IEEE Microwave Magazine - September/October 2018 - 88
placed on top of a resonant circuit, such as a patch
antenna, acting as a sensor. The MUT will alter the
antenna's effective permittivity e r, eff , resulting in a
measurable shift in operating frequency from which the
properties of the MUT can be extracted. This method
enables very fast determination of substrate parameters, although at the cost of reduced accuracy.
Coping with Material Parameter Uncertainty:
Rules of Thumb and Stochastic Tools
Textile materials are not fabricated with microwave
applications in mind. This leads to variations among
batches and even samples. Moreover, material properties are sensitive to changing operating conditions. By
including stochastic design frameworks in the full-wave,
computer-aided design (CAD) process [55], uncertainty
and variability can be mitigated. Knowledge concerning
how material parameters are distributed under various
operating conditions is highly valuable to the designer,
as these help avoid unnecessary overspecification of the
antenna requirements.
Moisture content, which can change due to heat
and humidity in the surrounding environment, is
an important factor to take into account. Water has a
high permittivity, e r . 78 [56], compared to the much
lower dielectric constant of textile materials, typically on the order of 1-2. Therefore, the moisture
content of a substrate can significantly affect the
performance of wearable components susceptible to
changes in permittivity, such as antennas and filters.
A substrate with a moisture regain of less than 3% is
therefore recommended.
Design Requirements
Selection of Materials and
Fabrication Technology
Preliminary Design of Building Blocks
Antenna System
Active Circuits
Power Supply
Stochastic Framework-Based Circuit/
Full-Wave Cooptimization
Validation in Realistic Conditions
High-Performance, Low-Cost Wearable System
(a)
Figure 3. A stochastic framework-based holistic design
paradigm: from design requirements to high-performance,
low-cost, smart wearable systems.
88
d
Material
Description
fr
tan
3-D fabrics
3-D spacer knit;
100% polyester
1.10-
1.13
0.004-
0.018
Cordura
Plain weave; 100%
PA 6.6, PU-coated
1.58
0.008
Neoprene
Neoprene laminated
with jersey; 100%
polyester on both
faces
1.37
0.001
Knit 100%
polyester; PU-coated
(face side), carded
(reverse side)
1.45
PA: polyamide; PU: polyurethane.
-2
|S11| (dB)
Table 1. The dielectric properties of some
common textile materials [54].
Fake leather
0
-4
-6
-8
-10
2
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9
Frequency (GHz)
3
Measurement
Simulation
(b)
0.012
Figure 4. Destructive methods yield very reliable results
but are unsuited for assessing large volumes. For (a) a
substrate integrated waveguide resonator realized on the
MUT, (b) the measured and simulated reflection coefficient
agree well [53].
September/October 2018
Table of Contents for the Digital Edition of IEEE Microwave Magazine - September/October 2018
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