomponents with or without the coil misalignment. The peak of
the radiation noise measured at 0.5 m from the coil issuppressed from 52.9 dBμA to 39.5 dBμA. The proposed
method is also effective even if the misalignment of the coils
occurred. Moreover, the decrease of the maximum efficiency issmaller than the conventional spread spectrum method.
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