Synthesis of Fourth-Order Rectangular Coaxial Bandpass Filter With Four Transmission Zeros via CNC and Additive Manufacturing Techniques

In this paper, a fourth-order coaxial-line bandpass filter is designed using a novel rectangular coaxial transmission line resonator supported by low-cost additively manufactured insulator support. The filtering response is systematically synthesized, and the unloaded Q factor Quas well as the resonant frequency f0of the proposed resonator are thoroughly analyzed. Notably, the filter demonstrates the ability to generate four transmission zeros without requiring additional structural components, representing a competitive advantage in filter synthesis theory. The filter is fabricated with conductors machined via CNC techniques and insulator supports produced through low-cost additive manufacturing. Measured results show good agreement with simulated and synthesized responses, with a center frequency of 2.00 GHz and a fractional bandwidth of 6.50 %. Four transmission zeros are observed outside the passband, and the measured insertion loss is approximately 0.70 dB, indicating highly competitive performance.