Diplexer is a device used for either splitting a frequency band into two sub-bands or for combining two sub-bands into one wide band. It is popularly used in satellite communication systems to combine both the Tx and the Rx antennas on space craft. Microwave diplexer is commonly used in the Radio Frequency (RF) front end of cellular radio base stations to separate the Tx and the Rx channels.
Conventionally, microwave diplexers are achieved by connecting two separately designed filters together via an external energy distribution device. This connecting device could be a T-junction, a Y-junction, a circulator, a manifold or a common resonator. In this article, we present without an external junction a microwave diplexer , and replacing it with resonators which provide resonant poles in the diplexer response.
Here is a design flow and a simulation of a 10-pole (10th order) diplexer that has been successfully designed and simulated. The diplexer is composed of 2 poles from the dual-band filter, 4 poles from the Tx bandpass filter, and the remaining 4 poles from the Rx bandpass filter. The design was implemented using asynchronously tuned microstrip square open-loop resonators. Figure 1 shows the design of the diplexer using SolidWorks.
Figure1- Geometry of Multi-Coupled Resonator Microwave Diplexer
Fig. 2 shows the diplexer layout which consists of a multi-coupled resonator designed using SolidWorks. The diplexer fed by a 50 microstrip-line (width WF = 1.1 mm) for each port, the center frequency is 1.84 GHz, The RT/Duroid 6010LM substrate with a dielectric constant of 10.8, a loss tangent of 0.0023, and a substrate thickness of 1.27 mm was used for the simulation. The geometrical parameters of the multi coupled resonator diplexer (S1, S2, S3, S4, S5, S6, S7, S8, S9, t1 and t2) are defined in a file and they will be imported to SolidWorks as equations in order to facilitate and accelerate the workflow.
Figure2 - Diplexer layout and fabricated model 
The table below summriazes the diffrent dimensions (global variables) to be imported into SolidWorks.
Table1 - summarizes the list of dimensions to be imported into SolidWorks
The diplexer circuit model was simulated using HFWorks S-Parameters analysis. Figure 3, depicts the distribution of the electric field and figure 4 shows the simulated results S-parameters of the geometry of Multi-Coupled Resonator Microwave Diplexer.
Figure3 - Distribution of the electric field at 1.9GHz
Figure4 - Simulation responses of the diplexer circuit model with HFWorks
Figure5 - Measured and simulated (with ADS) results of the diplexer circuit model
The measured results and the EM loss simulation results with HFWorks are presented, in Fig.4 and indicated that the isolation between the Tx and Rx bands is about 40 dB. The graphs clearly show a good agreement between the simulation and measurement.
 Augustine O. Nwajana, Kenneth S. K. Yeo, " Multi-Coupled Resonator Microwave Diplexer with High Isolation", 2016 European Microwave Conference, pp. 1167-1170, 4–6 Oct 2016, London, UK.