## Appendix A  
Before handling small-sized surfaces, such as DUTs, experimental configurations were tested and investigated with a large-sized reflecting surface, as is performed for RF calibration. Figure 11a has a horn antenna and an 8-by-16 array antenna on the line-of-sight (LoS), whose distance is 2 m. Two 8-by-16 array antennas see each other directly in Figure 11b. The TX side and RX side are connected to the signal generator and spectrum analyzer, respectively. The signal generator is used in the preliminary tests because its input power is adjustable by up to 15 dBm greater than that of the VNA. For the sake of surface sensing, the proposed scheme deals with relatively small reflecting surfaces, whereas the setup here adopts a relatively large reflecting geometry whose size is 1 m by 1 m. This is placed leaning against the wall to generate the reflection of the signal from the TX antenna to the RX antenna. The angles of incidence and reflection are set at 45 degrees and the travelling distance from the TX to RX antenna is 2 m, as in Figure 11c,d. The received signal strength taken in the spectrum analyzer for the four cases is presented in Figure 11e,f. For the setup using the horn and the array antenna, the signal strength of the LoS case overlaps that of the reflection by the large metal plane, as shown in Figure 11e. The value is \(-35\) dBm. A similar effect is observed in Figure 11f, where the reflected wave caused by the large metal scatterer has almost the same received power level as that of the LoS between the two array antennas. Based on these experiments, the surface-sensing antenna integrated with the 5G antenna is adopted to sense the smaller-sized metal- and metamaterial-surfaced structures. Back to the origin of the proposed idea, more explanations need to be given regarding the basics of the mm-wave antenna, apart from Section 2, and concerning the reflecting metasurface chosen as an object for RF sensing. Firstly, for the communication link made at 28 GHz, a 1D array antenna is designed as in Figure 3a and in the following. It has a feed-line different from the conventional one to make the RF signals stronger. One of the best ways to make RF signals that emanate from the antenna strong is to increment the directivity of the radiated beam. The shape of the feed-line matters for the 1D array antenna, unlike with larger apertures or 2D arrays. The main feeding TX-line is parallel to the orientation of the patches, as this is the commonly used method. Contrarily, the feed-line is bent at a right angle, and most of the TX-line is proposed to be orthogonal to the orientation of the patches, as shown in Figure A2a. It aims to suppress the interruption of the waves from the feeding TX-line by twisting the polarization. Figure 14(b) shows the changed beam-width or, in other words, the improvement in directivity, which renders the gain to an increase from 5 to 8.dBi. This provides positive effects on the mm-wave communication. Secondly, researchers might choose to use shared aperture metamaterials and sub-arrays instead of the proposed antenna structure, which may seem simple. When surveying technical articles, like [21; 22], a number of them seen to give intriguing functions for metamaterial antennas or sub-arrays for mm-wave frequencies. In line with their proposals, a variety of mm-wave metamaterial antennas have been designed in this lab, but due to a tendency for the volume of each of the structures with accessories becoming large, an inexpensive and effective one was suggested instead. As a reviewer of this paper, noted shared aperture antennas or sub-arrays should be tried instead of our structure, and they implied that there are novel antennas better than ours, so we have spared time to design antennas different from the proposed structure. Shared apertures introduced in [21; 22] might have been tried at the beginning. [PERSON] et al. 's antenna works for the Ku band and W band. [PERSON] et al. obtained far-fields from 3.4 to 3.8 GHz for a high-band and from 0.69 to 0.96 GHz for a low-band.