Fig. 10. Simulations showing the two designs bent at 90 and mounted on a
tissue-equivalent phantom: (a) extended shielding ground conductor design; and
(b) original design. The space between the antennas and phantoms was 1 mm.
TABLE III
SIMULATED ON-BODY EFFECTS OF THE TWO DESIGNS. THE RESULTS ARE
CALCULATED AT THEIR FREE SPACE RESONANT FREQUENCY WHEN BENT
INTO CURVED SHAPE. (1.413 GHz IS FOR THE DESIGN WITH EDGE EXTENSION
AND 1.401 GHz IS FOR THE DESIGN WITHOUT EDGE EXTENSION)
was also studied. In the simulation model, human tissue was
modeled as a flat muscle-equivalent phantom with dimensions
of 250 mm 150 mm 50 mm, and the antenna was located
1 mm above the tissue. The body tissue absorbs some radiated
power, and the SAR value is relatively crucial to WBANs. For
WBAN applications, devices are made as small as possible to
facilitatewearability, and battery sizes are typically limited.
Sensor devices must have extremely low power consumption
to ensure that they can operate for long periods. Furthermore,
their transmission ranges are typically designed to be no more
than 3 m, therefore, the output power of the transmitter was
set to -10 dBm , according to the link budget of the system
[23]. The average SAR is shown in Fig. 11. The peak value of
the calculated SAR at 1.465 GHz was 1.183 W/kg3, which is
below the FCC restrictions. However, a method for lowering
the SAR was also studied. The SAR field distribution is closely
related to the shape of the antenna and its ground plane. As
shown in Fig. 11(a), the SARfield distribution of a square patch
antenna with a square ground plane exhibits a hot spot under
the antenna. By contrast, the SAR field distribution of a patch
antenna shaped like an adhesive bandage exhibits two hot spots
near its two long sides, as shown in Fig. 11(b). To lower the
peak value of the SAR field distribution, two semicircular metal
segments were added to the bottom ground plane to disturb the
electric field. As shown in Fig. 11(c), after adding the two semicircular
metal segments, the SAR field became more uniform
and the peak SAR value decreased by 45% from 1.183 W/kg3
to 0.65 W/kg3. Due to the offset feed of the antenna, the two
semicircular segments had to be asymmetrically positioned to
achieve a more favorable result. The adhesive-bandage-like
antenna with the 8 mm extended shielding ground plane met