## 2 State of the Art  
With the debut of the antenna in the 1970s, the concept of the microstrip antenna was initially established for the first time. An increase in the number of study articles has been written about the concept by several researchers from different countries [8; 9; 10]. The development of innovative designs for microstrip antennas has been prompted by the rise in the number of applications that make use of wireless communication. The objective of these designs is to attain a high level of performance in modern communication systems. The filters and diplexers are components that are utilized in modern communication systems, and each of these components performs substantial and equally vital duties. A well-designed diplexer should not only be priced reasonably, but it should also be very effective in terms of performance. All of these qualities should be present. In the conventional method of manufacturing microstrip diplexers, the operations of first making two separate filters and then constructing a matching circuit are carried out. This is the standard way. The easiest approach is to make use of a combination of two band-pass filters (BPFs), as stated in reference [11; 12; 13; 14]. As a result of their architectural design, microstrip diplexers are designed to be utilized in a wide number of applications that involve wireless communication. During the process of designing and creating the diplexer, several potential structures have been proposed. When it comes to RF diplexers constructed based on diverse filter designs [15], there are a few different approaches that may be applied; however, the three methods that are listed below are the ones that are utilized the most frequently:  
One of the first strategies is the utilization of two BPFs, each of which has a center frequency that is distinct from the other. The employment of a bandpass filter for one channel and a bandstop filter for the other channel, both of which are designed to reject the same frequency band, is another instance of this. The interaction between these filters when employed together is important. For example, using two band-stop filters, each with a different center frequency, may be essential in certain instances. Each diplexer is equipped with two channels, and each of these channels runs at a different frequency to accommodate a certain group of applications. To give you an  
Figure 1: Diplexer in RF front end. example, the authors of the reported paper in [16], introduced a diplexer that was developed specifically for WiMAX wireless communications. Instead, the authors of [17] came up with the idea for a diplexer during the same year. Both GSM-4G and frequency division duplex applications were taken into consideration during the development of this diplexer. A microstrip diplexer filter with good isolation has been reported in [18]. The short stubs-load resonator technique, upon which this filter was built, is based on a feeding line coupling approach. The building of this filter was made possible utilizing this method. Specifically, the filter was made to provide transmission zero at a lower frequency than the original frequency. Its primary aim during development was to meet the needs of wireless communications requiring an exceptionally high degree of isolation. It is designed for use in those areas and features two channels that operate at 3 and 5 GHz. In 2019, a small microstrip diplexer built on a dual closed-loop stepped impedance resonator (DCLSIR) was described by the authors of [19]. The DCLSIR stands for the building block of this diplexer. The necessary properties of the microstrip diplexer can be achieved by merging two DCLSIR-BFFs to be operated at 8.3/10 GHz. To create a new structure for design purposes, the authors of [20] proposed the idea of a planar dual-channel microstrip BPF in 2020. Given that the suggested diplexer has large fractional bandwidths (FBWs %) of 34.3% for the first channel and 38.6% for the second channel. It could be used in broadband applications.