## 2 Materials and Methods
### Characterization of Plane Wave Attenuation  
In this subsection, the setup and the procedure that was followed for the characterization of electric field attenuation in the fabricated phantoms are shown. In Figure 3, the setup used in the anechoic chamber to measure the electromagnetic field inside the phantom is described.  
Figure 2: Setup for measuring the dielectric properties of liquid phantoms. (**a**) PNA. (**b**) Dielectric probe. (**c**) Platform to move the sample. (**d**) Tank to store the phantom. (**e**) Heater and mixer to fabricate the phantom.  
The transmitter horn antenna (model 3164-03 Dual Polarized Horn from ETS Lindgren [32]) (Figure 3D) allowed both horizontal and vertical polarization without the need of rotating the antenna physically. The receiver antenna 1 (ant1) was designed to work at 402 MHz (MedRadio band) and ISM 2.45 GHz and the antenna 2 (ant2) was designed to work at ISM 868 MHz [23; 33]. Both antennas showed good matching at 5.8 GHz. The antennas in the described setup were used as field probes when inserted in the phantom. It has been corroborated by simulations that their normalized radiation patterns were kept mostly constant with penetration into the phantom (see Appendix A).  
As can be seen, to ensure that no reflections were produced near the phantom, the supporting table was surrounded with absorbing materials (Figure 3A, (1)). The impact of the feeding cable on the measurement was reduced by several means: firstly by reducing the cross section exposed to the plane wave on the z axis (see axes in Figure 1), secondly by covering the feeding cable with a ferrite-based absorber (Figure 3B, (4)) and thirdly by placing a foam-based absorber in front of it (Figure 3B, (1)). It was very important that the plane wave impinged upon the cable cross section and not transversally. This guaranteed that the disturbance in the measurement results was kept under a certain limit. To ensure that cable position was correct, a foam structure was placed along the backside of the phantom (Figure 3B, (6)). The ruler (Figure 3B, (2)) and the structure to move the antenna  
Figure 3: Setup used in Queen Mary University of London’s anechoic chamber for the characterization of plane wave attenuation. (**A**,**B**) Test bed of the phantom with a probe antenna inside. (1) Absorbing panels, (2) ruler, (3) structure for moving the antenna, (4) feeding cable with ferrite-based absorbing material, (5) antenna, (6) backside foam structure, (7) polystyrene box to ensure that the transmitter antenna and the implanted antenna are at the same height. (**C**) The two implanted broadband antennas employed to cover the desired frequency ranges. (**D**) The transmitting horn antenna.  
along (Figure 3B, (3)) were used to measure the insertion depth of the antenna into the phantom in the direction of the plane wave radiation (z axis). They were not metallic.  
With this setup, ant1 and ant2 were alternatively placed within the phantom at a certain distance from the front interface (Z = 0) and the S\({}_{21}\) was measured with respect to the transmitting antenna located at 3.32 m (Figure 3). Implanted and transmitting antennas were placed at the same height. A procedure was followed to measure the S\({}_{21}\) parameter for both polarizations for each position, while progressively introducing ant1 and ant2 in the liquid phantom every 2 mm (for 2.45 GHz and 5.8 GHz) and every 3 mm (for 402 MHz and 868 MHz) in the z direction. A change in polarization was made in the transmitter side. Normalization was performed with respect to the measurement on the interface of the phantom. This was done to de-embed the transmitted field magnitude (different with operational frequencies) and the effect of the probe antennas for comparison with the model. In all the measurements, the S\({}_{11}\) parameter was checked to ensure that the antenna was matched and that the obtained results were, consequently, reliable.