Design on fish-bone shaped tri-plate transmission line filter at millimeter wave lengths

The tri-plate transmission line is usually fabricated by etching copper-coated dielectric substrates to support the center conductor. However, the dielectric loss of the dielectric substrates is no longer negligible increasing operational frequencies, and this fact causes difficulty for loss reduction of filters at millimeter-wave frequencies. With this in mind, the tri-plate transmission line structure to be able to support the center conductor without the dielectric substrates was devised and a bandpass filter made by only metal strip was designed and fabricated as a trial at 30 GHz.