# New Miniature Narrow Band Microstrip Diplexer for Recent Wireless Communications
## 4 Results and Discussion  
The described microstrip diplexer was designed, optimized, and simulated with the electromagnetic (EM) Sonnet simulator. Scattering parameters for diplexers with a Kappa-Rogers substrate, as simulated, are shown in Figures 7 and 8. The transmit (Tx) filter has an insertion loss of 0.7 dB and a return loss of 21.2 dB at 2.84 GHz. The center frequency of the receive (Rx) filter is 4.08 GHz, and its insertion loss is 0.9 dB, while its return loss is roughly 17 dB. Furthermore, over a frequency range of 1 to 6 GHz, there is greater than 35 dB of band isolation between the two channels. The bandwidths ranges are 2.82-2.86 and 4.02-4.11 GHz for the Tx and Rx filters, respectively, which are highly narrow bandwidths. _S11_ response is a union of _S22_ and _S33_ responses as explained by Figure 8. Figure 9 depicts _S11_ responses for projected diplexer based on coupling space (g). As we can observe from this figure, the _S11_ response in the second channel becomes more effective as g decreases due to a tight coupling effect. Most changes are distinguished in the second channel, but minor _S11_ variations in the first channel exist, especially in the \(-3\) dB region.  
Figure 6: The structure and dimensions of the microstrip diplexer based on Kappa substrate, all dimensions in mm unit. L1 = 3.5, L2 = 0.8, W1 = 0.2, W2 = 0.2, L3 = 6.6, L4 = 9.9, L5 = 4.9, L6 = 4.2, W3 = 1.1, W4 = 1.1, W4 = 1.1, W5 = 3.9, W6 = 4.3, W7 = 3.3, U = 1, g = 0.1, W8 = 1, W9 = 5.4, W10 = 5.6, W11 = 3.6, W12 = 0.8, L7 = 1, L8 = 2.2, L9 = 6.3.  
Figures 10 and 11 depict the current intensity distribution for the projected microstrip diplexer. The highest values of magnetic strength are 42 and 53 A/M, which appeared in the corresponding resonators at 2.84 and 4.08 GHz for both cases, respectively. Figure 12 depicts the group delay for all S parameters for the projected microstrip diplexer. Negative group delay (GD) values are apparent in Table 1. The NGD does not contradict causality, which means that the diplexer may foretell the future location of the pulse based on its current position. PGD can be mitigated and its variability reduced with the use of NGD circuits.  
Figure 8: Simulated _S22_, _S33_, and _S23_ scattering responses for the projected diplexer based on the Kappa substrate.  
Figure 7: Simulated _S11_, _S12_, and _S13_ scattering responses for the diplexer based on the Kappa substrate.  
Figure 10: Current intensity distribution @ 2.84 GHz.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{S Parameter} & \multicolumn{1}{c}{Group Delay (ns)} & \multicolumn{1}{c}{Frequency (GHz)} \\ \hline \(S11\) & \(-3.824\) & \(4.08\) \\ \hline \(S21\) & \(-8.765\) & \(3.5\) \\ \hline \(S31\) & \(-6.688\), \(-6.604\) & \(2.703\), \(2.923\) \\ \hline \(S32\) & \(-2.263\), \(0.4766\) & \(3.469\), \(4.508\) \\ \hline \end{tabular}
\end{table}
Table 1: Negative group delay values.  
Figure 9: Simulated \(S11\) responses for the projected diplexer based on the coupling space (g).  
The developed microstrip in this paper has the lowest compactness and the greatest band isolation between Tx and Rx channels as compared with [18; 19; 20; 21; 22; 23; 24; 25], as explained by Table 2. Additionally, it has the lowest fractional bandwidths as compared with reported diplexers in [18; 19; 20; 21; 22; 23; 24; 25].  
Figure 11: Current intensity distribution @ 4.08 GHz.  
Figure 12: Group delay response for the designed diplexer.  
Figure 13 is a photograph of the microstrip diplexer after it was fully fabricated. In order to confirm the validity of the design simulations and test the operation of the diplexer, it was run through its paces on a Vector Network Analyzer (VNA). Figure 14 displays the measured and simulated frequency responses of the proposed diplexer. The simulated and measured results are consistent with one another. There are small differences because of VNA setup factors including the size of the diplexer, the quality of the SMA soldering, and the effect of coaxial cable losses.