This chapter presents a microstrip implementation of the diplexer circuit model established in Chapter 4. The microstrip square open-loop resonator (SOLR) method reported in Hong and Lancaster (1996) has been utilised in the implementation of the microwave diplexer. The SOLR is very popular in the implementation of microwave passive devices including filters and diplexers, due to its compact size. In this chapter, the SOLR is employed in the implementation of the 10th order (10-pole) microwave diplexer established in Chapter 4. The simulation and measurement results show good agreement, with a band isolation of about 50 dB achieved between the transmit (Tx) and the receive (Rx) bands of the diplexer.
Microstrip Square Open-Loop Resonator
The microstrip square open-loop resonator (SOLR) components have compact size, simpler structure, and are flexible to construct when compared to waveguide cavity components (Hong & Lancaster, 1996). The compact size of the microstrip SOLR has led to its popularity in filter design. The microstrip SOLR is achieved by simply folding a half-wavelength resonator into a square-shaped resonator, having an open gap on one of its four sides as shown in Figure 1.
Figure 1. Microstrip square open-loop resonator structure
The length of one side of the resonator is approximately equal to one-eighth of a wavelength (λ/8). When two of the SOLR structures are placed side-by-side, with a separation distance, s; proximity coupling due to fringe fields results. The nature and the strength of the existing coupling depend on the nature and the extent of the fringe fields. It has been shown that at resonance of the fundamental mode, each of the microstrip SOLR possess the maximum electric field density at the side with the open gap. Similarly, the side opposite the open gap has the maximum magnetic field density (Hong & Lancaster, 1996). The coupling coefficient between any two microstrip SOLRs depends on the separation distance, s. An increase in the value of s, leads to a corresponding decrease in the coupling coefficient and vice versa. Figure 2 shows the different possible kinds of couplings that can be achieved with the microstrio SOLR.
Figure 2. Microstrip square open-loop resonator, (a) uncoupled, (b) electric coupling, (c) magnetic coupling, (d) mix coupling
The microstrip SOLRs utilised in realising the microwave diplexer presented in this chapter, were designed to have the dimensions shown in Figure 3. The transmit band resonator (Tx), the receive band resonator (Rx), and the energy distributor resonator (ED), all correspond to the BPF1, the BPF2, and the DBF component filters, respectively as explained in Chapter 4.
Figure 3. Microstrip square open-loop resonator dimensions, (a) transmit band dimension, (b) energy distributor dimension, (c) receive band dimension
All dimensions were achieved based on the design specifications (that is, the centre frequencies of the respective component filters). The full-wave simulation layout and responses, in ADS Momentum, for achieving the Tx, Rx and ED resonators dimensions is shown in Figure 4, where fTx, fRx and fED are the fundamental resonant frequencies for the Tx, Rx and ED resonators, respectively.
Figure 4. Full-wave simulation layout and responses for the Tx, the Rx, and the ED resonators at their respective fundamental resonant frequencies