## 2 State of the Art
For that reason, it is suitable for use in applications that necessitate broadband speeds. In contrast to the 1.6 GHz used by L-band, the WiMAX resonant frequency is 3 GHz. The microstrip diplexer was developed to be utilized in GSM applications. Before it was put into operation, this diplexer was supposed to operate at frequencies of 0.78 GHz and 1.85 GHz, respectively [21]. The thin cells and patch served as the design keywords of this device. Because the resonator that has been proposed has a symmetrical construction, research into both the odd and even modes must be undertaken to gather knowledge concerning the resonator behavior. The diplexer has provided high levels of isolation, low levels of insertion losses, and very good return losses. An innovative compacted substrate integrated waveguide (SIW) diplexer with square cavities was proposed in [22] for C-band application. This diplexer operates at a frequency that is lower than the cutoff frequency that is determined by the cavity. It is responsible for applying the evanescent-mode propagation mechanism so that it can transmit signals. When the center frequency is adjusted to 6.16 GHz and 7.72 GHz, respectively, insertion losses of 0.83 dB and 0.77 dB are detected in the lower and higher passbands. These frequencies correspond to the lower and higher passbands, respectively. About 5.1% fractional bandwidth is provided for every passband provided by the diplexer that has been presented regardless of the passband. In addition to that, it provides an isolation level of 22 dB. In [23], the authors have developed a microstrip diplexer that is not only extremely isolated but also extremely small in terms of the electrical components that it contains. Specifically, this specific diplexer is intended to be utilized in the Radio Occultation (RO) System. It is utilized in a significant manner to investigate a wide range of properties that are associated with the atmospheres of planetary systems. Many of these features are the subject of ongoing research. Research on planetary atmospheric properties is currently underway using the radio occultation technique. The onboard RO transmitter depends on the frequency synthesizers to ensure its proper functioning. This transmitter operates in two distinct frequency bands, with one at 6.7 GHz and the other at 8.4 GHz. A miniature narrow-band microstrip diplexer with two channels has been perceived in [24] using an FR4 substrate. Two bandpass filters, each operating in a specific frequency range, connect at a single junction to create it. The standard components of every BPF include input and output feedlines, a resonator with meandering lines, a step impedance resonator (SIR), and a uniform impedance resonator (UIR). In a recent study, an artificial neural network (ANN) was utilized to aid in the build-up and design of a high-performance microstrip diplexer [25]. An ANN model takes into account various factors. These factors to consider are the input frequency, substrate type, substrate thickness, and physical dimensions. Another output from this model is the generation of group delay values (GD), along with S-parameters. Neural networks trained via back propagation usually function within a frequency range of 1.4 to 3 GHz. To construct multifunction integrated diplexers that are capable of carrying out operations such as frequency selection, frequency division, and power division, the paper reported in [26] came up with an innovative circuit model. This unique circuit model was developed to develop multifunction integrated diplexers. The highly integrated multifunctional diplexer that has been presented is composed of dual simple multimode resonators that operate at distinct frequencies, in addition to dual isolation resistors. This diplexer is designed to perform multiple functions simultaneously for integrating the two multimode resonators based on the distributed coupling method. The responses have shown both the second- and third-level order. Both of these orders have been given consideration.