# A Frequency-Reconfigurable Dual-Band RF Crossover Based on Coupled Lines and Open Stubs
## 4 Implementation and Measurement  
The proposed dual-band frequency-reconfigurable crossover was fabricated using printed circuit board (PCB) technology. The design was accomplished by utilizing a RO4003C substrate that has a dielectric constant (\(\epsilon_{\text{r}}\)) of 3.55; its loss tangent (\(\tan\delta\)) is 0.0027; and the thickness (h) selected is 1.524 mm. As for the two varactor diodes employed in the design, SMV1405 was chosen. The varactor diode's equivalent circuit is shown in Figure 5. This diode has an inductance (L) of 0.7 nH, a resistance (R) of 0.8 \(\Omega\), and its capacitance (C) can be altered from 2.67 pF to 0.63 pF. The 2.67 pF capacitance can be achieved as the DC biasing voltage is 0 V, and as the voltage increases, the capacitance decreases. Once the biasing voltage reaches 30 V, the capacitance becomes 0.63 pF. The dimensions used in the fabrication process of the proposed crossover are displayed in Table 2. Figure 6 shows the proposed design circuit layout along with the fabricated prototype. The size of the proposed crossover (without considering the port lines and biasing circuits) is 45.5 mm \(\times\) 29.4 mm. Measurements of the fabricated dual-band frequency-reconfigurable crossover were conducted via a network analyzer. Figure 7 shows the S-parameters (\(\left|\text{S}_{11}\right|_{\text{dB}}\), \(\left|\text{S}_{21}\right|_{\text{dB}}\), \(\left|\text{S}_{31}\right|_{\text{dB}}\), and \(\left|\text{S}_{41}\right|_{\text{dB}}\)) and their variation along the alternation of the varactor diode's capacitance. \begin{table}
\begin{tabular}{c c c c c c c} \hline \(w_{p}\) & 3.42 mm & \(w_{4}\) & 3 mm & \(l_{2}\) & 9.1 mm & \(l_{slot}\) & 4.8 mm \\ \hline \(w_{1}\) & 0.5 mm & \(w_{slot}\) & 9.53 mm & \(l_{3}\) & 4 mm & \(g_{1}\) & 0.4 mm \\ \hline \(w_{2}\) & 2.5 mm & \(l_{p}\) & 19.93 mm & \(l_{4}\) & 9 mm & \(g_{2}\) & 1.2 mm \\ \hline \(w_{3}\) & 0.8 mm & \(l_{1}\) & 20.56 mm & \(l_{5}\) & 1.5 mm & \(R_{bias}\) & 30 k\(\Omega\) \\ \hline \end{tabular}
\end{table} 
Table 2: Dimensions of the dual-band frequency-reconfigurable crossover. Figure 5: SMV1405 varactor diode equivalent circuit (R = 0.8 \(\Omega\), L = 0.7 nH, C = 0.63 pF \(-\) 2.67 pF). Figure 6: The proposed design: (**a**) circuit layout; (**b**) fabricated prototype. Tables 3 and 4 show a summary of the capacitance, operating frequencies, and obtained S-parameters at the low-frequency band and at the high-frequency band, respectively. The capacitance of the two varactor diodes is changed by altering the biasing voltage from 0 V, which gives 2.67 pF, to 30 V, which corresponds to 0.63 pF. The measurement of the fabricated dual-band frequency-reconfigurable crossover showed tunable frequency bands starting from 2.06 GHz to 2.40 GHz and from 5.44 GHz to 5.84 GHz. As for the return loss of the proposed design, it shows better performance in the high band compared to the lower band. \(\left|\mathrm{S_{11}}\right|_{\mathrm{dB}}\) was found to be greater than 15 dB for the lower band and more than 20 dB for the higher band. In contrast, the insertion loss was better at the lower band. \(\left|\mathrm{S_{31}}\right|_{\mathrm{dB}}\) was below 1.12 dB at the lower band, while it reached 1.6 dB at the higher band. In both cases, the isolation was above 15 dB. Regarding the tuning range for both the lower and higher bands, it has been calculated using Equation (23) and found to be 15.3% and 7.1%, respectively.