Two experimental SDR projects for MT7921AU adapters

Two experimental projects turn specifically supported MT7921AU/MT7961 USB Wi-Fi adapters into software-defined radio (SDR) receivers. The practical choice comes down to the adapter’s USB identity, the frequency coverage you need, and whether you want to work with a temporary USB program or a patched Linux driver and firmware.

SDR moves much of a radio’s signal processing into software. Both projects capture I/Q samples, two components of a radio signal that software can process to inspect its spectrum and other properties. Their documented modes are experimental, and their stated receiver figures are not calibrated performance guarantees.

Check the adapter’s USB identity first

A chipset name alone does not tell you whether a particular dongle matches either project’s supported hardware.

BirriaSDR identifies a tested COMFAST adapter with USB ID 3574:6211 and separately lists generic-ID 0e8d:7961 boards. Its tuning range differs by board: 2156–2610 MHz for the tested COMFAST adapter and 2190–2610 MHz for the generic-ID boards. Mtk79ICAP identifies MT7921AU/MT7961 USB sticks with ID 0e8d:7961.

Check the ID reported for your adapter against the project’s listed hardware before choosing a setup. The documented compatibility applies to those identified devices, not automatically to every adapter built around the MT7921AU.

Sampling and tuning are different measurements

BirriaSDR uses 14-bit complex I/Q sampling at a fixed 40 MSps, across a 40 MHz sampled span. Its supported tuning range describes where the receiver can be set; it does not mean the entire range has calibrated sensitivity or frequency accuracy.

Mtk79ICAP documents 20, 40 and 80 MHz modes, with corresponding sample rates of 20.0008, 40.0016 and 80.0032 MS/s, and coverage in the 2.4 and 5 GHz bands. Its 80 MHz mode supports continuous capture in dense format; without that format, 80 MHz operation at 5 GHz is limited to snapshots.

A spectrum-and-waterfall display such as Gqrx’s makes radio activity visible as a spectrum plot and a history of signal energy over time. The display is a useful way to understand what SDR software presents; it does not identify which receiver hardware produced a particular capture.

Choose a setup path

BirriaSDR talks to the adapter through libusb, rather than a modified Linux Wi-Fi driver. Its small program runs from volatile device memory: the project says it does not write to flash or eFuse. The USB interface must be free for libusb, and the device must not already have a vendor ROM patch loaded.

Mtk79ICAP takes a more involved Linux route. It requires patched wireless firmware, modified Linux mt76 modules, and kernel headers for the running kernel. The patcher accepts a specified distribution firmware build, and the modules need rebuilding after kernel updates. Its repository also describes removing the patched firmware file to restore the distribution’s stock file for a subsequent load.

The trade-off is practical: BirriaSDR avoids persistent flash or eFuse writes, while Mtk79ICAP documents 5 GHz coverage and an 80 MHz mode through a driver-and-firmware setup. Choose based on the hardware ID and the host changes you are willing to manage, rather than the chipset label alone.

Monitor capture and measurement limits

Mtk79ICAP says monitor-mode Wi-Fi frame capture can continue while SDR streaming when the chip’s automatic gain control (AGC) is enabled. With fixed gain, the chip stops decoding monitor-mode frames during streaming.

The receiver figures also need to be read as project specifications, not calibrated measurements. BirriaSDR says sensitivity, gain and frequency accuracy are uncalibrated; its filters can also let signals outside the sampled span alias into the capture. Mtk79ICAP describes its signal levels as uncalibrated and says some register settings were reverse-engineered. Neither project’s stated sampling mode establishes uniform, calibrated reception across its tuning coverage.