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Radiation Effects of Vest and Whip VHF-LB Antenna on Human Body A. Jafargholi, M. Kamyab Dept. Electrical Engineering K.N. Toosi University of Technology Tehran, Iran L. Farhoudi Antenna and EMI/EMC Lab. Shahid Beheshti Institute of Communication Tehran, Iran Abstract—In this paper we investigated the radiation effects of vest and whip VHF-LB antenna on human body. In order to simulate the human body we proposed two simple models. Measurements results confirm the simulations. I. INTRODUCTION All life on Earth has adapted to survive in an environment of weak, natural low frequency electromagnetic fields (in addition to the Earth’s static geomagnetic field). Natural low-frequency EM fields come from two main sources: the sun, and thunderstorm activity. But in the last 100 years, man-made fields at much higher intensities and with a very different spectral distribution have altered this natural EM background in ways that are not yet fully understood. Much more research is needed to assess the biological effects of EMR. Wearable antennas have recently received growing interest due to the introduction of personal communications technology [1-4]. In the future, e.g. clothing may have a variety of consumer. Antennas play a paramount role in an optimal design of the wearable or hand-held units used in personal electronics. This has resulted in demand for flexible fabric antennas, which can be easily attached to a piece of clothing. The textile antennas can be constructed using ordinary fabric as dielectric and e.g. conductive tape or fabric as conducting elements. This enables the antenna to be flexible and lightweight. Hence, the proposed antenna structure is easy to attach to clothing and the structure does not limit the possible antenna placements. Conventionally, antennas that are rigid and massive have limited amount of places they can be attached to without being uncomfortable to the user. Some textile antennas have already been introduced [4, 5]. However, in these antennas the focus has been in constructing an antenna that works properly. In [5] a new VHF-LB vest antenna is introduced. This antenna configuration is so flexible and very lightweight that the user does not feel any change in her/his cloth condition. In this paper first we review SAR definition as an important factor in our investigation. In section three we compared the simulation results for both vest and whip antenna and finally we proposed two EM models for human body in section four. II. EM EFFECTS ON HUMAN BODY AND SAR (SPECIFIC ABSORPTION RATE) It has been known since the early days of radio that RF energy can cause injuries by heating body tissue. In extreme cases, RF-induced heating can cause blindness, sterility and other serious health problems. These heat-related health hazards are called thermal effects. In addition, there is evidence that magnetic fields may produce biologic effects at energy levels too low to cause body heating. The proposition that these thermal effects may produce harmful health consequences has produced a great deal of research. Body size also determines the frequency at which most RF energy is absorbed. For example a baby’s smaller head resonates near 700 MHz. Over the resonance frequency, less RF heating generally occurs. However, additional longitudinal resonances occur at about 1 GHz near the body surface. In addition to intensity, the frequency of an electromagnetic wave can be important to determine how much energy will be absorbed and, therefore, the potential for harm. The quantity used to characterize the rate of absorption is called "Specific Absorption Rate" or "SAR," and usually expressed in units of W/kg. In the far-field (several wavelengths distance from the source) whole-body absorption of RF energy by an adult standing human has been shown to occur at a maximum rate in the frequency range of 70-100 MHz, depending on the size, shape and height of the individual. This called resonance frequency. The amount of Resonance is a natural phenomenon and one well-known example is what happens when you strike a tuning fork. It has been tuned to resonate at a particular frequency, for musical purposes, no matter how hard it is hit. But any nearby object that resonates naturally at the same frequency will also start to vibrate in sympathy with the tuning fork. This will be heard as a humming sound. 1-4244-1039-8/07/$25.00 ©2007 IEEE 1

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Page 1: 04216965

Radiation Effects of Vest and Whip VHF-LB Antenna on Human Body

A. Jafargholi, M. Kamyab Dept. Electrical Engineering

K.N. Toosi University of Technology Tehran, Iran

L. Farhoudi Antenna and EMI/EMC Lab.

Shahid Beheshti Institute of Communication Tehran, Iran

Abstract—In this paper we investigated the radiation effects of vest and whip VHF-LB antenna on human body. In order to simulate the human body we proposed two simple models. Measurements results confirm the simulations.

I. INTRODUCTION All life on Earth has adapted to survive in an

environment of weak, natural low frequency electromagnetic fields (in addition to the Earth’s static geomagnetic field). Natural low-frequency EM fields come from two main sources: the sun, and thunderstorm activity. But in the last 100 years, man-made fields at much higher intensities and with a very different spectral distribution have altered this natural EM background in ways that are not yet fully understood. Much more research is needed to assess the biological effects of EMR.

Wearable antennas have recently received growing interest due to the introduction of personal communications technology [1-4]. In the future, e.g. clothing may have a variety of consumer. Antennas play a paramount role in an optimal design of the wearable or hand-held units used in personal electronics. This has resulted in demand for flexible fabric antennas, which can be easily attached to a piece of clothing. The textile antennas can be constructed using ordinary fabric as dielectric and e.g. conductive tape or fabric as conducting elements. This enables the antenna to be flexible and lightweight.

Hence, the proposed antenna structure is easy to attach to clothing and the structure does not limit the possible antenna placements. Conventionally, antennas that are rigid and massive have limited amount of places they can be attached to without being uncomfortable to the user. Some textile antennas have already been introduced [4, 5]. However, in these antennas the focus has been in constructing an antenna that works properly. In [5] a new VHF-LB vest antenna is introduced. This antenna configuration is so flexible and very lightweight that the user does not feel any change in her/his cloth condition.

In this paper first we review SAR definition as an important factor in our investigation. In section three we compared the simulation results for both vest and whip antenna and finally we proposed two EM models for human body in section four.

II. EM EFFECTS ON HUMAN BODY AND SAR (SPECIFIC ABSORPTION RATE)

It has been known since the early days of radio that RF energy can cause injuries by heating body tissue. In extreme cases, RF-induced heating can cause blindness, sterility and other serious health problems. These heat-related health hazards are called thermal effects. In addition, there is evidence that magnetic fields may produce biologic effects at energy levels too low to cause body heating. The proposition that these thermal effects may produce harmful health consequences has produced a great deal of research. Body size also determines the frequency at which most RF energy is absorbed. For example a baby’s smaller head resonates near 700 MHz. Over the resonance frequency, less RF heating generally occurs. However, additional longitudinal resonances occur at about 1 GHz near the body surface.

In addition to intensity, the frequency of an electromagnetic wave can be important to determine how much energy will be absorbed and, therefore, the potential for harm. The quantity used to characterize the rate of absorption is called "Specific Absorption Rate" or "SAR," and usually expressed in units of W/kg. In the far-field (several wavelengths distance from the source) whole-body absorption of RF energy by an adult standing human has been shown to occur at a maximum rate in the frequency range of 70-100 MHz, depending on the size, shape and height of the individual. This called resonance frequency. The amount of Resonance is a natural phenomenon and one well-known example is what happens when you strike a tuning fork. It has been tuned to resonate at a particular frequency, for musical purposes, no matter how hard it is hit. But any nearby object that resonates naturally at the same frequency will also start to vibrate in sympathy with the tuning fork. This will be heard as a humming sound.

1-4244-1039-8/07/$25.00 ©2007 IEEE 1

Page 2: 04216965

The principle applies to wine glasses, cells, bones in fact everything under the sun has its own resonant frequency. The human body will also resonate. When the height of a body is about half a wavelength, energy is absorbed maximally, the body acting as a half wave dipole antenna. The peak absorption for an adult man is about 70 MHz (VHF band).

But the head itself resonates at a much higher frequency, about 1 GHz (microwave communication band). At higher frequencies still, the basic mechanism of interaction is molecular rotation when the polar charges are separated in space. Such polar molecules tend to align themselves in the electric field and, as this oscillates, the molecules tend to follow these oscillations; in the process, this energy is dissipated as heat - this is the basis of microwave cooking.

In the other hand, SAR is at maximum level under these conditions. Because of the resonance phenomenon, safety standards have taken account of the frequency dependence of whole-body human absorption, and the most restrictive limits on exposure are found in this frequency range (the very high frequency or "VHF" frequency range). SAR is a quantity that describes the amount of absorbed radiated effect for a specific material at a certain frequency. The quantity SAR is a quantity that describes the amount of absorbed radiated effect for a specific material at a certain frequency. It can be derived from either the temperature gain or electric field.

The method used for measurements derives the radiated effect from the electric field since the difference in temperature is too small to measure in the cellular phone frequency band due to the low energies involved at these low frequencies (i.e. non-ionizing radiation). SAR has become the most frequently used quantity involved when health issues are discussed. According to the ANSI/IEEE (American National Standard Institute/Institute of Electrical and Electronics Engineers) standard the maximum SAR averaged over 1 g should not exceed 1.6 W/kg and that the whole body mass averaged SAR should not exceed 0.08 W/kg. ICNIRP’s guidelines for SAR in the head average over a 10 gr cube should not exceed 2 W/kg. As define

3

EPtTcSAR abs

ρσ

ρ==

∂∂= (1)

Where

tT

∂∂ - Changes in heat over time [degreeK /s]

c - Specific heat capacity [J(kg degreeK)-1]

σ - Conductivity [S/m]

ρ - Density [kg/m3]

E - Electric field strength [V/m]

Pabs - Absorbed power within the 1 or 10 gr cube [W]

III. HUMAN BODY MODEL We simulate the body by two models, namely cube and

complete models. In cube body model we assume that the human head is composed from skull, brain and eyes. Hands and legs are composed from muscle and bone and finally the body has been constructed from muscle, bone, heart, liver, kidney, lung, stomach and water, etc. In table 1, the specification of proposed human body models are presented. In this table we present the electromagnetic parameters (permittivity and conductivity), weight and volume of each model organs at VHF-LB frequencies. In tables 1-3 we proposed characteristics of two models that have been used for simulations. By using tables 1 and 2, we obtained the complete model properties and by using tables 1 and 3 the cube body model characteristics have been derived.

TABLE I. HUMAN BODY EM PROPERTIES

Organ Size Mass ε

400 MHz

σ 400

MHz

% of human weight

Brain 1600 cm3 1-1.5 kg 49.7 0.59 2%

Heart 12×8-9×6 cm3 250-350 gr 66 0.97 0.5% Kidney 12×5×6 cm3 150 gr 66.4 1.1 0.5%

Liver 20-22.5×15-17.5×10-12.5

cm3 1.3-3 kg 51.2 0.65 3.3%

Lung 25×30×5 cm3 625 gr (R), 567 gr (L) 54.6 0.68 1.8%

Stomach 50 mlit – 1 lit 480-520 gr 67.5 1.00 0.8% Eyes - 50 gr 57.7 1.00 0.1%

Muscle - 6-7 kg 58.8 0.84 10% Skull - 1.8-2.4 kg 17.8 0.16 3%

Intestine - 1.8-2.4 kg 66.1 1.9 3% Bone - 5-7 kg 13.1 0.09 8.3%

Water - - 81 0.001 66%

TABLE II. COMPLETE BODY MODEL

Permittivity Conductivity Body Part

% of human weight VHF-LB 400 MHz VHF-LB 400 MHz

Bone 66.6

Han

ds

Muscle 33.3 31.47 0.3867

Bone 25

Leg

s

Muscle 75

67.7 48.55

0.75 0.67

Body Table 3 65 66.4 0.75 1.1

Bone 45

Brain 45

Hea

d

Eye 10

60 44.15 0.6 0.4375

TABLE III. CUBE BODY MODEL

Permittivity Conductivity Body Part

% of human weight VHF-LB 400 MHz VHF-LB 400 MHz

Muscle 45

Bod

y

Bone 55 55 36.25 0.525 0.456

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Page 3: 04216965

IV. VEST AND WHIP SIMULATION COMPARISON A rigid whip antenna has been transformed into a

wearable, flexible, textile based antenna. It has advantages over the 80 cm whip antenna in that it is body conformal and visually covert, not compromising the soldier’s silhouette. Preliminary evaluation performance and safety of the vest antenna has been tested at the Shahid Beheshti Institute of Communication, Tehran, Iran, and it had good results.

In figure 1, two designed vest antennas for VHF-LB are shown. As it shown in this figure the strip vest antenna composed from two separate loops and a shield in back and underneath protect the body form the radiation hazards.

In order to have a good comprehension about these designs and the whip antenna’s performance we simulate these antennas in VHF-LB frequency with HFSS simulator.

Simulation shows, figure 2, that in vest antenna the maximum SAR is located at the bottom of the body and it is so far from the head and in comparison with the whip antenna SAR decreases about 5-14 times.

The vest antenna was tested successfully in some different positions of body including standing, kneeling, and prone. Received signal levels are shown in figure 3. Same as simulations results the vest antenna’s gain is 5 dB lower than the whip antenna’s gain.

To compare antennas’ safety we have checked specific absorbency rates and near H filed in simulation and our prototype. In table 4, measurement of near H field and simulation results for both whip and vest antenna is shown. As it shown in this table, the intensity of whip antenna’s near filed and therefore its effect on human body is more than vest antenna.

Simulation and measurement results show that the proposed human body models are suitable but it should be mentioned that in cube model length of legs has not been considered.

Furthermore in whip antenna maximum SAR has occurred in head but in vest antenna, the simulations show that maximum level of absorption has occurred in bottom and we can decrease its effects by using a shield plate. In order to measure the near H filed, we used R&S’s near filed probes and EMI receiver.

The transmitter in both tests was near the user’s right ear. Antenna’s feed in whip case is near the user’s head but in vest antenna, feed’s location is near the bottom.

As it shown in table 5, in 3000 azimuth there is -4 to -8 dB attenuation this result shows the effect of legs on the radiation pattern. With cube body model we will have a perfect omni directional radiation pattern but in presence of legs, when frequency increases to the body resonance frequency (70 MHz) this effect became more clear.

Figure 1. Vest (left) and whip (right) antenna simulation

Figure 2. SAR simulation results for vest (Up) and whip (Down) antenna

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Page 4: 04216965

TABLE IV. NEAR FIELD H, SIMULATION AND MEASUREMENT RESULTS FOR VEST AND WHIP ANTENNA

30 35 40 45 50 55 60 65 70 75 80-80

-75

-70

-65

-60

-55

-50

Frequency [MHz]

Rec

eive

d P

ower

[dB

w]

Received Power Comparison For Whip and Vest Antenna

Whip AntennaVest Antenna

Figure 3. Antenna transmitting measurement test level

TABLE V. DESIGNED VEST ANTENNAS COMPARISON

Parameters Strip Vest Antenna

Jacket Antenna

Whip Antenna

Gain -1.6 to -6 dBi -4 to -8 dBi 1 to -3 dBi

Emag 7.76 V/m 14.88 V/m 37.7 V/m

Pattern -7 dB in 3000 -4 dB in 3000 -8 dB in 3000

Maximum Average SAR 0.027 W/Kg 0.076 W/Kg 0.378 W/Kg

V. CONCLUSION In this paper we investigated the radiation effects of vest

and whip VHF-LB antenna on human body. In order to simulate the human body we proposed two simple models. Measurements results confirm the simulations. Received signal levels in measurements, show same results as simulations that the vest antenna’s gain is 5 dB lower than

the whip antenna’s gain. Simulations and measurements results show that the intensity of whip antenna’s near filed and therefore its effect on human body is more than vest antenna.

REFERENCES [1] P. Salonen, L. Sydanheimo, M. Keskilammi, M. Kivikoski, Planar

inverted-F antenna for wearable applications, IEEE 3th International Symposium on Wearable Computers, San Francisco, CA, USA, 1999,95-100.

[2] P. J. Massey, Mobile phone fabric antennas integrated within clothing, IEE 11th International Conference on Antennas and Propagation, 2001,344-347.

[3] Salonen, P., Hurme, H., A Novel Fabric WLAN Antenna for Wearable Applications, 2003 IEEE Antennas and Propagation Society International Symposium, Columbus, Ohio, USA, vol. 2, pp. 700 - 703,2003.

[4] M. Tanaka, J.-H. Jang, Wearable Microstrip Antenna, 2003 IEEE Antennas and Propagation Society Intemational Symposium, Columbus, Ohio, USA, vol. 2, pp. 704 - 707, 2003.

Standard value

Simulation Results H (A/m)

Measurement Results H

(dBm)

(A/m) (dBm) Vest Whip Vest Whip

Measurement Points

0.16 -15.9 0.09 0.49 -19 -13 Head (right)

0.16 -15.9 0.09 0.066 -32 -14 Head (left)

0.16 -15.9 0.016 0.054 -30 -16 Head (front)

0.16 -15.9 0.012 0.064 -34 -18 Head (back)

0.16 -15.9 0.011 0.028 -31 -14 Near the Heart

0.16 -15.9 0.028 0.028 -26 -14 Front of Body

0.16 -15.9 0.028 0.028 -45 -19 Back of Body

0.16 -15.9 0.4 0.49 -14 -12 Near the Antenna Feed

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