An experimental Android proof-of-concept demonstrating electromagnetic side-channel transmission (TEMPEST) via smartphone internal Class-D audio amplifiers.

By synthesizing inaudible 21 kHz ultrasonic audio pulses at 100% media volume, the app forces the internal Class-D H-Bridge driver to switch at high peak currents. The high-order PWM switching harmonics leak through unshielded speaker coils, creating a detectable RF comb spectrum around the 2-meter amateur radio band (144.000 MHz).

- Mechanism: High-current PWM switching modulation via standard Android AudioTrackAPI.

- Carrier Frequency: Comb spectrum across VHF (Centering around 144.000 MHz depending on the phone's internal PWM switching clock).

- Acoustics: Inaudible (21 kHz tone, 48 kHz sampling rate). No audible sound is emitted.

- Modulation: OOK (On-Off Keying) / CW Morse code.

- Permissions Required: Zero (No Root, No Camera, No Storage permissions required).

- Frequency: Tune your receiver to 144.000 MHz (Search in 12.5 kHz / 25 kHz steps between 144.000 – 146.000 MHz if needed).

- Modulation: AM (Amplitude Modulation) mode is essential. FM limiters strip amplitude envelopes.

- Squelch: Set Squelch = 0 (Open static noise) or hold the MONIbutton.

- Bandwidth: Select WIDE (25 kHz) to account for clock jitter and spectral spreading.

- Remove any thick conductive cases.

- Set Media Volume to 100% (Amps will sleep/attenuate if volume is low).

- Physically touch the transceiver's antenna tip to the bottom speaker grill (next to the USB-C port).

WhatsApp.Video.2026-09-22.at.21.58.40.mp4

Efe Işık (TA1EEI)

- ORCID: 0009-0001-1628-2183

- Amateur Radio Callsign: TA1EEI

- Email: 2007efeisik@gmail.com

If you use this proof-of-concept or refer to this research in academic/technical work, please cite it as:

@software{isik2026classd_transmitter,

author = {Işık, Efe},

title = {ClassD-VHF-Transmitter: Mobile Audio-to-VHF Covert Channel},

year = {2026},

publisher = {Zenodo},

doi = {10.5281/zenodo.22903894},

url = {[https://doi.org/10.5281/zenodo.22903894](https://doi.org/10.5281/zenodo.22903894)}

}