Thomas M. Weller

A 2.45 GHz Phased Array Antenna Unit Cell Fabricated Using 3-D Multi-Layer Direct Digital Manufacturing

This paper reports on the design, fabrication and characterization of a 3-D printed RF front end for a 2.45 GHz phased array unit cell. The printed unit cell, which includes a circularly-polarized dipole antenna, a miniaturized capacitive-loaded open-loop resonator filter and a 4-bit phase shifter, is fabricated using a direct digital manufacturing (DDM) approach that integrates fused deposition of thermoplastic substrates with micro-dispensing for deposition of conductive traces. The individual components are combined in a passive phased array antenna unit cell comprised of seven stacked substrate layers with seven conductor layers. The measured return loss of the unit cell is > 12 dB across the 2.45 GHz ISM band and the measured gain is -11 dBi including all components. Experimental and simulation-based characterization is performed to investigate electrical properties of as-printed materials, in particular the inhomogeneity of printed thick-film conductors and substrate surface roughness. The results demonstrate the strong potential for fully-printed RF front ends for light weight, low cost, conformal and readily customized applications.

A study on 3D-printed coplanar waveguide with meshed and finite ground planes

Direct digital manufacturing technology is gaining increasing attention for RF and microwave applications. The introduction of new materials and processes utilized by this technology has triggered the need to study their high frequency performance. Challenges with this type of fabrication include the intrinsic surface roughness of materials produced using fused deposition modeling (FDM), control of the metallization thickness, minimum feature sizes, and the tolerance on the component dimensions. In this work, a fully printed coplanar waveguide (CPW) is fabricated and characterized. In addition, finite ground CPW (FGCPW) and a meshed ground CPW are implemented to reduce ink usage and printing time. To the best of the authors knowledge this is the first study of a meshed ground CPW.

Meshed rectangular waveguide for high power, low loss and reduced weight applications

Additive manufacturing technologies are increasingly being demonstrated to be useful for microwave circuits, showing improved performance in multiple cases. In this work, a meshed rectangular waveguide structure 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.

High-k and low-loss thermoplastic composites for Fused Deposition Modeling and their application to 3D-printed Ku-band antennas

Four types of high-permittivity and low-loss electromagnetic composites for Fused Deposition Modeling (FDM) based on Cyclo-Olefin Polymer (COP) and Acrylonitrile Butadiene Styrene (ABS), reinforced by Ba_0.55Sr_0.45TiO₃, MgCaTiO₂, or TiO₂ co-fired micro-fillers, have been prepared and characterized up to 17 GHz using cavity resonators. Thin-sheet specimens made of 25% v/v COP-MgCaTiO₂, prepared by FDM, exhibit a relative permittivity ε_r of ~ 4.74 and loss tangent tanδ_d lower than 0.0018, while 25% v/v COP-Ba_0.55Sr_0.45TiO₃ specimens exhibit relative permittivity ε_r of ~ 4.92 and loss tangent tanδ_d lower than 0.0115. Meanwhile, 30% v/v COP-TiO₂ specimens exhibit a relative permittivity ε_r of ~ 4.56 and a record low loss tangent tanδ_d lower than 0.0014, while the 6% v/v ABS-Ba_0.55Sr_0.45TiO₃ specimens demonstrate relative permittivity ε_r of ~ 3.98 and loss tangent tanδd below 0.0088. Rectangular edge-fed patch antennas operating at ~ 17 GHz are fabricated using 3D printing to compare the size and performance against designs that used commercial microwave laminates.

Simultaneous RF electrical conductivity and topography mapping of smooth and rough conductive traces using microwave microscopy to identify localized variations

This paper presents a near-field microwave microscope (NFMM) capable of simultaneous non-contact imaging of electrical conductivity (σ) and topography across the surface of microwave circuits without the need of a distance sensor. The microscope monitors the resonant frequency of a dielectric resonator-based microwave probe to acquire the surface topography, and the quality factor to determine the electrical conductivity. Conductivity and topography images of copper foil reveal an average conductivity of about 4e7 S/m and an arithmetic roughness of 0.2μm, respectively. Measured average roughness and conductivity of CB028 silver paste are 0.7e6 S/m and 1.1μm, respectively. The NFMM data reveal significant and correlated variation in surface features and conductivity across the surface of the printed CB028 films. The topography and conductivity images obtained demonstrate that the NFMM can be employed for localized characterization of smooth and rough conductive materials used in microwave devices.

3D tag with improved read range for UHF RFID applications using Additive Manufacturing

In this paper a 3D tag antenna geometry is proposed for UHF RFID systems. Two antennas were built over different dielectric materials with similar properties using both Direct Digital Manufacturing and traditional photolithography and copper etching. The impedance matching between the antenna arms and the passive RFID integrated circuit was accomplished with the H-slot matching technique with a simulated 10 dB return loss bandwidth that allows the tag to operate in the American and European ISM RFID bands of 902-928 MHz and 864-868 MHz, respectively. The antennas were compared to commercially available tags in size, weight and read distance, showing a read range improvement of 136% (for a threshold power of 30 dBm) with respect to the best tag tested. A reduction in length of 78 % with respect to a planar 2D model was also achieved. The radiation patterns were measured, showing an omni-directional beam pattern.

Fabrication, Modeling, and Application of Ceramic-Thermoplastic Composites for Fused Deposition Modeling of Microwave Components

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Ka-band characterization and RF design of Acrynolitrile Butadiene Styrene (ABS)

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High-k and low-loss polymer composites with co-fired Nd and Mg-Ca titanates for 3D RF and microwave printed devices: Fabrication and characterization

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Propagation Characteristics and Modeling of Meshed Ground Coplanar Waveguide

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