## 1 Introduction  
Rat-race couplers (RRCs) have been widely used in microwave circuits for many years. These couplers are essential components of many applications, including power dividers, mixers, and amplifiers, due to their excellent isolation, phase balance, and broadband operation. However, harmonic distortion can significantly degrade the performance of microwave circuits, especially when high power levels are involved. Therefore, it is necessary to design compact RRCs with harmonic suppression to improve the performance of microwave circuits. Compact rat-race couplers have attracted significant attention in recent years due to the small size they occupying microwave devices and their low cost. In particular, researchers have focused on developing microwave devices, such as compact rat-race couplers with harmonic suppression capabilities, to reduce unwanted harmonic signals generated by nonlinear devices in microwave and RF systems [1, 2, 3]. This literature review aims to provide an overview of recent research on compact rat-race couplers with harmonic suppression capabilities. A common approach to achieving harmonic suppression in compact rat-race couplers and power dividers involves the introduction of different resonators [4, 5] and filters [6, 7]. For example, in [8], the authors proposed a balanced rat-race coupler by presenting the quad-mode resonator. The coupler could achieve a filtering response up to the second harmonic. In [9], a compact rat-race coupler with a stepped-impedance resonator (SIR) was proposed. This also provided a wide stop band and harmonic suppression. This coupler could achieve a high level of suppression for the higher harmonics. Another approach to achieving harmonic suppression is to use novel coupler structures. For example, in [10], a compact rat-race coupler with an asymmetrical branch-line structure was proposed. The coupler achieved more than 20 dB suppression of the second, third, and fourth harmonics over a wide frequency range. In [11], a compact rat-race coupler with a modified branch-line structure and shunt open stubs was proposed. The coupler achieved a wide suppression band in the frequency response. In addition to the compact rat-race couplers, other coupler structures have also been proposed for harmonic suppression. In [12], a compact Wilkinson power divider with harmonic suppression capabilities was proposed. This power divider achieved suppression for the second harmonic. In [13], a compact coupler with harmonic suppression capabilities was proposed using meandered lines. The coupler could achieve a suppression greater than 20 dB suppression for the second and third harmonics over a wide frequency range. Study [14] introduced a compact rat-race coupler design with harmonic suppression using stubs and meandered lines and exhibited a wide bandwidth and high isolation. Additionally, in [15], a compact rat-race coupler with harmonic suppression was introduced using meandered lines and synthesized lines. This featured a simple and compact design and good performance. A rat-race coupler with harmonic suppression based on folded structures was proposed in [16] and this offered a compact size and high harmonic suppression level. Moreover, [17], proposed a compact rat-race coupler with harmonic suppression using capacitor loading. The method featured a simple and effective design and high harmonic suppression performance. In [18], a compact rat-race coupler with harmonic suppression was introduced using a ring resonator. This coupler offers 3 rd harmonic suppression and a wide bandwidth. Moreover, the utilization of photonic crystals [19; 20; 21] can be applied for higher-frequency operations for microwave circuits [22; 23; 24]. Additionally, recently, neural network techniques have been used to improve the performance of electronic circuits [25; 26; 27] that have been applied in the design of BPFs and couplers [28; 29; 30; 31]. Among the neural networks, RBF networks are useful tools to model nonlinear systems, such as microwave systems.