Milimeterwave Patch Antenna

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TECHNOLOGY FORCES (Technol. forces): Journal of Engineering and Sciences January-June 2010

Abstract In this paper we present millimeterwave

patch antenna designs for medical implants. The

designs are based on the transmission line model and

are simulated in CST. We first present a 31.5GHz

patch antenna design based on a RT6002 substrate (

=2.94 ). The antenna has a small form factor and

exhibits a high return loss and directivity. However,

it does suffer from high attenuation loss and cannot

be used for deep tissue implants. We next propose a

5.85 GHz design that is based on a super-high

permittivity substrate ( = 80). This design retains

the small size of the 31.5GHz antenna but greatly

improves the link budget. This improvement can result

in greater data bandwidth or higher penetration depth

within the body. Link budget analysis has shown that

even under the worst case scenario data rates of up to

51.43kbits/sec can be achieved over a 25kHz channel.

I - INTRODUCTIONhe design of implanted antennas has received

considerable attention from the research community.

The design of these antennas is quite challenging,

as there is a limit on the amount on power that can

be transmitted as well as on the size of these devices. The

limitation on the transmit power is due to the amount of

battery power available as well as due to concerns about

exposure to electromagnetic radiation.

A number of different antenna designs have been

considered for medical implants [1]-[4]. In [2] spiral and

serpentine antenna designs have been considered and the

authors have simulated the performance of these antennasusing a single block of muscle and a realistic human

shoulder. The results are also verified experimentally

using a tissue simulant material composed of TX-151,

sugar, salt and water. A similar analysis is performed in

[3] for spiral and planar inverted-F (PIFA) antenna.

However, the authors have primarily focused on the human

brain using a six-layer model (brain, CSF, Dura, bone, fat

and skin). In both these papers the authors have considered

a frequency of 402-405 MHz that has been recommended

by the European Radio Communications Committee (ERC)

for ultra-low-power, active, medical implants.

II ANTENNA DESIGNAt 402MHz the wavelength of an electromagnetic wave

is about 0.75m. It is obvious that any antenna with dimensions

comparable to this wavelength cannot be used for an implant.

The technique usually used to overcome this problem is todesign a conducting surface that is spiraled along the surface

of the substrate. The resonant frequency of the microstrip is

then proportional to the total length of the spiral and not to

the length of any individual element. Although this results

in a size reduction but its still not quite suitable for an implant

(see table 2). We have investigated the idea of using a

rectangular patch antenna designed to operate at millimeter

wave frequencies ( l


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we have considered.

III - SIMULATION RESULTSThe antenna is placed within a three-layer body consisting

of skin, fat and air and energized by a waveguide port

with a normalized power of 1WF . We used air as the

cavity around the antenna; however, in practice, someother non-conducting medium might be used.

Fig 1. Patch implanted inside the model

(4mm Skin, 4mm Fat, 1mm cavity).

Tissue Permittivity Conductivity (S/m)

5.85G Hz 31.5G Hz 5.8 5GHz 31 .5G Hz

Skin 35.0720 14.7980 3.7602 27.9710

Fat 4.9503 3.5970 0.2964 1.8636

Muscle 48.4180 22.2700 5.0212 36.8130

Table 1: Dielectric properties of human tissue at

5.85GHz and 31.5GHz.

(a) 31.5GHz Design (er =2.94,tand=0.0012 )

For an isotropic radiator with a transmit power of 1W the

power density at a distance of 1m is

and for the embedded patch with a gain of -46.5dB the

power density would be reduced to

which is equal to the result obtained through p-field

simulation (Fig 4). Now if the effective area Ae of the

antenna is known then the received power Prat a particular

distance can be easily calculated. The effective area [7]

of a half wave dipole is given as

where Gris the receive antenna gain. This gives a received

power of -106.95dBW or -76.95dBm.

In the absence of the body the antenna has a power density

of -3.63dBW/m2

at a distance of 1m. Therefore there is

a power loss of 53.87dB within the body. Similar results

are obtained for the reverse link using ray-tracing where

a 52.40dB loss is observed within the layers of skin and

fat (Fig 3). However, it must be noted that, the wireless

communication channel between an implant and an external

device is not symmetric and the link budget in one direction

might be quite different from that in the other direction.

Fig 2. Return loss of the antenna with and without the

body.

Fig 3. Relative Electric field strength 20log10 (/E10lEol) .

External device to implant communication.

Fig 5. P-field at 5.85GHz (a) H-plane (b) E-plane.


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Fig 5. P-field at 5.85GHz (a) H-plane (b) E-plane.

(b) 5.85 GHz Design ( er =80.0,tand=8.6x10-4

)

We next investigated the idea of using a super high

permittivity (SHP) substrate that reduces the size of the

patch at the expense of radiation efficiency. The material

that we have used is ADT80 which was originally

fabricated for hyperthermia applications [9]. It was found

that with this substrate the patch dimensions at a frequency

of 5.85GHz would be quite comparable to that of the

31.5GHz antenna with RT6002 as the substrate. Simulation

results have shown that there are two basic drawbacks of

using this technique.

1. The patch has very low radiation efficiency (13.96%).

2. The patch has low directivity (2.01dBi).

Furthermore when implanted within the body the patch

does not have a p-field maximum along the broadside.

However, as expected, there is lesser attenuation within

the body (10.50dB).

It is observed that there is a gain of 25.10dB over the

31.5GHz patch antenna in the forward direction. The

embedded patch has a return loss of more than 15dB and

experiences a frequency detuning of less than 1%. Thereceived power for a half wave dipole is 67.18dBW - .

IV-LINK BUDGET ANALYSISLet us consider the worst case scenario where the signal

power is -76.90dBm. We keep an additional 10dB fade

margin (ITU-R recommendations) giving us an average

received power of -86.90dBm. The noise power can be

calculated as

N = kTB (5)

where k is the Boltzman constant, T is the equivalent

noise temperature in degree Kelvin and B is the bandwidthin Hz. At a standard room temperature of 20C the noise

power for a 25kHz channel is calculated as -129.95dBm.

According to ITU-R recommendations a 20dB margin is

added to account for other sources of interference resulting

in a noise floor of -109.95dBm. This gives us a signal to

noise ratio of approximately 23dB. If we take the SAR

requirements into consideration then the signal to noise

ratio is reduced to about 5dB.

We can now calculate the capacity of the 25kHz channel as

C = Blog2 (1=SNR) (6)

This gives us a theoretical maximum data rate of

51.43kbits/sec. It seems that this should be sufficient for

most telemetering applications like exercise EEG diagnosis

which requires 40kbps (4k sample rate and 10 bits

resolution). It is quite obvious that any additional

data sources can be accommodated by increasing the

bandwidth of the channel or by improving the SNR.

V - CONCLUSIONThe 31.5GHz patch design is quite attractive in terms of

its size and radiation characteristics. However, the high

attenuation loss within the body reduces the link budgetand makes it unsuitable for deep tissue implants. The link

budget can be greatly improved by using an SHP substrate.

This improvement can result in greater data bandwidth

or higher penetration depth within the body. It can also

be used to reduce the transmit power to extremely low

levels thus satisfying the most stringent SAR requirements.

Finally, it must be noted that the return loss of the antenna

increases inside the body and there is also some frequency

detuning. Higher return loss is a desirable characteristic

but frequency detuning is not and can be removed by

adjusting the dimensions of the antenna such that the null

occurs at the desired frequency.

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REFERENCES

[1] Y. Ahmed, Y. Hao and C. Pirini, A 31.5GHz Patch

Antenna Design for Medical Implants, Special Issue

of

International Journal on Antennas and Propagation, Sep.2008.

[2] P. Soontornpipit, C. M. Furse, and Y. C. Chung, Design

of Implantable Microstrip Antennas for Communication

with Medical Implants, IEEE Transactions on

Microwave Theory and Techniques, vol. 52, no. 8,

pp.1944-1951, Aug. 2004.

[3] J. Kim, and Y. R. Samii, Implanted Antennas Inside

a Human Body: Simulations, Designs, and

Characterizations, IEEE Transactions on Microwave

Theory and Techniques, vol. 52, no. 8, pp. 1934-1943,

Aug. 2004.

[4] M. Norris, and J. Richard, Sub-Miniature Antenna

Design for Wireless Implants, Proceedings of the IETSeminar on Antennas and Propagation for Body-Centric

Wireless Communications, pp. 57-62, April 2007.

[5] H. Higgins, Body Implant Communications Is it a

Reality?, Proceedings of the IET Seminar on Antennas

and Propagation for Body-Centric Wireless

Communications, pp. 33-36, April 2007.

[6] C. A. Balanis, Antenna Theory: Analysis and Design,

John Wiley, 2nd Ed., 1997. [6] D. M. Pozar, Microstrip

Antennas, Proceedings of the IEEE, vol. 80, no. 1,

Jan. 1992.

[7] J. D. Kraus, Electromagnetics, McGraw-Hill, 3rd Ed.,

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[8] M. Okoniewski, and M. A. Stuchly, A Study of the

Handset Antenna and Human Body Interaction, IEEE

Transactions on Microwave Theory and Techniques,vol. 44, no. 10, pp. 1855-1864, Oct. 1996.

[9] D. Andreuccetti, M. Bini, A. Ignesti, R. Olmi, S. Priori

and R. Vanni, High Permittivity Patch Radiator for

Single and Multi-Element Hyperthermia Applicators,

IEEE Transactions on Biomedical Engineering, vol.

40, no. 7, July 1993.

[10] C. M. Hsu, T. C. Yo, C. M. Lee and C. H. Luo, The

Low Power Biotelemetry Architecture for Implantable

Applications, IEEE APS Symposium, pp. 3193-3196,

July 2007.

f For the 10gm tissue that we have used the transmit power

should be less than 16mW (1.6W/kg x0.010kg). This wouldreduce our link budget by approximately 18dB.

The formula given in the text uses directivity instead of

gain;

however, this is only valid if there is no loss in radiation.

CST (Computer Simulation Technology) considers the

antenna and

the body as the radiator. Therefore, the gain is the gain of

the complete

structure and not just the designed antenna.

Table 2. Comparison of six different antennas designed for medical implants. The size of our antenna is

governed by the dimensions of the ground plane.

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Milimeterwave Patch Antenna