Radio Receiver (RX)

🟢 Start — zero knowledge, plain words. 🟡 Hands-on — building or buying, specifics and tradeoffs. 🔴 Specialist — the physics and math behind it.

🟢 Start. The receiver is a tiny board with an antenna that catches the pilot's commands and hands them to the flight controller. Today's standards are ExpressLRS (ELRS) — open source, cheap, enormous range — and TBS Crossfire. A receiver is "bound" to one specific radio so nobody else can take over your drone.

🟡 Hands-on. ELRS 2.4 GHz means small antennas and high packet rates (low latency); the 868/915 MHz variants (and Crossfire) penetrate obstacles better at the cost of antenna size. Watch two numbers in the OSD: RSSI dBm (signal strength) and LQ (percentage of packets that arrived) — falling LQ with good RSSI means interference, not distance. The absolutely critical part: configure and ground-test failsafe — what the drone does after signal loss (motor cut or GPS Rescue). Without it, one lost link = one flyaway.

🔴 Specialist. ELRS uses LoRa modulation, and the tradeoff is fundamental: fewer packets per second = narrower noise bandwidth = better sensitivity = more range:

Every −3 dB of sensitivity ≈ 1.4× range. Quick budget: 100 mW (20 dBm) + two 2 dBi antennas, 2 km at 2.4 GHz → FSPL ≈ 106 dB → $P_{rx} \approx -82$ dBm → 26 dB of margin at 250 Hz. That's why control dies long after video — by design.