## 5 Discussions
For example, taking  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **FPC Antenna** & **Antenna Booster** \\ \hline \multicolumn{3}{c}{Big \\ Area of the antenna part} & 96 mm \(\times\) 21 mm & Small12 mm \(\times\) 3 mm \\ \multicolumn{3}{c}{Average size *: 89.6 mm \(\times\) 25.3 mm} \\ \hline Automatic Pick and Place & No & Yes \\ \hline \multicolumn{3}{c}{Predictable performance} & No, performance depends on micro-coaxial and FPC antenna position & Yes \\ \hline \multicolumn{3}{c}{Fits in small IoT devices (less than 96 mm in length)} & No & Yes \\ \hline \multicolumn{3}{c}{Enables thin devices} & No, it needs to be about 20 mm over the PCB to obtain reasonable efficiency & Yes, the antenna booster height is only 2.4 mm \\ \hline \multicolumn{3}{c}{BoM} & Adhesive, FPC, coaxial line and receptacles at the FPC antenna and PCB & Antenna booster and a matching network all placed on the PCB \\ \hline \multicolumn{3}{c}{Price (1000 units)} & \(\sim\)3-X & X \\ \hline \multicolumn{3}{c}{70} & \multicolumn{1}{c}{} \\ \multicolumn{3}{c}{65 x 42} & \multicolumn{1}{c}{} \\ \multicolumn{3}{c}{mm\({}^{2}\)} & \multicolumn{1}{c}{} \\ \multicolumn{3}{c}{**Antenna booster**} & \multicolumn{1}{c}{} \\ \multicolumn{3}{c}{56.5}  
as a reference the FPC antenna at 10 mm over the ground plane, the antenna booster is 6.3 dB, 4.3 dB, and 7.6 dB more efficient on average at the low-frequency region for the small, medium, and large PCB, respectively. In the high-frequency region (1710-2690 MHz), the difference is not as severe, but still, the antenna booster exceeds the FPC antenna's performance. The poor efficiency performance of the FPC antenna on the ground plane is the result of a severe mismatching due to the proximity of the conductive antenna portion to the ground plane (Figure 15). As an illustrative case, the S\({}_{11}\) for both technologies are compared considering the big 130 mm \(\times\) 60 mm PCB. When the FPC is at the same 2.4 mm height as the height of the antenna booster, the S\({}_{11}\) at 900 MHz is only \(-\)0.61 dB which represents that only the 13.1% of the transceiver available power is delivered to the antenna. However, the S\({}_{11}\) at 900 MHz is \(-\)12 dB for the antenna booster, representing that 93.7% of the available power is delivered to the antenna. For the 10 mm and 20 mm height, S\({}_{11}\) is still very poor for the FPC antenna, being worse than \(-\)2 dB. At the high-frequency region (1.71-2.69 GHz), the S\({}_{11}\) for the FPC antenna is not worse than that in the low-frequency region but still presents deficient matching values compared to the antenna booster, which achieves an S\({}_{11}\) well below \(-\)6 dB across the whole 1.71-2.69 GHz frequency region. The question is whether the performance can be again recovered by using a matching network. The matter is addressed by computing the potential bandwidth, which calculates the bandwidth an antenna system can reach with input impedance data. Potential bandwidth has been computed with Optenni-Lab [15]. The issue is that the potential bandwidth of the FPC on-PCB decreases drastically, as expected from the basic antenna theory of antennas electrically close to conductive planes. The potential bandwidth is compared for the off-PCB and on-PCB for two frequencies (Table 8). As shown, the potential bandwidth for the off-PCB is aligned with the required bandwidth at the low-frequency region (698-960 MHz = 31.6%) and the high-frequency region (1710-2690 MHz = 44.5%). However, for the on-PCB, the potential bandwidth is very poor (\(\sim\)1%) at both 900 MHz and 2200 MHz, meaning that bandwidth cannot be recovered even with the addition of a matching network. Figure 15: Measured S\({}_{11}\) for the 12 mm \(\times\) 3 mm \(\times\) 2.4 mm (h) antenna booster and FPC at three different heights over the ground plane (2.4 mm, which is the same height as the antenna booster, 10 mm and 20 mm). PCB is 131 mm \(\times\) 60 mm. As observed for both technologies, the ground plane impacts, especially in the low-frequency region.