ESP32-A1: Wi-Fi and BT Dual Antenna Solution

Bison Science addresses the issue  that arise when a radio device with a traditional antenna is placed against a material that reduces the gain of that antenna or the material that acts as a ground plane. Examples might include radios that are placed against a concrete wall or ceiling, mounted to a metal surface, placed flat on the ground, or held against a body. We designed the ESP32-A1 antenna to meet the following requirements:

Antenna Requirements

• Two antennas for seamless communication.
• Easy to attach and remove.
• No modification of the current ESP32-M1 Reach Out design.
• Simple physical design with no mechanical complexity.
• Not larger than the ESP32-M1 Reach Out board.
• Not affected by the ground plane or by any material
• Reasonable gain and directivity.
• Both antennas need to radiate independently.
• Useful for other radios, not just the ESP32-M1 Reach Out board.

Outline

Taking on the challenge described above, the design started with a simple square patch, on an FR-4 substrate. The thickness was fixed to 1.6 mm for ease-of-manufacturing and to keep the antenna further from the ground plane. 

Optimizing further, we switched from an inset feed to a via feeding on the bottom of the PCB. This is similar to a coaxial feed but the SMA connector is kept at the edge of the board, which allowed for a long microstrip line between the ground plane on the bottom layer. (MPAs need a solid ground under the patch for good gain.) With the feeding method worked out and room for two adjacent antennas, the design advanced for two patches. Many design changes and optimization performed, and finally obtained the correct resonance and gain.

Unfortunately, the gain of two MPAs on a small FR4 PCB was not promising, so we opted for a better antenna substrate: Roger RO4350. Given the same thickness, we ended up with a slightly larger patch, but still fit within PCB dimension.

The images below shows assembled antenna on ESP32-M1. ESP32-A1 gain was measured in an anechoic chamber and the gain pattern at 2.45 GHz is approximately 2.7 dBi and 55% efficiency for Wi-Fi patch and approximately 0.7 dBi and 44% efficiency for Bluetooth.