Broadband microstrip-to-CPW feedthru design using air cavities

In the paper, a broadband microstrip-to-CPW feedthrough from DC to 40 GHz is designed based on air cavities etched on the RT/duroid substrate. Because air cavities etched on the substrate can decrease locally the effective relative dielectric constant, they can be used to change impedance, capacitance and inductance, especially compressing cutoff frequency of the first high-order model into higher frequency. Compared with many designs, because the air cavities are etched on the substrate and not under the strip, it is easy to fabricate this feedthrough. In the fabrication the feedthrough without air cavities is first fabricated, and then the air cavities are dug in the substrate. Based on the theory of air cavities, many similar models can be design. Numerical simulations valid that our idea is effective.