Graduation Year

2017

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Electrical Engineering

Major Professor

Thomas M. Weller, Ph.D.

Committee Member

Kenneth Church, Ph.D.

Committee Member

Nathan Crane, Ph.D.

Committee Member

Gokhan Mumcu, Ph.D.

Committee Member

Arthur David Snider, Ph.D.

Committee Member

Jing Wang, Ph.D.

Keywords

Additive manufacturing, picosecond laser machining, mesh, modeling, 3D-printing

Abstract

The potential of Additive Manufacturing (AM) for microwave and mm-wave applications is increasingly being revealed thanks to recent advancements in research. AM empowers engineers with new capabilities to manufacture complex conformal geometries quicker and at lower costs. It allows, for instance, the embedding of RF front ends into functional structures. In this dissertation, two aspects of AM are explored: (a) The development and characterization of techniques that improve the performance of AM microwave circuits and antennas, and (b) study of complex geometries, such as meshed structures, as an alternative to reduce material usage, cost, and weight of the components.

Micro-dispensing of silver paste (CB028) is extensively used in this work as a viable approach for manufacturing microwave planar transmission lines. However, the performance and upper-frequency range of these lines are limited by the cross-sectional shape and electrical conductivity of the printed paste, as well as the achievable minimum feature size which is typically around 100 μm. In this work a picosecond Nd:YAG laser is used to machine slots in a 20-25 μm-thick layer of silver paste (Dupont CB028) that is micro-dispensed on a Rogers RT5870 substrate, producing coplanar waveguide transmission lines with 16-20 μm-wide slots. It is shown that the laser solidifies 2 μm wide region along the edges of the slots, thus significantly increasing the effective conductivity of the film and improving the attenuation constant of the lines. The extracted attenuation constant at 20 GHz for laser machined CB028 is 0.74 dB/cm. CPW resonators and filters show that the effective conductivity is in the range from 10 MS/m to 30 MS/m, which represents a 100x improvement when compared to the values obtained with the exclusive use of micro-dispensing.

Another main aspect of this dissertation is the study of meshed structures in coplanar waveguides. For most AM processes the materials utilized for the conductive layer are the most expensive ones; hence, there is value in minimizing the conductor surface area used in a circuit. In this work, the approach of meshed ground coplanar waveguide (MGCPW) is analyzed by simulating, fabricating and measuring a broad set of meshed ground geometry sizes. Furthermore, a physical-mathematical model is presented, which predicts the characteristic impedance and the capacitance per unit length of MGCPW with less than 5.4% error compared to simulated data. A set of filters is designed and fabricated in order to demonstrate the approach. The main parameter affected by meshing the ground plane is the attenuation constant of the waveguide. It is shown that 50% mesh density in the ground plane of a MGCPW line can be used with less than 25% increase in the loss. In contrast, the loss of finite ground coplanar waveguide (FGCPW) can increase by as much as 108% when the ground size is reduced by the same factor (50%). Both 3D printing (micro-dispensing) and traditional printed circuit board manufacturing are used in this work, and most of the propagation characterization is performed at 4 GHz.

A meshing technique is also applied to rectangular waveguides, and its effects are studied. It is presented as an option for high power, low loss, but also reduced weight applications. A set of meshed Ku-band waveguides was fabricated using binder jetting 3D printing technology showing that the weight can be reduced by 22% with an increase in loss of only 5%, from 0.019 dB/cm for the solid part to 0.020 dB/cm average across the band with the meshed design. Further weight reduction is possible if higher loss is allowed. To demonstrate the concept, a comparison is made between non-meshed and meshed waveguide 4 pole Chebyshev filters.

Finally, the BJ technology is characterized for Ku-Band rectangular waveguide and reflector antenna applications. This technology is characterized using electron beam microscopy (SEM) and energy dispersive spectroscopy (EDS). The RF performance of the 3D printed circuits is benchmarked with Ka-band cavity resonators, waveguide sections, and a filter. An unloaded resonator Q of 616 is achieved, and the average attenuation of the WR-28 waveguide section is 4.3 dB/m. The BJ technology is tested with a meshed parabolic reflector antenna, where the illuminating horn, waveguide feed, and a filter are printed in a single piece. The antenna shows a peak gain of 24.56 dBi at 35 GHz.

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