A 430MHz Band Receiving Antenna for Microwave Power Transmission to Capsular Endoscope

T.Kumagai1, K.Saito

2, M.Takahashi

2, K.Ito

1

1Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

t.kuma@graduate.chiba-u.jp

ito.koichi@faculty.chiba-u.jp

2Research Center for Frontier Medical Engineering, Chiba University,

1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

kazuyuki_saito@faculty.chiba-u.jp

omei@faculty.chiba-u.jp

Abstract

Wireless power transmission is being investigated as a means to operate tiny medical devices such as the

capsular endoscope, which is able to exist for a long period during diagnostic procedures within the body. In this paper,

we examine the wireless power transmission to a capsular endoscope by microwaves to show its usability for medical

applications. A modified helical antenna inside the endoscope is proposed as a power receiving antenna, operating at

430 MHz band. By calculating a maximum received power in the stomach using such antenna, the results show that

adequate power can be well received.

1. Introduction

Wireless power transmission has been investigated as a mean to operate tiny medical equipments such as

capsular endoscope that exists for a long time in the body [1]. Capsular endoscope is non invasive and less

uncomfortable to patients, which differs from the existing wired endoscope. Hence, it is attracting attention in recent

years. Currently, a small battery inside the capsule is employed to operate the device. It is, however, risky to put such a

harmful material into the human body. In addition, only the power of battery is insufficient to maintain the high

performance.

This paper presents wireless power transmission to a capsular endoscope by using the microwave method. Such

a method is usually used in the SPS (Space Solar Power Satellite/station) plan [2] and the EV (Electric Vehicle) [3]. In

the microwave method, the performance of the device called the rectenna (rectifying antenna) that consists of an

antenna and rectifying circuit is very important. Summarized in this paper is the design for a power receiving antenna

that operates at 433.92 MHz one of the ISM (Industry Science Medical) bands. In order to obtain maximum received

power through the antenna, the antenna characteristics are numerically analyzed by use of the Finite-Difference Time-

Domain (FDTD) method. In terms of the operating frequency, 315-600 MHz band is preferred due to its transmission

characteristics and safety to the human body [4]. For this reason, 433.92 MHz is selected as operating frequency.

Capsular endoscope

26 mm

11 mm

Lens

Power

Information

(Image)

Power sourceExternal antenna

Capsular endoscope

Figure 1. Wireless power transmission system for capsular endoscope.

978-1-4244-6051-9/11/$26.00 ©2011 IEEE

2. Design of Antenna

The structure of the designed antenna (Figure 2) is shown; the antenna is assumed to be inserted into a capsular

endoscope, hence, a folded-helical sheet antenna with a diameter of 9.5 mm and height of 23 mm is designed. We

selected a folded-type antenna because the input impedance of the normal helical antenna is low, which creates

difficulty in impedance matching. By our design, one end of the antenna was shortened, while the other was opened.

This structure makes it possible to be matched at the resonant frequency. The size of the antenna is linked for the

capsular endoscope, which is currently produced by several manufacturers [5-7]. The size of the endoscope antenna is

26 mm in length and 11mm in diameter (Figure 1). The width of the sheet is 0.4 mm, and a feeding point is set at the

center of the antenna.

The input characteristics in terms of the reflection coefficient (S11) and the input impedance are numerically

analyzed. As shown in Figures 3 and 4, at 433.92 MHz of the desired operating frequency, the reflection coefficient was

-12.2 dB and the input impedance was 39.0 Ω and 14.5 Ω for resistance and reactance, respectively. The radiation

pattern is omnidirectional in x-z plane.

Glass

(εr=5.5)

Conductor

x

y

z

xy

z

9.5

x

y

z

23

Open

Short

Unit : mm

(a) Cubic view I (c) Top view(b) Cubic view II

0.4

Figure 2. Structure of receiving antenna.

-50

0

50

100

420 430 440 450

Inp

ut i

mp

edan

ce [Ω

]

Frequency [MHz]

Re.

Im.

-25

-20

-15

-10

-5

0

420 430 440 450

Ref

lect

ion

co

effi

cien

t[d

B]

Frequency [MHz]

Figure 3. Input impedance. Figure 4. Reflection coefficient.

20 20

100

100

(a) Side view (b) Top view

Air space

9 dipole antennas

Unit : mm

50

Conductor reflector

x

y

z

d

Air space

Muscle(εr=56.9, σ=0.81 S/m)

40

100

Receiving antenna

x

z

y

20 2

Figure 5. Calculation model.

3. Calculation Model

The maximum received power, which is obtained from the designed receiving antenna, is numerically calculated.

The calculation model is show in Figure 5, assumed to be the power transmission to a capsular endoscope in the

intestines. Two rectangular parallelepipeds are modeled as a human body. The size of the human body is infinite in y

and z-axes directions. There is an air space d mm from the body surface and the receiving antenna is placed at 2 mm

from the back of the human body. In addition, the electric constants used for the human body model is the value

(relative permittivity εr: 56.9 and conductivity σ: 0.81 S/m) which is the muscle's electric constant at 433.92 MHz and

body density is 1040 kg/m3.

Multiple dipole antennas are used as transmitting antennas and placed at 20 mm from the human body surface.

When electromagnetic waves are radiated to the human body, radiation exposure to the human body must be considered.

An evaluation standard for electromagnetic wave exposure called Specific Absorption Rate [W/kg] (SAR) is defined by

the following formula:

SAR = σ

ρ E

2 [W/kg] (1)

where, σ is the conductivity of human tissue [S/m], ρ is density of human tissue [kg/m3], and E is the electrical field

intensity (effective value) [V/m]. The standard limitations for average SAR per arbitrary 10g of tissues and whole-body

are defined by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) [8]. In the frequency

range from 10 MHz to 10 GHz, under an environment where the exposure to electromagnetic waves can be managed,

the limit value is 10 W/kg; while in a general public environment, the limit value is 2 W/kg. In this paper, we apply the

managed environment for SAR evaluation.

The radiated power from transmitting antenna could be higher by reducing SAR value. When one antenna is

simply used as a transmitting antenna, the SAR becomes higher due to the concentration of electromagnetic waves.

Hence 9 dipole antennas are employed to avoid it. The infinite conductor plate is set at 50 mm behind the transmitting

antenna to prevent leakage of the power radiated from the transmitting antenna to the exterior.

4. Result

Calculation results are summarized in Table 1. The radiated powers from transmitting antenna come from the

values when 10g-averaged maximum SAR which is defined by ICNIRP becomes 10 W/kg that is the limitation value.

We can see that the radiated powers increase according to the thickness of the body surface (d). It owes to the diffusion

of electromagnetic waves in large area and decreasing of the SAR. In contrast, the transmission efficiency calculated

from the radiated power of the transmitting antenna and the received power obtained by receiving antenna decrease by

increasing of d. Therefore, the results indicate that an inverse relationship between the radiated powers from

transmitting antenna and the transmission efficiency.

From these two results, the received powers are calculated. The results show that more than 110 mW can be

obtained by designed receiving antenna in each cases. It seems reasonable that these results are adequate values for

received power even though the rectification efficiency is another issue that should be considered [9].

Table 1. Calculation results.

d Radiated power from

transmitting antenna Received power

Transmission

efficiency

15 mm 2.63 W 112 mW 4.3 %

25 mm 4.72 W 113 mW 2.4 %

35 mm 7.81 W 126 mW 1.6 %

5. Conclusion

In this paper, we proposed a receiving antenna that can be inserted on a capsular endoscope for wireless power

transmission, and the received powers obtained by the designed antenna have been numerically analyzed. The results

show us that adequate power can generally be well received, thus the wireless power transmission by the microwave

method is a viable application. In the future, fabrication of the designed antenna and experimental testing will be

conducted to prove the validity of the calculation results.

6. References

1. Q. Niu, L. Wang, T. Dong, and H. Yang, “Application of a MEMS-based energy harvester for artificial heart wireless

energy transmission,” Proceedings of 2009, ISECS International Colloquium on CCCM, vol.1, pp.38–41, Aug.2009

2. C. T. Rodenbeck, M. Y. Li, and K. Chang, “A Phased-Array Architecture for Retrodirective Microwave Power

Transmission from the Space Solar Power Satellite,” Microwave Symposium, vol.3, pp.1679–1682, Jun.2004

3. N. Shinohara and H. Matsumoto. “Wireless Charging System by Microwave Power Transmission for Electric Motor

Vehicles,” the Institute of Electronics, Information and Communication Engineers (IEICE) Transaction on Electronics,

vol.J87-C, no.5, pp.433–443, Mar.2004

4. E. Murakuni, N.Higaki, and K Shiba, “Analysis of Human Body Electric Field Distribution Using the Transmitting

Antenna in an Implantable Device,” ITE Technical Report, vol.33, no.15, pp.13–18, Mar.2009, in Japanese.

5. Given Imaging Homepage, “PillCam SB,”

http://www.givenimaging.com/en-us/HealthCareProfessionals/Pages/PillCamSB.aspx.

6. Olympus Medical Systems Corp．Homepage, “Endo Capsule,”

http://www.onaka-kenko.com/endoscope-closeup/endoscope-technology/et_06.html.

7. RF SYSTEM lab. Homepage, “Sayaka,” http://www.rfsystemlab.com/sayaka/index.html.

8. ICNIRP, "Guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields (up to 300

GHz),” Health Phys., vol. 74, no. 4, pp.494–522, Apr. 1998.

9. T. Kotani, “Electronics pioneers the evolution of capsular endoscope,” Nikkei electronics, No.994, 2008.12.29,

pp.67-69, in Japanese.