Biomedical Engineering Reference
In-Depth Information
Fig. 7.5 Magnitude
S-parameter plot for the
implanted antenna position
Frequency (GHz)
the simulation software. Simulations recorded negative gain values for the antenna
after tissue absorption; this means that the signal emitted in free space is lower
than the radiated power from the implanted antenna. Hence, it is possible to excite
the antenna with a signal power higher than the outdoor allowable 41.3 dBm/MHz
power limit by increasing the excitation signal amplitude such that the power
emitted to the free space after tissue absorption lies within the FCC regulations.
Simulations are conducted in order to analyze the effect of this kind of manipu-
lation on the SAR and temperature variation.
The S-parameters of the antenna for the above WCE position is depicted in
Fig.
7.5
. The two dimensional polar plot of the far field gain of the antenna at
4 GHz are shown in Fig.
7.6
. The far field gain is shown on the x-y plane that
passes through the centre of antenna. The angle (Phi) in Fig.
7.6
is measured in an
anti-clockwise direction from the X-axis that passes through the centre of the
antenna. The three dimensional far field antenna gain is calculated using (
7.13
),
and converted into a two-dimensional plot using the gain values at the intersec-
tions with the x-y plane.
It can be seen from the far field results that the two-dimensional antenna gain is
-63 dBi. It was observed from the simulations that the maximum three-dimen-
sional gain is slightly higher than the two dimensional gain, and lies in the same
direction. The gain is significantly low because of the large power absorption by
the surrounding tissue mass. The recorded negative antenna gain after the tissue
absorption means that a power level that is significantly higher than the FCC
recommended spectral mask of -41.3 dBm/MHz for indoor transmission of the
IR-UWB signals can be used for the antenna excitation, given that the regulations
applied for SAR and SA are met [
49
,
50
]. The delivered power to the antenna can
be arranged such that the IR-UWB power level after the power loss due to tissue
absorption lies within the FCC approved spectral mask. Simulations are carried out
