## 4 Measurement  
Figure 14 shows the CRLH TL antenna combined with the non-Foster circuit. An SMA connector was used for connecting the CRLH TL antenna and non-Foster circuit. Figure 15 shows the received power measurement environment. Alaris's OMNI-A0245 antenna was used for a transmitting antenna. The gain of the transmitting antenna was from \(-7\) to \(-3\) dBi at 120-500 MHz [28]. One mW power was applied to the transmitting antenna. The distance between the transmitting antenna and receiving antenna was 10 m, above four times that of the wavelength (2.06 m) of the minimum operating frequency (145.5 MHz) to meet the far-field conditions.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Frequency** & **Capacitance** \\ **(MHz)** & **(pF)** \\ \hline
126 & 9.55 \\
131 & 9.61 \\
136 & 9.68 \\
141 & 9.76 \\
146 & 9.84 \\
151 & 9.94 \\
156 & 10.05 \\
161 & 10.18 \\
166 & 10.33 \\
171 & 10.51 \\
176 & 10.75 \\
181 & 11.06 \\
186 & 11.52 \\
191 & 12.32 \\
196 & 14.15 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Capacitance of the antenna versus frequency.  
Figure 14: Designed CRLH transmission line antenna.  
Figure 16 shows the measured reflection coefficient of the proposed antenna with and without the non-Foster. It shows that the CRLH TL antenna without the non-Foster circuit did not resonate in the military VHF band. However, the CRLH TL antenna with the non-Foster circuit showed a reflection coefficient less than \(-10\) dB in 145.5-500 MHz due to the impedance matching by the non-Foster circuit. The operating frequencies of the proposed antenna included the military VHF band. In order to investigate the loss due to the non-Foster circuit, the input impedance of the non-Foster circuit was measured as in Figure 17. The real part indicated the loss, showing that it ranged between 11-16 ohm which implied the loss originated from the non-Foster circuit itself was non-negligible. The imaginary part showed a traditional negative slope as the frequency increases (i.e., j/uc). We note that the reflection coefficient of the non-Foster circuit itself was high since the impedance had a big difference from the port impedance of the vector network analyzer, which was 50 + j0 ohm.  
Figure 16: Measured reflection coefficients.  
Figure 15: Received power measurement environment.  
Figure 18 shows the received power of the antenna. Since the non-Foster circuit was used to match the input impedance of the antenna, the received power was enhanced. Comparing the received power before and after applying the non-Foster circuit, the received power enhancement ratio was 17.3 dB on an average of 120 to 500 MHz. Figure 19 shows the proposed antenna gain calculated by [PERSON] transmission formula. The formula of [PERSON] transmission is as follows.  
\[P_{r}=G_{t}G_{r}P_{t}\big{(}\frac{\lambda}{4\pi r}\big{)}^{2} \tag{4}\]  
Equation (4) can be used to calculate the gain of the receiving antenna from the received power.  
\[G_{r}=\frac{P_{r}}{G_{t}P_{t}}\big{(}\frac{4\pi r}{\lambda}\big{)}^{2} \tag{5}\]  
By Equation (5), we can calculate the gain of the proposed antenna. The comparison between the simulated gain of the proposed antenna and the measured gain showed a similar tendency. It is worth noting that experimental results showed the non-Foster circuit became unstable as the power was increased. Thus, the proposed antenna was not suitable for the transmitting side, but the receiving side [29].  
Figure 17: The measured impedance of the non-Foster circuit.  
Figure 18: Measured received powers.  
Table 5 shows the comparison of our antenna with other CRLH TL antennas. Note that the bandwidth of our antenna was more or less than that of other antennas, but only our antenna was implemented in the VHF band using CRLH TL with the non-Foster matching circuit.