How a 1-Watt drone radio reaches 100 km: the modulation trick behind ExpressLRS

Drone control links are a marvel of radio engineering: using a transmitter with a maximum output of one watt, protocols like ExpressLRS (ELRS) can maintain control over a drone more than 100 kilometers away. A developer known as , who has been building a drone electronics stack from scratch, recently designed four open-source ELRS receivers and detailed the principles behind their extraordinary range.

The conventional way to increase radio range is to lower the frequency—lower frequencies penetrate materials better and attenuate less over distance. While ELRS can operate in the 900 MHz band, 's receivers primarily use the 2.4 GHz band. The real breakthrough lies in modulation. Unlike other drone control protocols, which typically use frequency modulation, or Wi-Fi, which uses phase and amplitude modulation, ELRS employs Chirp Spread Spectrum (CSS) modulation. This technique trades data rate for reliability: each bit is transmitted as a chirp—a linearly rising radio tone—and the data is encoded in the chirp's starting frequency. To decode, the receiver multiplies the incoming signal with an inverse chirp and then performs a fast Fourier transform, revealing the starting frequency. This process provides an equivalent processing gain of 24 dB, allowing the receiver to decode signals even when they are below the noise floor.

The hardware implementation is surprisingly simple: each receiver consists of an ESP32 microcontroller, an SX1281 radio chip, and a few peripherals. All four receivers operate in the 2.4 GHz band, with two additionally featuring 900 MHz antennas. Despite breaking conventional RF design rules—such as placing antennas close to ground planes and other components—all receivers performed reliably in tests. 's work demonstrates that sophisticated modulation, not brute-force power, is the key to long-range drone control.