# A Balanced Symmetrical Branch-Line Microstrip Coupler for 5G Applications
###### Abstract  
Symmetry in designing a microstrip coupler is crucial because it ensures balanced power division and minimizes unwanted coupling between the coupled lines. In this paper, a filtering branch-line coupler (BLC) with a simple symmetrical microstrip structure was designed, analyzed and fabricated. Based on a mathematical design procedure, the operating frequency was set at 5.2 GHz for WLAN and 5G applications. Moreover, an optimization method was used to improve the performance of the proposed design. It occupied an area of 83.2 mm\({}^{2}\). Its harmonics were suppressed up to 15.5 GHz with a maximum level of \(-15\) dB. Meanwhile, the isolation was better than \(-28\) dB. Another advantage of this design was its high phase balance, where the phase difference between its output ports was 270\({}^{\circ}\)\(\pm\) 0.1\({}^{\circ}\). To verify the design method and simulation results, the proposed coupler was fabricated and measured. The results show that all the simulation, design methods, and experimental results are in good agreement. Therefore, the proposed design can be easily used in designing high-performance microstrip-based communication systems. microstrip; coupler; phase; filtering response; 5G application; coupling factor +
Footnote †: journal:  
0000-0001-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1-1  
However, it occupies a very large implementation area without suppressing the harmonics. Two microstrip couplers are proposed in [15; 16] with large sizes, high insertion loss and high coupling factor. Using the T-shaped microstrip stubs, a coupler was designed in [17] for 5G high-band applications. The presented microstrip couplers in [18; 19] are suitable for Global System for Mobile Communications (GSM) applications. However, they could not attenuate the harmonics. Also, the proposed coupler in [18] is very large. In [20], a microstrip of \(-3\) dB BLC was achieved using open-circuited coupled lines. A microstrip coupler for use in Worldwide Interoperability for Microwave Access (WiMAX) applications is reported in [21], but it does not have a filtering frequency response. A summary of the advantages, disadvantages, design methods and applications of the above-reported couplers are listed in Table 1.  
In this work, we present a microstrip BLC with a novel structure. It has a filtering frequency response, which can suppress the 1 st and 2 nd harmonics up to 15.5 GHz. Operating at 5.2 GHz makes it suitable for IEEE 802.11a WLAN and mid-band 5G applications. A lowpass resonator is proposed and mathematically analyzed to use in the coupler structure. The proposed coupler is obtained through the direct integration of two lowpass filters (LPFs) and we did not change the dimensions of the LPFs in the final coupler structure. However, the additional optimization is performed to improve its performance. Our design has a high performance, and it can be easily integrated with the other high frequency circuits for designing RF communication systems. The final layout of the designed coupler is optimized to improve its performance. Good insertion loss, isolation, phase balance and coupling factor are obtained, while the return loss and the coupler size (83.2 mm\({}^{2}\)) are acceptable. This manuscript is organized as follows. Section 1 provides the introduction. Section 2 provides the presented mathematical analysis and optimization of the proposed resonator, filters and coupler. Section 3 presents