## 2 Materials and Methods
### System Sensitivity Evaluation Using Numerical Simulations  
We evaluated the performance of our imaging system shown in Figure 4 by modelling in CST Microwave Studio. We considered two sets of simulations for this purpose; the first set studies signal propagation inside the target-free tank filled with one of the immersion liquids, while the second also includes a cylindrical target with various contrasts. The target-free simulations allow us to compare the level of transmitted signals for the different immersion liquids in Figure 3, which should be above the noise of the VNA used in our measurements. To study the system's response to a target, we calculated the signal scattered from a 16 mm diameter cylindrical target immersed in the 90% glycerine-water mixture. To assess the effect of varying the target contrast to the received signals, we also compared the  
Figure 4: (**a**) Photo of the hardware system prototype used in our measurements; (**b**) Schematic of the imaging system hardware modelled in CST. signal from the water target to those of a metallic target modelled in CST as a perfect electric conductor (PEC), as well as a salt-water mixture. To better sample the signal levels transmitted in the tank filled with one of the immersion liquids considered in this study, we first simulated a 16-antenna array with a larger, 150 mm diameter. Figure 5 plots the received signal strength at 1.5 and 2.0 GHz as a function of receiver number, where receivers are numbered in the same order as in Figure 4b. Comparison of the two plots shows the strong level of attenuation that the signals experience as the frequency increases. As expected, we observe that the maximum signal strength is received when (low-loss) Triton X-100 is used as the background medium. The signal level for 80% glycerine-water is very low, below \(-140\) dB at 2.0 GHz, for example, but is considerably higher for the 90% mixture. The 90% corn syrup shows higher transmission levels than both glycerine mixtures, but is more difficult to handle experimentally (as is the Triton X-100). The results of Figure 5 suggest that we can use a 90% glycerine-water mixture as our system's immersion liquid if we wish to avoid handling issues with corn syrup and Triton X-100. We thus proceeded with simulating our exact measurement setup comprising an eight-antenna array of 130 mm diameter filled with the 90% glycerine-water mixture. The received signal strength is plotted as a function of receivers for every 500 MHz in Figure 6a. The same information is plotted in Figure 6b, when we introduce a strong PEC scatterer to validate the imaging system's sensitivity for the given target size. As expected, the curves of the received signal strength are not symmetric any longer, and the impact on the received signals correlates with the target's location shown in Figure 4b. In particular, the signal strength is visibly reduced at receivers for which the transmitted signal is \"blocked\" by the PEC scatterer. In line with our interest in microwave medical experiments, we introduced in our setup two different liquids for our target: water and a salt-water mixture (10 mg/mL concentration). These targets exhibit high contrast relative to the 90% glycerine-water background. Figure 7 compares the impact of the PEC with these two dielectric targets on the received signal strength at two representative frequencies, 1 GHz and 1.5 GHz. We observe very similar signal levels and trends for the water and salt-water targets, which are of course different from the PEC target which blocks the signal in the opposite antennas. We also calculated and plotted (in dB) ratios of the received signal strength with and without the target in Figure 8. While this difference is considerably stronger for the PEC target, it is also quite evident for the dielectric targets, for which the most notable difference is in the receivers across from the targets.