This paper first reviews manufacturing technologies for realizing air-filled metal-pipe rectangular waveguides (MPRWGs) and 3-D printing for microwave and millimeter-wave applications. Then, 3-D printed MPRWGs are investigated in detail. Two very different 3-D printing technologies have been considered: low-cost lower-resolution fused deposition modeling for microwave applications and higher-cost high-resolution stereolithography for millimeter-wave applications. Measurements against traceable standards in MPRWGs were performed by the U.K.’s National Physical Laboratory. It was found that the performance of the 3-D printed MPRWGs were comparable with those of standard waveguides. For example, across X-band (8–12 GHz), the dissipative attenuation ranges between 0.2 and 0.6 dB/m, with a worst case return loss of 32 dB; at W-band (75–110 GHz), the dissipative attenuation was 11 dB/m at the band edges, with a worst case return loss of 19 dB. Finally, a high-performance W-band sixth-order inductive iris bandpass filter, having a center frequency of 107.2 GHz and a 6.8-GHz bandwidth, was demonstrated. The measured insertion loss of the complete structure (filter, feed sections, and flanges) was only 0.95 dB at center frequency, giving an unloaded quality factor of 152—clearly demonstrating the potential of this low-cost manufacturing technology, offering the advantages of lightweight rapid prototyping/manufacturing and relatively very low cost when compared with traditional (micro)machining.

3-D Printed Metal-Pipe Rectangular Waveguides

This paper presents the modeling, design, fabrication, and measurement of microelectromechanical systems-enabled continuously tunable evanescent-mode electromagnetic cavity resonators and filters with very high unloaded quality factors (Qu). Integrated electrostatically actuated thin diaphragms are used, for the first time, for tuning the frequency of the resonators/filters. An example tunable resonator with 2.6:1 (5.0-1.9 GHz) tuning ratio and Qu of 300-650 is presented. A continuously tunable two-pole filter from 3.04 to 4.71 GHz with 0.7% bandwidth and insertion loss of 3.55-2.38 dB is also shown as a technology demonstrator. Mechanical stability measurements show that the tunable resonators/filters exhibit very low frequency drift (less than 0.5% for 3 h) under constant bias voltage. This paper significantly expands upon previously reported tunable resonators.

High- $Q$ Tunable Microwave Cavity Resonators and Filters Using SOI-Based RF MEMS Tuners

3-D printing allows increased design flexibility in the fabrication of microwave circuits and devices and is reaching a level of maturity that allows for functional parts. Little is known about the RF and microwave properties of the standard materials that have been developed for 3-D printing. This paper measures a wide variety of materials over a broad spectrum of frequencies from 1 MHz to 10 GHz using a variety of well-established measurement methods.

Broadband Microwave Frequency Characterization of 3-D Printed Materials

A novel synthesis technique to integrate high-Q 3-D filters with highly efficient slot antennas is presented in this paper. This technique allows for compact integration of 3-D filters and antennas with very high antenna efficiency and significantly reduced form factor of integrated RF front ends. Prototype four-pole Chebyshev cavity filters integrated with slot antennas are demonstrated at X-band using both coaxial and coplanar waveguide feeding. The center frequency and fractional bandwidth of the filter/antenna system with coaxial feeding are 9.96 GHz and 6.0%, respectively. Due to the high-Q factor (~850) of the cavity resonator, the efficiency of this filter/antenna system is measured to be 89%, compared with the measured S21 of -0.5 dB (89%) for an identical filter. This means a near 100% efficient slot antenna is achieved within this integrated filter/antenna system. The measured impedance matching, efficiency, gain, and radiation pattern closely agree with simulation results. Equivalent-circuit models of the integrated filter/antenna system are developed and verified with full-wave simulations. This technique can be applied for filter/antenna integration in all microwave, millimeter-wave, and submillimeter-wave frequency regions.

Compact Low-Loss Integration of High- $Q$ 3-D Filters With Highly Efficient Antennas

Waveguide slot array antennas can have high gain, since the power distribution network (waveguide) has low loss, enabling a large aperture size. Rapid response to requirements for frequency, gain, and skew angle inspired a search for an alternative to conventional, commercial slot array acquisition. The solution to this requirement is high-resolution 3-D printing of modified slot arrays combined with metal plating. Modification of the structure included removal of wall material in regions where these openings would not cause radiation, opening the structure to enable metal plating while not affecting gain. A 3-D-printed slot array requires no further assembly, unlike conventional waveguide-based arrays that require soldering or brazing, block machining, or plate assembled, brazed structures. Stereolithography 3-D printing of plastic slot array antennas, modified to enable metal plating, has been used to produce a 30 dBi realized gain slot array at 21 GHz for under $1000 USD in a few weeks. Metal plated, 3-D printed plastic slot arrays are also very lightweight in comparison with conventional metal structures. Performance of 3-D printed, metal plated slot arrays has been shown to be identical to conventional metal structures at frequencies up to 22 GHz.

3D Printed Waveguide Slot Array Antennas

High-Q evanescent-mode resonators and filters are realized by both silicon micromachining and layer-by-layer polymer processing. Capacitively loaded cavities can be reduced to a size much smaller than a wavelength, but still have a much higher unloaded Q than lumped elements. The loaded resonators are utilized for reduced-size filters with a low insertion loss enabled by the relatively high-Q factor. The small fabrication tolerances of silicon micromachining and polymer stereolithography processing enable the realization of highly loaded evanescent-mode resonators and filters. A 14-GHz resonator micromachined in silicon has a volume of 5 mm /spl times/ 5 mm /spl times/ 0.45 mm, representing a resonant frequency reduction of 66.8% compared to an empty cavity of the same dimensions. The polymer-based fabrication is used to create resonators of different three-dimensional geometries with Q's up to 1940 and frequency reductions up to 49.9%. An insertion loss of 0.83 dB is measured in a 1.69% bandwidth filter created by polymer processing with a frequency reduction of 47% compared to an unloaded cavity. The frequency sensitivity to fabrication tolerances of these structures is also analyzed.

Precision fabrication techniques and analysis on high-Q evanescent-mode resonators and filters of different geometries

Horn antennas are created using laser-based layer-by-layer stereolithography processing in which three-dimensional structures can be synthesized directly from three-dimensional CAD files. The horns are used as a simple demonstration of how to create three-dimensional objects using rapidly deposited two-dimensional layers of polymer. Two types of horns are created using the polymer fabrication and then metallized using a conductive ink seed layer. This ink can be further metallized by the use of electroplating, however, the effects are shown to be minimal due to the small current density at all locations within the horn. Two types of horn operating in Ku band are realized to test the capability of this technique to create precise features without adversely affecting the pattern and the matching to the horn. The average return loss from the horns is above 20 dB throughout the band of operation and the measured gains (10.15 dB and 14.58 dB) are accurate within 5% with respect to the simulated values.

Layer-by-layer stereolithography of three-dimensional antennas

Widely tunable high-Q evanescent-mode resonators are created using layer-by-layer polymer stereolithography process. The ability of polymer stereolithography process to fabricate thin flexible 3D structures with small fabrication tolerances enables the realization of highly loaded tunable evanescent-mode resonators. A tunable frequency range of 5.4-10.9 GHz, which corresponds to an equivalent capacitance tunability of 400%, is obtained with quality factors ranging from 745 to 1,580 over the tunable frequency range.

S. Hajela

Papers

Precision fabrication techniques and analysis on high-Q evanescent-mode resonators and filters of different geometries

Layer-by-layer stereolithography of three-dimensional antennas

Widely tunable high-Q evanescent-mode resonators using flexible polymer substrates