# Design of a Modified Compact Coupler with Unwanted Harmonics Suppression for L-Band Applications
###### Abstract  
This paper presents the design, analysis, and fabrication of a new miniaturized microstrip branch line coupler (BLC) with high harmonics suppression. The T-shaped resonators, open stubs cross-shaped resonators and radial stubs are used in the proposed coupler design. The designed BLC operates at 1 GHz frequency, which can suppress up to 5 th spurious harmonics with a 20 dB level of attenuation. High miniaturization of about 86% is obtained for the proposed BLC, which is corresponding to the normalized size of 0.009 \(\lambda\)g\({}^{2}\). The measured values of isolation and return loss are obtained 28 dB and 29 dB, respectively, while the measured insertion loss of better than 0.2 dB is achieved at the operating frequency. Additionally, the operating bandwidth of the designed coupler ranges from 0.905 GHz up to 1.105 GHz, which shows a 200 MHz operating bandwidth or a fractional bandwidth (FBW) of 20%. The presented BLC is fabricated and measured, where the measurements confirm the simulated results. The designed coupler shows desirable performance compared to the recent designed couplers. p 2022 11 1747 174 1  
in [15] shows about 74% size reduction, but it has a large insertion loss of about 1 dB and only 2 nd harmonic suppression. A squared coupler is presented in [16], which can suppress only 2 nd harmonic using the slow wave technique. In [17], a wide band three sections BLC is presented. High size reduction was achieved in this coupler; however, only the 2 nd harmonic is suppressed in this work. Using open subs in the main branches of the coupler is a technique which has been widely used for harmonic suppression and size reduction [18; 19; 20; 21; 22; 23; 24]. In this technique, squared or radial open stubs can be used in each main quarter wavelength lines. These open stubs can produce transmission zeros and provide wide stopband for the coupler. Additionally, the applied open stubs reduce the overall size of the coupler. The presented coupler in [18] can suppress up to 5 th harmonics, but it is not very compact. Another H-shaped resonator coupler is designed in [19]. The applied H-shaped resonators result in 64% size reduction, but the obtained stopband is not very wide. A coupler with T-shaped resonators is presented in [20]. This coupler has harmonic suppression up to 8 th harmonics and good size reduction. However, the insertion loss of the coupler is not acceptable. Radial subs are used to design a coupler in [21], which resulted in harmonics suppression and acceptable size reduction, but the second harmonic is not well attenuated. In [22], a compact microstrip coupler with good harmonic suppression is introduced using eight open-ended stubs, placed inside the coupler structure, which effectively reduces the circuit size about 80%. This structure works at 0.96 GHz and suppresses 2 nd to 6 th harmonics, but this device has complex structure and high insertion loss (0.3 dB). In [23], a microstrip coupler is designed at 1 GHz, using spiral open stubs, which resulted in 63% size reduction. This device only suppresses 2 nd and 3 rd harmonics and has high insertion loss (0.3 dB), which is undesirable. In [24], a miniaturized coupler at 2.1 GHz is designed with 63% size reduction using the spiral T-shaped transmission line. This device only suppresses 2 nd and 3 rd harmonics and has high insertion loss (0.9 dB), which is not acceptable. In the coupler design, dimensions of the applied lines have a direct effect on the coupler performance. In order to find the best performance, the length and width of the applied stubs should be obtained. To realize the best dimensions of the stubs, neural networks and artificial intelligence can be used [25; 26]. In [27], a neural network design approach is presented for filters and resonators, which can be used for microwave couplers. In [28], a compact band pass filter with 7 th-order harmonics suppression is proposed, which has a compact size and provides a wide rejection band. This filter can be used easily in the coupler structure.