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A direct synthesis procedure is presented for the design of transmission line low-/high-pass filters with series stubs. The proposed prototype consists of a cascade of series stubs of electrical length thetasc alternating with uniform transmission lines of electrical length 2thetasc. An exact network synthesis technique is applied on the fully frequency-distributed transmission line filter structures. By short-circuiting these series stubs, a low-pass prototype can be established to synthesise the filters with both predictable in-band and out-of-band behaviours. Instead, if the series stubs are open-circuited, a quasi-high-pass or wide band-pass filter can be designed with specified frequency bandwidth and centre frequency. Based on the formulation of a transfer function, a set of closed-form design formula is derived so that the synthesis design can be carried out for the proposed filters with Chebyshev equal-ripple frequency responses in the concerned low- and high-pass bands. As design examples, two five-pole planar filters based on the low- and high-pass topologies are implemented by using hybrid slotline and coplanar-waveguide structures. After final optimisation is executed via full-wave electromagnetic software to take all the discontinuities into accounts, two designed filters are fabricated and measured to verify the proposed synthesis approach and confirm the theoretical predictions.
Date of Publication: June 2009