Last week two projects showed that the 2.4 GHz radio in many ESP32 chips can hand raw samples to software, turning a few-euro board into a receive-only software-defined radio (SDR). Now the transmit side has arrived. Developer cyber-murmel (credited as marble by Hackaday) published the first builds of esp-sdr-zephyr on 11 October 2026, with v0.1.3 following on 12 October, and Hackaday covered it on 13 October.

What the firmware does

esp-sdr-zephyr builds on the receive-only ESP-SDR work from the ESPARGOS team and adds transmission through the radio's DAC, all on the Zephyr RTOS. The pre-built release targets one board, the Seeed XIAO ESP32S3 with 8 MB of flash. It ships as a multi-app image with MCUboot, and you pick an app from the board's USB shell.

For the PC side there is a SoapySDR module built for Ubuntu 24.04 on x86-64; on other systems you build it from source. In gqrx you use the device string driver=esdr,soapy=1 and an input rate of 250,000 samples per second, the most that USB full speed carries. Hackaday's demo had an ESP32-S3 and an ESP32-C6 exchanging text in a Hellschreiber-style mode, which has nothing to do with Wi-Fi.

Limits worth knowing

The README lists a 100-6,000 MHz API range, but the S3's PLL only locks at roughly 1.8-2.8 GHz. Hackaday first reported 100 MHz and corrected that after the developer explained it was an API artifact. So this is a 2.4 GHz band tool, not a cheap wideband SDR, and nobody has published independent RF measurements yet.

The project also has an experimental software 802.15.4 PHY, the radio layer under Zigbee and Thread. According to the maintainer's notes, summarised by esp32.co.uk, two-way frame tests against an ESP32-C6 ran over coax, clear-channel assessment always reports "clear", address filtering, RSSI and calibrated transmit power are unfinished, and an OpenThread build compiles but has not been tested over the air. It will not turn an S3 into a Zigbee coordinator or Thread border router.

While the SDR runs, the library owns the Wi-Fi radio, so the board cannot scan, connect or run an access point at the same time. The code is GPL-3.0-or-later.

Should you try it?

If you like RF experiments, it is a very cheap way to see raw 2.4 GHz signals and debug your own Bluetooth or 802.15.4 boards. Use a spare XIAO, not a sensor that runs your house. Transmitting is regulated. The developer's own waterfall came from a cabled link through a 30 dB attenuator; do the same, or stay within your local radio rules.