Abstract
High Q-factor open-box mode resonances have been found in the microwave measurements of several 3-D printed dielectric-filled metal-pipe rectangular waveguides (MPRWGs). These parasitic Fabry-Perot eigenmodes are confined by the conductive walls in the transverse plane of the MPRWG and partially confined by the air-dielectric and dielectric-air boundaries in the longitudinal direction. The excitation of open-box modes was previously speculated to be due to the inhomogeneous and/or anisotropic nature of the 3-D printed dielectric-fillers. This has now been confirmed, by representing the inhomogeneous and anisotropic nature of the woodpile-like dielectric structure (physical realm), with an anisotropic dielectric constant tensor (simulation realm). Analytical and numerical eigenmode solvers, previously used by the authors with MPRWGs, are applied here to parallel-plate waveguides (PPWGs) and circular waveguides (CWGs); identifying all the individual parasitic open-box modes. With the former, its TM11 mode exhibits an ultra-high Q-factor of approximately 2,300 at X-band, which is considerably higher than those found with other modes and in other waveguide structures. Finally, a numerical full-wave frequency-domain simulator that employs the dielectric constant tensor is introduced in this paper. This new modeling technique independently confirms that open-box modes are excited in 3-D printed dielectric-filled MPRWG, PPWG and CWG structures. This paper provides the foundations for accurately modeling parasitic resonances associated with inhomogeneities and anisotropy in 3-D printed microwave components; not just the metal-walled waveguide structures considered here, but the methodology could also be extended to generic 3-D printed dielectric waveguides and substrate-based transmission lines.
Highlights
Over the past decade, 3-D printing of microwave components has gained increasing popularity, due to the availability of affordable 3-D printers that provide a cost-effective solution for prototyping microwave components [1]
Open-box mode resonances have been found in microwave measurements of 3-D printed cuboid samples within metal-pipe rectangular waveguides (MPRWGs)
These parasitic Fabry-Pérot eigenmodes were either ignored or not rigorously investigated. These parasitic resonances can exhibit very high Q-factors, which may offer the potential for future exploitation
Summary
3-D printing of microwave components has gained increasing popularity, due to the availability of affordable 3-D printers that provide a cost-effective solution for prototyping microwave components [1]. In 2015, using an ultraviolet (UV)-cured ink-jet type 3-D printer, the U.S Airforce Institute of Technology investigated both solid (homogenous) and periodic (inhomogeneous) polytetrafluoroethylene (PTFE) cuboids; the latter introduces periodic airgaps (rectangular inclusions) to create a biaxial anisotropic dielectric constant [7] With this biaxial anisotropic material, X-band measurements of a dielectricfilled WR-90 MPRWG clearly show an open-box mode resonance at 10.4 GHz (not mentioned in their report). Open-box modes were not found with conductive PLA cuboids, as they are heavily damped by the presence of carbon black powder [6] These open-box modes were investigated using both analytical and numerical eigenmode solvers, but our analyses were restricted to just MPRWGs. we used an irregular coarse tetrahedral meshing scheme within COMSOL Multiphysics® frequency-domain simulator to excite the open-box modes; representing an artificial form of inhomogeneity and/or anisotropy (to demonstrate proof of concept). Throughout this paper, our analytical methods adopt the simplified textbook power-loss approximations
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