Home / Signals
Dossier 06 / Radio room
Two links keep a drone honest: a command link that must not lie, and a video link that may fade but should fail loud. Both are radio. Both obey the same old rules about power, antennas, and line of sight.
Uplink
Early radios sent a pulse per channel down a wire – PWM – then a train of pulses on one wire – PPM. Digital serial links (SBUS, CRSF, FPort) packed those channels into a packet the flight controller can read at hundreds of hertz. The handheld still feels like a wartime box: two gimbals, switches, a bind button. Under the plastic it is a small computer talking LoRa or a proprietary modem.
ExpressLRS is open, cheap, and fast on 2.4 GHz or 900 MHz. TBS Crossfire and similar 900 MHz systems trade some update rate for foliage and distance. Older 2.4 GHz frequency-hopping radios still fly trainers. The frequency is the band; the protocol is the language. A receiver must speak the same language as the transmitter.
Failsafe is the dead-man’s handle. If packets stop, the receiver tells the flight controller to drop, hover, or return – whatever you set on the bench. An unset failsafe is an unbriefed crew.
Downlink
| System | Band | Character |
|---|---|---|
| Analog FM | Typically 5.8 GHz | Snows when weak, almost no delay, cheap modules, messy when many pilots share a field. |
| HDZero | 5.8 GHz digital | Built to keep analog-like latency with a cleaner picture. Breaks into blocks instead of snow. |
| Walksnail / Avatar | 5.8 GHz digital | High detail, goggle record, slightly more delay than the fastest analog. |
| DJI O3 / O4 class | Vendor digital | Very clean image and strong penetration; closed ecosystem; latency low enough for most flying, not always for the sharpest race. |
| Wi-Fi / LTE apps | 2.4 / 5 GHz or cellular | Fine for slow camera drones. Too late and too fragile for acro FPV. |
Polarization matters. A circularly polarized cloverleaf on the aircraft and a matching patch or omni on the goggles shrug off the rolls that would fade a linear whip. Diversity receivers listen on two antennas and pick the cleaner one – two ears instead of one.
Range is not a number on a box. It is height, trees, water, other transmitters, and whether the aircraft’s antenna is blanked by carbon. A wearable control station with a 25 km radio still needs a path. The 2026 Crossbow Block 2 story on the home board is engineering for that path: smaller box, same link, optional displaced antennas.
Good command links send data back: pack voltage, GPS, link quality. That is the wireless instrument panel. Some digital video systems piggyback this on the video radio. Some keep it on the control radio. Either way the pilot should see volts before the motor song changes.
