Thank you to Efe Işık (TA1EEI) for writing in and sharing with us his open-source project showing how to turn a modern Android smartphone’s internal Class-D speaker amplifier into a silent, intentional VHF Morse transmitter. Efe writes:

By pushing inaudible 21 kHz full-amplitude PCM audio via Android's AudioTrack, the amplifier's PWM switching circuit radiates near-field RF harmonics into the 144 MHz (VHF) band. The signal is strong enough to be received in AM mode on common SDRs and BK4819-based handheld transceivers (such as the Quansheng UV-K5) simply by placing the antenna near the speaker grill.

Everything is open-source, requires zero Android permissions, and includes instructions on receiver setup and frequency alignment.

With the software running on the Android device, simply use an SDR or handheld radio to look for the signal in the 144 - 146 MHz region. Increasing the volume can help increase transmission power.

This reminds us of the TrojPix project, a more sophisticated method that uses a program on a PC to imperceptibly modulate pixels on a screen, effectively turning video cable/screen EMI into an air-gapped data transfer path.