Microstrip patch antennas and antenna arrays may be the least visible but most widely used microwave antennas. They are elegant in their simplicity, formed by feeding a voltage to a metal conductor patch atop a dielectric layer that isolates it from a ground plane. Microstrip patch antennas have been formed with air as the dielectric. More typically, however, they are fabricated on printed-circuit-board (PCB) materials using photolithography to etch fine features into thin conductive metal films. Because PCB materials are important components for microstrip patch antennas and antenna arrays, their characteristics should be carefully considered when designing a microstrip antenna, including the PCB’s conductive metal for the patch and ground plane. Copper is popular since it is light in weight and low in cost, with high conductivity. PCB manufacturing processes also contribute to how well a microstrip patch antenna will perform since a manufacturing process can determine circuit tolerances and add ingredients, such as a final plated finish, to the PCB which can impact antenna performance. Understanding how PCB materials work together and how they are shaped into a final design by manufacturing processes can help achieve desired performance goals for a microstrip patch antenna or antenna array even when reaching for millimeter-wave frequencies.
The size of a microstrip patch antenna is inversely proportional to frequency, with smaller patches at higher frequencies. Because of this relationship, with longer wavelengths at lower frequencies, microstrip patch antennas are too large to be practical at lower frequencies. The length, L, of a rectangular microstrip patch is typically one-third to one-half the wavelength of a frequency of interest, i.e., L= λ/3 to λ/2. This results in large resonant patches at frequencies less than 500 MHz. At 100 MHz, a rectangular microstrip patch would be about 1 m long, a large PCB. Higher frequencies result in smaller patches, which make more sense when integrating onto PCBs. When more gain and directivity is needed than supplied by a single patch, arrays or series connections of patches can be designed and fabricated. Microstrip antennas are attractive for use at microwave through millimeter-wave frequencies where they can be integrated with other circuitry on a PCB.
Any choice of PCB material for an electronic design with a microstrip patch antenna should consider the effects of the PCB’s material “ingredients,” such as its dielectric core, its conductive metals, and its final surface finish, and how they will affect antenna and circuit performance and behavior. The permittivity of the dielectric material, for example, will influence the dimensions of a one-third to one-half-wavelength patch. The thickness of the dielectric material will determine the height of the electromagnetic (EM) field radiated by the antenna as a result of a voltage applied to the microstrip patch’s feedline.
Most of a microstrip patch antenna’s EM fields lie between the patch and the ground plane, although EM fields also radiate from the lengthwise edges or “L” dimension of the rectangular patch (with no EM radiation from the edges of the rectangular path in the width or “W” dimension). The PCB’s final plated finish can have a strong influence on the nature of these EM fields from the edges of the patch. For example, a PCB with copper conductor lends its own conductivity characteristics to a microstrip patch antenna, but the conductivity will be complicated by the nature of the final plated finish. A popular and proven final plated finish is electroless nickel immersion gold (ENIG) in which thick nickel is combined with thin gold. While protecting the conductors, nickel is a ferromagnetic material and suffers magnetic losses. It has only about one-quarter the conductivity of copper, adding to conductor loss and impacting phase response especially at higher frequencies. The loss of conductivity from an ENIG finish will impact the fields generated by a microstrip patch antenna, more noticeably at higher microwave and millimeter-wave frequencies.
The choice of PCB conductor finish is a factor to consider in a microstrip patch antenna due to how the finish changes the conductor loss at the edges of the patch, even at frequencies below 6 GHz. For an ENIG finish applied to a copper conductor patch, the edges consist of a changing percentage of copper, gold, and nickel compared to the more consistent blend of the three conductive metals on the top surface of a copper patch. Many PCB final plated finishes are applied by an immersion process, including immersion tin or immersion silver finishes. The processes result in extremely thin coatings with minimal variations in thickness across the patch. This is especially significant at higher, millimeter-wave frequencies where thickness variations can impact small-wavelength signals.
Design for Manufacturing
High-performance microstrip patch antennas result from high-grade circuit materials treated with high-precision manufacturing processes. While microstrip antennas may be unrealistic at lower RF and microwave frequencies, the appeal of using the technology at higher, millimeter-wave frequencies must be practical, with antenna designs backed by manufacturing processes capable of creating hardware with repeatable dimensions and within realistic budgets. Just as microstrip patches are quite larger at lower frequencies, they can be excessively diminutive at higher millimeter-wave frequencies, such as the series-fed patch antennas commonly used for 77-GHz automotive radar systems. At such high frequencies and small wavelengths, variations in material properties can combine with variations in manufacturing processes for unacceptable deviations in antenna performance. Microstrip patch antenna designs must take into account not only PCB material characteristics but the limitations of manufacturing processes that will transform those PCB materials into microstrip patch antennas and antenna arrays.
As an example, an ideal rectangular microstrip radiating patch has sharp 90-deg. corners. But during fabrication, when a copper conductor is etched to form a patch according to design parameters, the etching process typically produces rounded corners. Rounded corners with radii of 2 to 5 mils may have minimal impact at lower frequencies, but a rounded corner with 3-mil radius will impact signal wavelengths at millimeter-wave frequencies, potentially causing degenerate EM modes and variations in a microstrip patch antenna’s radiation pattern. The effects of a 2-to-5-mil rounded corner can be modeled with commercial EM computer-aided-design (CAD) simulation software, but the simulated performance may not align with actual performance if the antenna is not manufactured to the same tolerances prescribed in a CAD tool.
At millimeter-wave frequencies such as 77 GHz, circuit material quality and manufacturing processes must also minimize variations in circuitry leading to and from microstrip patches, especially for millimeter-wave applications. Circuit junctions and interconnections, such as feedlines to the microstrip patch, must be closely impedance matched (typically 50 Ω) to minimize signal reflections that result in EM radiation losses at those junctions. Additionally, there are different conductor widths used for impedance transforming. For series-fed patch antennas used in 77 GHz radars, small-wavelength signals are carried by narrow conductors and significant variations in conductor width can degrade antenna performance. A PCB fabrication process capable of etching linewidths with ±0.5-mil tolerance may be noteworthy, but it may lack the precision and accuracy needed at millimeter-wave frequencies. For microstrip feedlines with nominal conductor widths of 4 to 5 mils, a ±0.5-mil (1-mil total) variation represents a 20% or more variation in circuit geometry and possibly in antenna radiation pattern.
Understanding the effects of conductors and dielectric materials on the generation of EM radiation from a microstrip patch antenna or antenna array can help predict the amount of gain and directivity from a patch with carefully controlled dimensions at a particular frequency of interest. But any PCB manufacturing process must also be capable of delivering the fine dimensions of the patch and its associated circuits with the tightest tolerances possible or the predictions from the simulations will be off. By accounting for both material behavior and manufacturing process variations, microstrip patch antennas and antenna arrays can be produced repeatably and cost-effectively for all frequencies of interest, including for the growing numbers of applications at millimeter-wave frequencies.
Producing a microstrip patch antenna or any high frequency component on a PCB such as high-performance circuit materials from Rogers Corp. requires a practical design that can be mass-produced with modern manufacturing methods. Superior materials support high performance, but they also must be properly and efficiently assembled into the final component to form a practical solution. To help circuit, component, device, and system designers better work with high-performance circuit materials for high frequency analog, high speed digital (HSD), and mixed-signal circuits, Rogers Corp. is assembling an educational ebook on their circuit materials and the optimum transformation of those materials into practical component and circuit solutions, such as microstrip patch antennas and antenna arrays. The ebook will explore circuit materials for a wide range of industries, including aerospace, commercial, industrial, medical, military, and space applications, and offer advice on selecting the right material for an application and “design for manufacturing” guidelines to turn that material into practical product solutions, at millimeter-wave frequencies and beyond.
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