## 3 Results  
For all the EM simulations, we used the commercial FDTD software Sim4 Life (Zurich, Switzerland) [53]. All simulations were performed with the conducting surface set as a perfect electric conductor with 0 mm thickness; the database for materials provided by the software was set to match the electric properties of the dielectric materials.  
EM simulations were performed with the design shown in Figure 5. The arrangement had a width and length of 80 and 225 mm, respectively. The dielectric material had a thickness of 5 mm. The excitation source was attached to a base line perpendicular to the strip lines, and this base was used to apply the current to the strip lines. The dimensions of the base were 80 by 2 mm, and the source was placed at the center of the base. The microstrip arrangement was excited with a voltage source, having a Gaussian pulse with a frequency centered at 210 MHz and bandwidth of 500 MHz in order to cover all the frequencies of interest.  
The goal of the design was to achieve tuning frequencies at 300, 280, 121, and 78 MHz. The dielectric material for the lower frequency lines (121 and 78 MHz) was set as Rogers RT/duroid 6010.2 LM with a relative permittivity of 10.7 and conductivity of 0.00128. The dielectric material for the high frequency lines (280 and 300 MHz) was set as Rogers RT/duroid 6002 with a relative permittivity of 2.94 and conductivity of 0.000668. Each dielectric material had a width of 20 mm and length of 225 mm. The conductor strip lines were set at the centers of each substrate. The separation between the lines was 20 mm, and the dielectric materials were interleaved so that one strip line operating at low frequency was positioned next to a line operating at high frequency.  
The loading condition was set with parallel capacitors. From line 1 to line 4, the corresponding capacitors were 80, 11, 100, and 15 pF. In addition, we added a parallel capacitor at the beginning of the strip lines operating at lower frequencies, allowing them to be tuned at such low frequencies, and the values of the capacitors were 65 and 18 pF. Table 1 shows a summary of the dielectric materials and the capacitor values for each of the lines operating at the corresponding frequencies.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Line** & **Frequency** & **Dielectric** & **Loading** & **Parallel** \\
**Number** & **[MHz]** & **Permittivity** & **Capactor [pF]** & **Capactor [pF]** \\ \hline
1 & 121 & 10.7 & 80 & 18 \\
2 & 300 & 2.94 & 11 & - \\
3 & 78 & 10.7 & 100 & 65 \\
4 & 280 & 2.94 & 15 & - \\ \hline \end{tabular}
\end{table}
Table 1: The composition of the dielectric materials and capacitors for each conducting line.  
Figure 5: The proposed design of the coplanar interleaved gapped microstrips consisting of four conducting lines.  
To visualize the operation of the arrangement, we also included the surface current density for each frequency in Figure 9. Each line operates at the respective frequency, while maintaining low coupling with the neighboring lines. From Figure 9, it can be noticed that the current waveform is less uniform for the case of the 300 and 280 MHz lines. This is the result of the combination of the four frequencies, which affects the optimization of the wavelength at these frequencies. However, the effectiveness of the designs still holds, as shown in the axial view in Figure 7.