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Laser Formed Fabrication of Conformal and Non-Conformal Millimeter and sub-Millimeter Wave Antennas

Description:

TECHNOLOGY AREA(S): Materials 

OBJECTIVE: The objective is to leverage recent advances in laser forming of metals to develop a cost effective manufacturing capability for conformal and non-conformal millimeter and sub-millimeter wave antennas that have complex shape, smooth surfaces, micron scale features, and bulk metal conductivity. 

DESCRIPTION: Of interest to the Army is the potential for a given technology to merge capabilities and operate in contested environments which is a goal of the Network C3I Army Modernization Priority with which this topic aligns. Ultra-wideband (UWB) antennas that operate with several decades of bandwidth and cover millimeter and sub-millimeter wave (mmW and s-mmW) frequencies enable backend hardware flexibility without replacing the antenna. What’s more, UWB antennas with complex shape that have polarization diversity can merge capabilities that require different polarizations and operate at different frequency bands. Fabricating antennas at mmW frequencies is challenging given the small features, especially when the antenna has a complex shape, such as a double curved surface. MMW antennas require micron-scale precision and high electrical conductivity. Additive manufacturing (AM) techniques are the current cost effective means for fabricating antennas of the prior description at sub-mmW frequencies, but the capability of AM to fabricate mmW antennas is limited. While AM offers a fine level of control such that complex 3D geometries can be manufactured, the technology does have some particular limitations such as less than bulk metal conductivity inks and pastes that require plating and rough surfaces which can be detrimental to the antenna performance [1] – [4]. Multi-scale prints, where a large antenna has small features, can also pose a challenge for AM techniques, and the time to print multi-scale antennas can be extensive. Some of these challenges may not be so significant for some antennas, but is significant for antennas such as the ultra-wideband (UWB) and polarization diverse multi-arm conical sinuous antenna [5], the impulse radiating UWB TEM horn [6], or the high-power and high-gain slot array in waveguide [7] designed for millimeter and sub-millimeter wave (mmW and s-mmW) frequencies. As such, a technology gap exists. A potential solution to the technology gap is a fabrication capability based on laser forming metal sheets. Laser forming is a method by which sheets of metal can be made to bend or buckle by the application of localized heat through a laser [8 – 9]. Judiciously choosing the locations and paths that the laser will heat can be used to create 3D shapes such as cuboids, coils, and doubly curved surfaces (e.g. the conical sinuous antenna and the TEM horn) [10]. Combining laser folding with laser cutting and welding allows for the manufacturing of closed geometries with features cut into them (e.g. the slot array in waveguide). Remaining research questions to be answered include thermal optimization of the process, improving sidewall roughness of cut surfaces, incorporation of welding, and process repeatability. In addition, developing the process such that the antenna technology developer can move a design from a computational electromagnetic modeling tool, such as HFSS, FEKO, or CST, to the laser device would better facilitate rapid prototyping of conformal antennas with small features. Using laser induced heat to deform metallic sheets like copper, steel, and brass to make 3D shapes has been demonstrated [10 – 13]. Application of the technique to a slot array in waveguide at w-band was also attempted but the lack of a combined laser welding function prevented the antenna from being completely closed [13]. It has also demonstrated that aluminum foam can be bent using laser heating [14]. The laser formed fabrication of antennas would prove beneficial to the commercial sector. The new fabrication capability could prove cost effective and would expand the domain of antenna types that can be fabricated at mmW frequencies. The capability would also allow for the fabrication of smoother, more precise, and subsequently better performing conformal antennas. 

PHASE I: Phase I shall explore the technical merit and feasibility of a laser forming based fabrication concept for mmW and s-mmW antennas to be executed in Phase II. The concept should prioritize commercially available laser systems. A study will address the research questions regarding optimal laser parameters for forming steel, aluminum, brass, and copper sheets with thickness ranging from 10 um to 1 mm to predict laser settings (i.e. power, exposure time, etc...) that are needed to optically cut, form, and weld the metal sheet into a 3D geometry and quantify the angle of resolution for bending and buckling. The laser cutting must achieve positional accuracy of no more than 10 microns, RMS surface roughness and line edge roughness less than 1 micron for both internal and external features, and minimum line width and spacing of 10 um. Laser forming angle resolution must be no more than one degree. The welding process must be within roughness spec, structurally robust, and maintain high electrical conductivity (>50% bulk conductivity). Documented process demonstration and study results are the primary deliverable. 

PHASE II: Phase II will build on the Phase I concept by refining the process to fabricate and deliver two RF antenna prototypes that demonstrate the precision and repeatability of the Phase I process. The antenna designs are an upper W-band longitudinal shunt slot array and a 4-arm C- to upper Ka-band conical sinuous antenna. Antennas should be characterized through laboratory measurements of the reflection coefficient, radiation pattern, and efficiency. The measured results of the antennas and devices should be compared to simulation results acquired by using computational electromagnetic software such as CST, FEKO, or HFSS. The measured reflection coefficient should be better than -10 dB and the gain should be within 3 dB of the simulated results. The end of Phase II should demonstrate antennas using a complete laser forming manufacturing capability and deliver said antennas for evaluation. 

PHASE III: Phase III will focus on the commercialization of fabrication technology for mmW and s-mmW antennas. The final process should demonstrate consistency and repeatability in manufacturing antennas for both military and commercial applications. Commercialization of the technology would be of use to the antenna development community as a whole by providing a cost effective means to producing high performance conformal antennas at mmW and s-mmW frequencies. 

REFERENCES: 

1: G. Zhang, et al., "Investigation on 3-D-Printing Technologies for Millimeter-Wave Terahertz Applications," Proceedings of the IEEE, Vol. 105, No. 4. April 2017.

KEYWORDS: Laser Forming, Sub-millimeter Wave, Millimeter Wave, Conformal, Antennas, Manufacturing Processes 

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