## 3 Results and Discussions
This distribution of artifacts and features was in common to all the simulations with different antenna grids, however with minor variations. Figure 5: Confocal map for a 6 cm deep target at 0.6 GHz with a \(3\times 3\) antenna grid. (**a**) x-parallel slice and (**b**) z-parallel slice. Expected target position is indicated by a red circle. the target and those antennas was comparable to the wavelength of the electromagnetic pulse. This can be observed in Figure 4d), where the signal coming from the central antenna was much more intense than that for the other antennas and therefore contributed more heavily to the formation of the confocal map. For shallow targets, there was a large difference in distances from the target between the central antenna and the lateral ones. However, this phenomenon became weaker and weaker with the depth (see the Supplementary Information SI) in agreement with the fact that only at short distances the target still resided in the antenna near-field. In fact, these artifacts appeared at the lowest frequencies or at the lowest depths. The appearance of a ghost target (Figure 6a) at higher depths instead was associated with the low dielectric contrast between the target and the surrounding medium. In fact, the ghost represented the backscattering given by the bottom of the target. As it can be seen from Figure 4, the second echo was temporally separated from the first one, but still displayed similar amplitude. This was a consequence of the low attenuation of the electromagnetic wave inside the target compared with the surrounding medium. Since the dielectric permittivity of the medium increased with the frequency, the time separation between the first and the second echo, dependent on the phase velocity, became larger at larger frequencies (see SI). Furthermore, the intensity of the second echo was actually amplified by the integration performed in the reconstruction algorithm (DAS). Additionally, the ghost disappeared when the target was substitute with a perfectly conductive one for which there is no echo at the bottom of the target (see SI). Summarizing, this artifact was present only at intermediated frequencies for high target depths and because of the strong attenuation of the medium at high frequencies (see Figure 1b), it was not visible at high frequencies for deep targets. The fact that the target appeared much smaller and at shallower positions than the original one (Figure 6b) was connected with the shortening of the wavelength at high frequencies. In fact, as it can be seen from Figure 4 the modulations in intensity belonging to a particular echo extended for a certain length of time which translated to a certain span in space in the confocal map. This span was connected with the wavelength of the signal used: at higher frequencies, i.e., smaller wavelength, the time extend of the modulation was shorter (see SI for an example), which is in agreement with the idea that shorter wavelengths correspond to higher spatial resolution. Considering the increase in the attenuation \(\alpha\) at high frequencies (Figure 1b) in these conditions it was only possible to localize the very top part of the target. Increasing the depth of the target, the electromagnetic attenuation \(\alpha\) given by the lung tissue was too high to allow any signal to travel back from the target, which is in agreement with the fact that the disappearing of the target from the confocal map (Figure 6c) happened only at the highest frequencies and at the largest depths. To quantify in which frequency range and nodule depth the cancer localization was possible, the confocal map was split into two regions \(V_{In}\) and \(V_{Out}\); the first corresponds to the position of the expected target, the latter covers the remaining of the confocal map. To account for blurring effects, \(V_{In}\) was taken as 10% larger than the real target.