# Potential of Sub-GHz Wireless for Future IoT Wearables and Design of Compact 915 MHz Antenna
## 3 System Design
### Antenna Topology  
In this section, the design of an antenna integrated into the wrist-worn wireless SpO2 sensor device is discussed in detail. A challenge for the implementation of wireless communication in Sub-GHz bands in medical wearables is represented by the constraints on the antenna size. For instance, a short/medium range application at the MedRadio band or 433 MHz ISM band leads to a large antenna size. In fact, the free-space quarter-wavelength \(\lambda_{0}/4\) in these frequency bands varies from 16.4 to 18.7 cm. As for 915 MHz ISM band and 869.5 MHz Short Range Device band, \(\lambda_{0}/4\) varies from 8.1 to 8.7 cm. Electrically small antennas [28] are therefore required at these frequencies for wrist-worn devices that are only several cm squared in size.  
The antenna is realized as a Printed Circuit Board (PCB) referred to as the AU and is shown with a red dashed outline in Figure 7a. The AU is placed on top of the PU and SU. All three functional units fit inside an enclosure of inside dimensions of \(W_{E}\times H_{E}\). The antenna is realized using standard low-cost, FR-4 material with thickness \(t=1.54\) mm, \(\epsilon_{\mathrm{r}}=4.17\), tan\(\delta=0.016\) and a copper thickness of 35 \(\upmu\)m. The antenna feed is provided by an SMA connector located at Point A and connects to a 50 \(\Omega\) grounded coplanar waveguide (GCPW) of width \(W_{F}\) and spacing \(S\). The GCPW then connects to the input of a \(\pi\)-type matching network shown at Point B. The output of the matching network drives the input of the radiator at Point C via a wire link \(WL_{1}\) that is an effective short-circuit at 915 MHz. Removal of \(WL_{1}\) enables direct measurement of the antenna input impedance at Point C using a Vector Network Analyzer (VNA). The chosen antenna topology was a Planar Inverted-F Antenna (PIFA), which is a variant of the quarter-wavelength monopole antenna, where the monopole is folded to be parallel to the ground plane and hence to reduce the height of the structure [66]. The chosen configuration presents a reduced radiated power in the direction of the ground plane as illustrated in Figure 7b which thus can be exploited as a partial-shield for the human body as in [67] as the PU, SU and coin-cell are placed between the radiator and body and are all at AC Ground potential. Isolation is also achieved by maintaining a physical separation (\(l\)) between the radiator and human body. The radiator element was designed using a curved inverted-F antenna (IFA) topology that follows the contour of the enclosure and achieves resonance close to the target frequency of 915 MHz. A shunt inductive microstrip branch is placed between point C and E similar to the classic IFA configuration to counteract the added capacitance that the curved radiator introduces. The top side radiator and ground plane  
Figure 6: Wrist-worn wireless SpO2 sensor: (**a**) Exploded view of the Sensor Device; (**b**) Sensor device on voxel-based phantom arm.  
geometries are mirrored on the bottom side with a series of 300 \(\upmu\)m diameter plated-through via-holes used to connect both sides. The matching network was then designed in the circuit domain using Ansys Designer [68].  
During Full-wave EM simulation, the resonant response of the antenna depends on two key parameters. The first is the total length of the radiating element denoted \(l_{\text{CD}}\). This length is defined as the total length of the radiating element, defined along the center line and can be seen from points C to D in Figure 7a. The second is the total length of the inductive branch from points C to E in Figure 7a and denoted \(l_{\text{CE}}\).  
Figure 7: (a) Geometry of the proposed antenna: top view (**Left** side) and side view cross-section (**Right** side not to scale); (b) Improving antenna-body isolation.