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7/29/2019 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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TECHNOLOGY FORCES (Technol. forces): Journal of Engineering and Sciences January-June 2010
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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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TECHNOLOGY FORCES (Technol. forces): Journal of Engineering and Sciences January-June 2010
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.
33
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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
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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
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[6] C. A. Balanis, Antenna Theory: Analysis and Design,
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Antennas, Proceedings of the IEEE, vol. 80, no. 1,
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[7] J. D. Kraus, Electromagnetics, McGraw-Hill, 3rd Ed.,
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[8] M. Okoniewski, and M. A. Stuchly, A Study of the
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[9] D. Andreuccetti, M. Bini, A. Ignesti, R. Olmi, S. Priori
and R. Vanni, High Permittivity Patch Radiator for
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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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