Adam Gulyas Documents His Homebrew 2.4GHz PCB Patch Antenna Project

Engineer-in-training Adam Gulyas has shared his work on creating a patch antenna for the 2.4GHz band, printed onto a PCB — and the precision mathematics which underpin the creation of what is, effectively, a rectangle of copper.

"I wanted some experience designing antennas, so I started with the simplest one, a patch antenna," Gulyas explains of the project. "I found some design equations in the textbook Antenna Theory: Analysis and Design 3rd Edition, pp 816-820, equations 14-1 to 14-7. I created a spreadsheet titled Patch Antenna Calculator to calculate the equations."

Those equations are key: While a PCB patch antenna can be created as simply as drawing a rectangle in your favoured PCB design application, the size and shape of the rectangle is vital for tuning it to a specific frequency — in the case of Gulyas' creation, the 2.4GHz industrial, scientific, and medical (ISM) band which is licence-free in most nations.

Once the antenna had been designed, it needed to be tested using a vector network analyser (VNA) — a tool which produces plots reflecting the antenna's performance. "It can be seen that, while the antenna is resonant at 2.44 GHz, S11 is only around -5 dB and the input impedance is 123 + j57 Ω, which are both too high," Gulyas writes. "Surprisingly, the antenna is also resonant at 3.767 GHz and 4.66 GHz, both of which show a better input impedance than the design frequency.

"Based on this information, I decided that a better way to feed the antenna was needed. I took one of the PCBs I made and cut off the wave guide, leaving just the patch antenna. Then I fed it with a coax feed, as shown above, at the very edge of the antenna to find Zin(0). With the insert distance determined, I roughly modified another antenna to confirm this approach.

"That improved things a lot. Minimum S11 is -31 dB, which is great, and minimum input impedance is 52.47 + j1.39 Ω. Much better than 90% of the energy is disappearing into the antenna. Is it actually radiating? Only an expensive test can definitively answer that. The main issue is that it's not tuned very well, which could be due to the imprecise hole I drilled."

While the new feeding method is an improvement, a comment by researcher Rushiraj Jawale suggests that there is still work to be done: "If you use the EMTALK website calculator, you can see it also gives edge impedance of the antenna which is usually very high (in order of hundreds of ohms) and you're feeding the edge of that antenna using a 50 ohms microstrip line hence, it will never match and won't give you the intended S11," he writes.

"You either have to use a insert feed or a coax feed by drilling a hole in the bottom or use a quarter wave impedance transformer if you want to feed the patch on the edge. This explains why you got a good result with the coaxial line feed."

The full project write-up can be found on Hackaday.io.