### Use of a RF Amplifier to Increase the Range of Reading Data from the Identifier  
Another method of testing to increase the range of information penetration was the use of the ZFL-500 LN+ signal amplifier from Mini-Circuit for the frequency range from 0.1 MHz to 500 MHz. The supply voltage of the amplifier during the tests was approx. 12 V DC. Its view and the characteristics of the gain as a function of frequency are shown in Figure 18.  
Figure 18: (**a**) The ZFL-500 LN+ amplifier and (**b**) its frequency gain characteristic—according to the amplifier manufacturer’s data [21]. Figure 17: Recorded signal waveforms corresponding to distance measurements: (**a**) 3.0 cm (\(m=27\%\)), (**b**) 15 cm (\(m=14\%\)) and (**c**) 25 cm (\(m=7\%\)). The measuring equipment listed in Table 6 was used to carry out the measurements. The measuring system used in the measurements is shown in Figure 19.  
During the tests, the ID card was placed at a distance of 3 cm from the reader. The coil was placed at different distances from the identifier and on the basis of the obtained oscillograms the depth factor and signal level were determined as a function of the distance of the coil from the identifier. The values of \(U_{max}\) and \(U_{min}\) read with the use of an oscilloscope and the calculated values of the modulation depth factor m are presented in Table 7, while examples of images recorded by the oscilloscope are shown in Figure 20. \begin{table}
\begin{tabular}{c c c} \hline
**Device** & **Type** & **Producer** \\ \hline Oscilloscope & DSOX1102A & Keysight \\ Spectrum analyzer & & Colorado Springs, CO, USA \\ (10 kHz-150 MHz) & 3588A & Hewlett Packard \\ Reader LF-band RFID & & Everett, WA, USA \\ and HF-band RFID & NS106 RFID Reader Writer & YiToo \\ ID card & NTAG213 & Guangzhou, China \\ Signal amplifier & ZFL-500 LN+ & Mini-Circuit \\  & & New York, NY, USA \\  & & Military Communication \\ Receiving coil & & Institute-National Research \\ (65 \(\times\) 35 mm, 6 coils) & & Institute, Zegrze \\  & & Poludniowe, Poland \\ \hline \end{tabular}
\end{table} 
Table 6: List of apparatus used to carry out the measurements. \begin{table}
\begin{tabular}{c c c c} \hline
**Distance d** & \multicolumn{3}{c}{**Signal Parameters**} \\ \cline{2-3}
**[cm]** & \(\mathbf{U_{max}}[\mathbf{V}]\) & \(\mathbf{U_{min}}[\mathbf{V}]\) &
\begin{tabular}{c} **Modulation** \\ **Depth Factor m [\%]** \\ \end{tabular} \\ \hline
3.0 & 13.3 & 9.7 & 16.0 \\
5.0 & 11.0 & 8.4 & 13.0 \\
10.0 & 8.2 & 6.3 & 13.1 \\
20.0 & 6.3 & 4.9 & 13.0 \\
25.0 & 4.2 & 3.2 & 14.0 \\
30.0 & 3.1 & 2.7 & 7.0 \\ \hline \end{tabular}
\end{table} 
Table 7: Recorded signal parameters and calculated modulation depth factor value for selected receiver coil-to-ID distances for ZFL-500 LN+ use case. Figure 19: Measuring system for determining the modulation depth factor using a signal amplifier. Figure 20: Cont. The use of the amplifier in the receiver circuit allowed to increase the communication range to 30 cm. The shape of the time waveform of the signal allows for the identification of distinctive features important from the point of view of the possibility of non-invasive information acquisition. Receiving the signal from the identifier from such a distance, in  
Figure 20: Recorded signal waveforms corresponding to distance measurements: (**a**) 3.0 cm (\(m=16\%\)), (**b**) 20 cm (\(m=13\%\)) (**c**) 25 cm (\(m=14\%\)) and (**d**) 30 cm (\(m=7\%\)). order to copy or retransmit it, may already pose a significant threat to the data contained in the identifier. Of the three methods of reading data from the NFC identifier presented, the method using an amplifier seems to be the most effective. However, a much simpler and cheaper solution, and comparable in terms of effectiveness, is the use of a resonance system. The benefits resulting from the use of the resonant system are observed especially for distances \(d\) not exceeding 20 cm, when the signal readability (modulation depth factor) is comparable or even greater than in the case of using an amplifier.