## 4 Discussion  
We present a design and preliminary investigation of a wideband MWT system, which was based on small, custom-made printed monopole antennas. These antennas operate efficiently between 1.0 and 3.0 GHz when immersed in liquids commonly used in microwave imaging. We also assessed the signal strength received by our array using CAD-based 3-D CST simulations of our system, and introduced two different dielectric targets (water and a water-salt mixture) to study numerically their effect on the recorded signal strength. We chose an eight-antenna array for our current experimental setup, and present results that show good agreement between simulations and experimental measurements. Finally, we used our in-house DBIM-TwIST algorithm to reconstruct the images from the acquired data, and show that we can successfully reconstruct the considered dielectric targets. The proposed system and reconstruction method differentiate this work significantly from previously proposed MWT systems, such as those in [10; 11; 12; 13]. First, the hardware allows the use of a wide range of frequencies, 1.0-3.0 GHz, which is critical in order to guarantee robustness and enhanced resolution for the DBIM-TwIST algorithm [25]. Moreover, using printed monopoles allows a simple modelling of the antennas as point sources by the forward solver of our algorithm. More importantly, the DBIM-TwIST approach can produce images of enhanced resolution relative to approaches based on traditional solvers, such as the CGLS (see Figure 13). On the other hand, limitations of the present work include having considered a cylindrical geometry without variations along the height of the tank, and a data acquisition process without a multi-port system, which is prone to long data acquisition times and experimental errors. To overcome these limitations, our ongoing and future work will refine the data acquisition and inversion process further, and assess the system's imaging abilities for various different experimental scenarios. Our next prototype will feature a multi-port system and multiple antenna rings with a minimum of 12 antennas for each cylindrical ring, which is closer to the optimal number, as argued in [34]. We will also perform experimental and numerical studies to assess the system's ability to reconstruct \"weak\" targets with a small contrast relative to the background medium, as well as investigate more complicated imaging scenarios which are relevant to realistic medical  
Figure 13: Reconstructed real and imaginary parts of the complex permittivity for the region inside the antenna array of Figure 4 in the presence of two water-filled targets. Data are generated from CST simulations. The top row corresponds to the use of the TwIST solver, and the bottom to the use of the CGLS solver. The figure shows single-frequency reconstructions at (**a**) 1.0 GHz and (**b**) 2.2 GHz. imaging problems. Ultimately, our goal is to create a prototype that could be used in medical imaging applications of interest [36], such as cancer or stroke detection and treatment monitoring. Conceptualization, P.K. ; Methodology, S.A., M.K., I.S. and P.K. ; Software, Z.M. and Z.G. ; Validation, S.A., M.K., I.S. and Z.G. ; Resources, P.K., E.K. and G.P. ; Writing--Original Draft Preparation, P.K., Z.G. and S.A.; Supervision, P.K. ; Project Administration, P.K., E.K. and G.P. ; Funding Acquisition, P.K. and E.K. This research was funded in part by Innovate UK grant number 103920, and in part by the Engineering and Physical Sciences Research Council grant number EP/R013918/1. This work has been developed in the framework of COST Action TD1301, MiMed. The authors would like to thank [PERSON] and [PERSON] for hosting [PERSON] (via a MiMed Short Terms Scientific Mission) in their lab, and sharing their valuable expertise in microwave imaging experiments. The authors would also like to thank [PERSON] for hosting [PERSON] (via a MiMed Short Terms Scientific Mission) at Dartmouth College, and and training him in experimental microwave imaging.