## 3 Performance Analysis for On-PCB Conditions
### PCB 131 mm \(\times\) 60 mm  
Several heights of the FPC above the ground plane of the PCB have been considered: 2.4 mm (the same height as the antenna booster), 5 mm, 10 mm, 15 mm, and 20 mm (Figure 4). No more cases have been considered beyond 20 mm since IoT devices should be small and present slim profiles. In this sense, 20 mm is regarded as a limit. Besides height, different FPC antenna positions have been analysed since the IoT designer has any degree of freedom to allocate the FPC antenna on the device. Three positions have been considered: the top edge, middle, and bottom edge (Figure 4). Therefore, for the FPC antenna case, there are five height cases and three positions. As a result, there are 15 different measurements. To simplify the results, the best location among the top, middle, and bottom edges in terms of total efficiency is represented for each FPC antenna height. As a result, there are five total efficiency measurements, one per FPC height and one total efficiency measurement for the antenna booster (Figure 5). This procedure is repeated for the medium and small PCBs. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Average Total Efficiency (\%)** & **698–960 MHz** & **1710–2690 MHz** \\ \hline FPC & 42.3 & 70.1 \\ Antenna Booster & 51.3 & 65.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Measured total efficiency for the FPC off-PCB and an antenna booster. Figure 3: Measured total efficiency for the embedded antenna booster and the FPC. When mounted on the PCB, total efficiency results for the FPC antenna drastically change from the off-PCB case. The total efficiency for the FPC antenna at 2.4 mm height (the same height as the antenna booster) is only 0.8% in the low-frequency region. In contrast, it is 52.0% for the antenna booster, representing a ratio of 18 dB better total efficiency for the antenna booster. Other values are summarised in Table 2, where it can be concluded that:  
* The efficiency of the FPC antenna improves as the height increases, from 0.8% at h = 2.4 mm to 14.8% at h = 20 mm in the low-frequency region and similarly in the high-frequency region. These results are aligned with antenna theory, where a  
Figure 4: (**a**): The FPC antenna is mounted at different heights above the ground plane. A foam spacer is used. Total PCB, including the ground plane, is 131 mm \(\times\) 60 mm, which is the same size as the PCB size of the antenna booster. (**b**): Three positions have been tested for each height shown on the left: top, middle, and bottom edge. Figure 5: Measured total efficiency for the embedded antenna booster and the FPC antenna at different heights. Only the best efficiency among the three FPC positions (top, middle, and bottom) is shown for each height. horizontal-current element is not efficient when it is electrically close to a conductive plane [19]. * The total efficiency is always better at both frequency regions for the antenna booster, even when the FPC antenna is 20 mm above the PCB. * The FPC antenna at the same height as the antenna booster results in 18 dB better for the antenna booster at 698-960 MHz and 7.8 dB at 1710-2690 MHz, despite being the antenna booster eight times smaller in length than the FPC antenna. Finally, the FPC antenna is also analysed when it is vertically oriented on the same PCB (Figure 6). Results in terms of total efficiency again reflect the worst performance more accused in the low-frequency region, where the difference between measured average total efficiency is 4.3 dB more for the antenna booster solution (Figure 7, Table 3). Furthermore, not only the FPC antenna is again the worst in the vertical case, but it also forces the device to be at least 21 mm in height (the width of the FPC), which goes against the direction of designing thin IoT devices. On the contrary, the antenna booster solution is only 2.4 mm in height, enabling very thin IoT devices.