Design of Single-Branch Laminated Waveguide Diplexers Using Modal Orthogonality

This paper develops novel laminated waveguide diplexers by taking advantage of the modal orthogonality of multiple cavity modes. The uplink and downlink channels of the diplexer can share a common filter structure, which minimizes the number of the cavities. The feeding probes and coupling slots are adequately located to achieve the required coupling coefficients and external quality factors at both passbands simultaneously. In this way, the matching network, i.e., T-junction, adopted in the traditional diplexer design can be eliminated. In addition, by the symmetric but orthogonal field distributions of two operation modes, nearly 30 dB of isolation can be achieved even by sharing the common cavities. The cavities are vertically stacked in low-temperature cofired ceramic technology, which further miniaturizes the circuit size. Two diplexers with third-order Chebyshev responses are designed and fabricated with different operation modes to show the flexibility in the band allocation. Experimental and simulated results are presented, both validating the design concept of the proposed diplexers.