# A Comparative Study of On-Body Radio-Frequency Links in the 420 MHz-2.4 GHz Range
## 2 Materials and Methods
### RF Technologies
#### 2.2.1 915 MHz Printed Folded Dipoles for UHF RFID  
UHF RFID is a wireless technology that operates in a frequency band around 900 MHz (in the United States: 902-928 MHz) and is mainly used for passive wireless identification of objects. However, the new generation of the UHF RFID standard also allows for passive transmission of sensor data from the tag, i.e., the remote, passive antenna-IC-sensor system, towards a reader, i.e., a powered system with an antenna and an RFID compatible radio ([PERSON], 1996). This makes this technology an interesting option in WBANs, since it allows one to place the energy source in a central hub and work with passive tag(s) on the other side of the link. We have used screen-printed, dipole antennas with a free-space power reflection coefficient (S\({}_{11}\)) below \(-10\) dB at 915 MHz according to the design presented in ([PERSON], 1996). Dipole antennas are frequently used in UHF RFID tags ([PERSON], 1996). The antennas are printed on a 125 \(\upmu\)m-thick polyethylene naphtalate (PEN) substrate (Q65 HA from Teijin Dupont Films, Wilmington, DE, USA) ([PERSON], 1996) using Creative Materials conductive Silver ink no. 12633 ([PERSON], 1996). The antennas are fed using a mechanically connected SMA connector (292-64A-06, Centric RF, Allen, TX, USA) which is adapted using a 0.96 \(\pm\) 0.02 mm thick plastic spacer underneath the center coax in order to eliminate the connector's capacitance. Figure 2 shows the screen-printed antennas, the connectors with spacer, their dimensions, and their measured S\({}_{11}\). The antennas were used to perform on-body path-loss measurements at 915 MHz. Two antennas were placed on the positions shown in Figure 1. A cardboard spacer with thickness 2.83 \(\pm\) 0.03 mm backed the antennas. The antennas were placed on the body in two configurations: orthogonal to the skin and parallel to the skin, see Figure 2f,g. In the orthogonal case, the connector and a 90\({}^{\circ}\) SMAconnector introduce 26.8 \(\pm\) 0.1 mm spacing in between the feed point of the antenna and the cardboard spacer. In the parallel case the connector introduces an additional 4.42 \(\pm\) 0.11 mm spacing from its bottom until the coax. A VNA (Agilent N5242A, PNA-X, Santa Clara, CA, USA) was used to register two-port S-parameters (S\({}_{11}\), S\({}_{12}\), S\({}_{21}\), and S\({}_{22}\)). For each position, 50 measurements were executed from 865 MHz to 965 MHz with 501 frequency steps (sweep time 5.3 ms). The S\({}_{21}\) and S\({}_{12}\) measurements were pooled and their average value (calculated in decibels) was determined as a function of separation distance (\(d\)), together with the standard deviation on this value. The 100 S\({}_{12/21}\) samples were used as the input for the path loss models listed in Equations (1) and (2). In addition to the reference scenario, the antennas at 915 MHz were also used to assess on-body path loss in four additional scenarios on the body (see Figure 2e):  
1. [label=(0)]
2. Chest: TX above the sternum and RX moved down in steps of 5 cm
3. Back: TX in between the shoulder blades and RX moved down in steps of 5 cm  
Figure 2: (**a**) Screen-printed folded dipole on PEN with antenna dimensions (mm). (**b**) Screen-printed folded dipole with the used mechanical SMA connector (**c**) The used mechanical SMA connector with the printed plastic spacer. (**d**) Measured S\({}_{11}\) of three printed antennas (black) and the UHF RFID frequency band (grey). (**e**) Additional on-body path-loss configurations used for measurements using this antenna. (**f**) On-body setup in the reference scenario with antennas orthogonal to the skin surface and (**g**) On-body setup in the reference scenario with antennas parallel to the skin surface. Leg: TX on the ankle and RX moved upward in steps of 5 cm
* Around the torso: TX on the sternum and RX moved around the shoulders on the same height in steps of 5 cm. The goal of these measurements was to compare the reference scenario to different on-body scenarios using the same frequency and antennas.