# A Balanced Symmetrical Branch-Line Microstrip Coupler for 5G Applications
## 3 Results and Comparison  
We obtained the simulation results by using ADS software and the measurements were performed using an HP8757A vector network analyzer. To conduct S-parameter measurements of a branch-line coupler using a vector network analyzer (VNA), the VNA was calibrated. This process was in line with a set of calibration standards that match the frequency range and impedance of the device under test (DUT). Common standards include open, short and load terminations, as well as through-and-line standards. Using the same standards, a full two-port calibration of the VNA was performed to compensate for any systematic errors in the measurement system, such as mismatch and transmission line effects. The VNA was then configured to perform S-parameter measurements in the appropriate frequency range and measurement format, for example, the phase and magnitude or real and imaginary components, as well as the number of points and frequency step sizes for the measurements. The BLC, which is the DUT, is then connected to the RF ports of the VNA with proper coaxial connections. To measure the return loss of the BLC, Port 1 of the BLC is connected to Port 1 of the VNA, while all the other ports are terminated with 50 ohm terminators. For the BLC insertion loss measurement, Port 1 of the VNA was connected to Port 1 of the BLC, and Port 2 of the VNA was connected to Port 2 of the BLC. Port 3 and Port 4 of the BLC were terminated through the 50 ohm terminators. Similarly, for the measurement of the BLC coupling loss, Port 1 of the VNA was connected to Port 1 of the BLC, and Port 2 of the VNA was connected to Port 3 of the BLC with Port 2 and Port 4 all terminated through 50 ohm terminators. Finally, for the measurement of the BLC's isolation loss, Port 1 of the VNA was connected to Port 1 of the BLC, and Port 2 of the VNA was connected to Port 4 of the BLC. Port 2 and Port 3 of  
Figure 7: The steps of our coupler design. the BLC were terminated with 50 ohm terminators. Due to the SMA and copper losses, the simulated losses are a little better than the measurements losses. The simulated and measured frequency responses are presented in Figure 8, while Figure 9 shows the simulated and measured phase difference between S\({}_{21}\) and S\({}_{31}\). The scattering parameters of proposed coupler at 5.1-5.3 GHz are depicted in Figure 10. Since the proposed structure is simple, the manufacturer error is minimal, which is an advantage. The proposed coupler works at 5.2 GHz (exactly at 5.19 GHz), which makes it suitable for WLAN and mid-band 5G (which covers 1 GHz to 6 GHz). At this operating frequency, the phase difference between S\({}_{21}\) and S\({}_{31}\) is 270\({}^{\circ}\)\(\pm\) 0.1\({}^{\circ}\). As shown in Figure 8, near the operating frequency, the best values of S\({}_{11}\), and S\({}_{41}\) (isolation) are \(-\)19.6 dB and \(-\)28.2 dB, respectively. Also, the best values of S\({}_{21}\) and S\({}_{31}\) near the operating frequency are \(-\)2.6 dB and \(-\)2.4 dB, respectively, which are at two different frequencies. Meanwhile, at the operating frequency S\({}_{21}\) and S\({}_{31}\) are \(-\)3.28 dB and 3.56 dB, respectively. The harmonics are suppressed by up to 15.5 GHz with a maximum level of \(-\)15 dB. Narrowband frequency responses of the proposed coupler are shown in Figure 10. Figure 11 shows the fabricated coupler. Figure 8: Simulated and measured frequency responses i.e. S\({}_{11}\), S\({}_{21}\) (blue lines) and S\({}_{41}\), S\({}_{31}\) (red lines). Figure 11: Fabricated Coupler. Figure 10: Narrowband frequency responses of the proposed coupler: (**a**) S\({}_{21}\) and S\({}_{31}\); (**b**) S\({}_{11}\) and S\({}_{41}\). Figure 9: Simulated and measured phase difference between S\({}_{21}\) and S\({}_{31}\). To prove the high performance of the proposed coupler, we compared it with previous works.