Designing a Patch Antenna to Add GPS to a Conference Badge

To most people antenna design and theory is much of a black art. However, with a little bit of eagerness and willing to learn anyone can build their own antenna design with good results. This is best demonstrated through Pepijn de Vos' journey on designing and prototyping a printed PCB antenna for GPS frequencies.

The motivation for the project came after speaking with a few colleagues at an MCH meetup whom worked with maps applications. MCH (May Contain Hackers) is a gathering that takes place in Amsterdam and is organized by volunteers. Its sole purpose is to bring like minded individuals together to share knowledge, technical advancements, experiments, hacks, and most importantly connect with other individuals with similar interests. Like many other technical meetups, badges were distributed to encourage learning and hacking.

The goal of this project was to add GPS capabilities to the badge. Furthermore, the true challenge and test was designing and prototyping a custom antenna for GPS frequencies. To begin, some background research was done which included reviewing patch antenna design and theory information from the following textbooks: Antenna Theory: Analysis and Design, Microstrip Antenna Design Handbook, and Circularly Polarized Antennas. Using information found from these texts, the dimensions for a rectangular path antenna were calculated for the L1 GPS frequency of 1574MHz. A short Python script was put together for the computations, which is shown below.

Once dimensions were determined, the next steps entailed calculating the 50 Ohm trace width needed for the specific PCB dielectric and determining how to make the antenna circularly polarized. It turns out designing a circularly polarized patch antenna is not exactly straight forward. However, it can be achieved by truncating the corners of the rectangular patch. With a good starting point determined for the antenna, the next steps involved lots of simulation. At first OpenEMS was used since it is open source and free. But after yielding odd results that did not make much sense, it was determined a different route would need to be taken. Having seen a lot of simulation work completed using Sonnet software elsewhere and determining free trial licenses are available, the rest of the simulation and design work was completed using Sonnet.

Working with Sonnet for the first time came with its own learning curve, though with the help of their support staff an optimal antenna design was converged upon in a short time. This was mainly done by running parametric sweeps on different geometries of the patch antenna. Naturally, the next step is to begin to bring the antenna to life. Using KiCAD a layout was created with the desired dimensions. The truncated patch antenna, a rectangular patch antenna, and a microstrip for calibration were all exported into Gerber files. These were then brought to the local hackerspace, Tkkrlab, to mill out a two sided PCB using a CNC machine.

The final steps included the exciting part of testing the design. First, the return loss or S11 was measured using a LiteVNA. This showed a S11 response that closely matched the simulated results. Next, the u-blox MAX-M10S GNSS module was acquired. This low power, asset tracking device will ultimately be what enables GPS on the badge. After tinkering with the I2C interface, the antenna and module combination were connected to the badge and NMEA messages began appearing on the display. The antenna was working, receiving satellite communications, and the location was found! Overall, the process of antenna design and prototyping yielded a successful PCB patch antenna for GPS frequencies.