Rapid Prototyping of Conformal Antenna Structures

Introduction: The size of portable and mobile devices is continuously shrinking. This requires new approaches for integrating the antennas into a system. Frequently, antennas are attached as an afterthought, and often in locations and orientations that expose them to easy damage. An alternative is to implement conformal antenna designs that are integrated in or on the housing of the mobile device. This offers considerable size and location flexibility in comparison to what can be accomplished by using traditional antenna structures. Conformal antennas exhibit uniquely thin and unobtrusive profiles, reduced sizes, and potentially improved performance, making them well-suited for use in unmanned sensor and distributed autonomous systems. However, the adoption of conformal geometries for antenna designs must be taken into account during the system design process, as it impacts the antenna input impedance and radiation patterns. Therefore, any approach to the development of conformal antennas will benefit from the ability to go through the design, fabrication, and optimization cycle quickly and inexpensively. In particular, the fabrication of conformal antennas can be extremely slow and expensive. Rapid prototyping techniques based on laser machining and laser direct-write offer a unique capability to greatly reduce the time spent in fabricating these structures. When integrated with RF testing hardware, a real-time tuning capability can be implemented that further shortens the development cycle from weeks to hours.

The term "direct-write" refers to any technique or process capable of removing, depositing, dispensing, or processing different types of materials over various surfaces following a preset pattern or layout.1 The ability to accomplish both pattern and material transfer processes simultaneously opens the door for the development and manufacture of next-generation commercial and defense microelectronic systems. The potential offered by direct-write techniques lies in their ability to transfer and/or process most materials over any surface with extreme precision, resulting in a functional structure or working device. Direct-write technologies do not compete with photolithography for size and scale, but rather add a complementary tool for specific applications requiring conformal patterning, rapid turnaround and/or pattern iteration. Among those techniques, laser direct-write offers several advantages for the rapid prototyping of conformal antenna structures. Figure 7 shows the laser direct-write system used for performing 3-D laser micromachining and laser deposition on spherical surfaces of lightweight substrates compatible with conformal antenna designs.

FIGURE 7
Laser direct-write set up adapted for laser micromachining of conformal antenna paterns.

Technical Approach: Because of its durability and robustness, a spherical fiberglass dome was chosen as the substrate for the implementation of a conformal GPS antenna (1575 MHz) design. The integration of the antenna and matching structure onto the surface of the dome required a slot-based design to give a broad, circularly polarized radiation pattern for operation over a wide range of orientations. The fiberglass domes were coated with an ~100 µm thick conductive silver epoxy layer from which the conformal crossed-slot antenna and the coplanar waveguide (CPW) feeds (Fig. 8) were generated by laser micromachining. The degree of roughness present in the surface of the fiberglass dome affected the uniformity by which the depth and width of the laser micromachined trenches could be controlled. To correct for these variations, individual components of the antenna design were tested and then tuned by laser machining the required matching elements (inductive loading on the antenna slots, series stubs on the CPW).

FIGURE 8
A prototype crossed-slot conformal antenna made by laser direct-write on a metal-coated figerglass dome; inset shows a section of the tuning stubs for the CPW.

The crossed slots are fed by using two CPW transmission lines oriented 45 deg from the slots. CPW is used because it requires only one conductor plane and because its radiation loss can be minimized by the appropriate choice of dimensions and the use of air-bridges. The tuning stubs are implemented using series CPW shorted stubs placed inside the center conductor of the CPW feed line. The laser processing is critical to this approach, since it allows the implementation of these stubs with high precision in the very limited space available and conformal to the surface. The CPW lines transition to female right-angle connectors that are mounted below the equator inside the dome, allowing connection to measurement equipment or to the 90-deg hybrid and low noise amplifier (LNA).

The initial design of the antenna was validated using a 3-D finite-element modeling software package.2 Figure 9 shows both the simulated geometry as well as a sample set of radiation patterns. The patterns are typical of a curved or drooping crossed dipole, and show both good gain and polarization performance over greater than one hemisphere, allowing for greater variability in device orientation. A major challenge in the development of conformal antennas on composites is the lack of knowledge of material parameters. In these simulations, the material properties and dimensions were estimated to give a first-order solution. In the actual fabrication of the antenna, the design and manufacturing steps are planned to allow tuning and deduction of the material parameters as the machining progresses. The rapid prototyping ability discussed here is critical to this approach, as it allows quick and frequent machine/measure cycles that would be nearly impossible with other techniques.

FIGURE 9
Co- and cross-polarization radiation patterns (Φ = 0, 90 cuts) for the loaded crossed slots on the spherical dome shown in the inset.²

Summary: Laser direct-write is a valuable technique for the rapidly prototyping conformal antennas on arbitrary surfaces. A conformal antenna consisting of two orthogonal crossed slots, designed for operation at 1575 MHz, was laser micromachined on a fiberglass dome coated with a silver conductive layer. The feed network for this antenna, containing two CPW transmission lines and matching circuits, was also laser micromachined. The individual elements of the design are functional and show the expected performance. The fully integrated package, including hybrids and LNA, is expected to be completed and tested in the near future.

[Sponsored by ONR]