Every capability your drone has flows through a radio link that physics is constantly trying to break. Understanding a handful of RF fundamentals — link budgets, Fresnel zones, antenna polarisation — separates teams that fly reliably at range from teams that collect unexplained failsafes. No RF degree required.
The three links on a working aircraft
- C2 (command & control): the safety-relevant channel — pilot commands and aircraft state. Low bandwidth, must be the last link standing.
- Telemetry/data: mission data, parameters, log streaming. Often shares the C2 radio at small scale.
- Video/payload downlink: bandwidth-hungry, its own guide.
Professional practice separates C2 from the bandwidth hogs so a saturated video link can never starve the channel that flies the aircraft — a principle your SORA assessor will also appreciate.
Frequency bands: the eternal trade
Lower frequency = better propagation and penetration per watt, less bandwidth. Higher = more bandwidth, shorter legs, more directionality.
| Band (EU focus) | Typical use | Legal power (typical) | Character |
|---|---|---|---|
| 433 MHz (ISM) | Long-range C2/telemetry | 10 mW ERP | Excellent reach per mW; narrow; crowded |
| 868 MHz (SRD) | C2/telemetry, RTK corrections | 25 mW ERP (500 mW sub-band w/ APC) | The EU workhorse; duty-cycle limits apply |
| 2.4 GHz (ISM) | C2 + moderate data, RC systems | 100 mW EIRP | Global harmonised; Wi-Fi congestion |
| 5.8 GHz (ISM) | Video, high-rate data | 25 mW EIRP (band-dependent) | Bandwidth-rich, line-of-sight only |
| LTE/5G licensed | BVLOS C2, video backhaul | Operator network | Coverage = someone else's problem (and SLA) |
Note the US differs (e.g. 915 MHz ISM at 1 W) — a reason many radio products ship regional variants, and a compliance checkbox for your CE marking.
Link budget: five lines of arithmetic that predict your range
RX power = TX power + antenna gains − path loss − losses
Path loss (free space, dB) = 20·log10(d_km) + 20·log10(f_MHz) + 32.45Example, 868 MHz at 10 km: path loss ≈ 32.45 + 20 + 58.8 ≈ 111 dB. With 14 dBm TX (25 mW), 2+2 dBi antennas, 2 dB cable losses: RX ≈ 14+4−111−2 = −95 dBm. A modern LoRa-class receiver at −110 dBm sensitivity leaves ~15 dB of fade margin — workable; comfortable is 20 dB. This five-minute calculation, done before buying radios, is the single highest-leverage RF habit a team can adopt.
Fresnel zones: why "line of sight" isn't enough
Radio needs an ellipsoid of clear space around the sight line — at 868 MHz over 10 km, the first Fresnel zone is ~29 m wide at midpoint. A ground antenna at head height sends half that ellipsoid into the dirt. Consequences: put the ground antenna on a mast (3–10 m transforms links), expect range to shrink dramatically over ridgelines and urban clutter, and plot terrain profiles for fixed routes.
Antennas: where cheap wins are hiding
- Polarisation match: a vertical whip on the ground and a horizontal one on a banking aircraft can cost 20 dB. Circular polarisation on one end, or diversity receivers, buys robustness through manoeuvres.
- Airframe masking: the aircraft's own carbon and battery shadow the antenna in some attitudes — the classic "link drops when flying away nose-down". Antenna placement flights belong in your test program.
- Ground tracking antennas: at serious range, a modest directional antenna on a tracker outperforms watts of omnidirectional shouting — and keeps you legal, since EU limits are ERP/EIRP (radiated, including gain).
1) RSSI/SNR logs vs distance — smooth decay or cliff? 2) Attitude correlation — masking/polarisation. 3) Ground antenna height — Fresnel. 4) Noise floor scan on site — interference. 5) Only then, hardware. Teams replace radios first; it's almost never the radios.
Redundancy for links that must not die
For operations beyond hobby distance: dual dissimilar C2 (e.g. 868 MHz primary + LTE secondary) with automatic failover, well-defined link-loss behaviour at every layer, and mission design that respects the coverage map. The BVLOS datalink guide continues this thread where the sight line ends.
Frequently asked questions
What frequency is best for a drone control link?
Lower frequencies travel further and penetrate obstacles better per watt: 433/868 MHz (EU) links commonly reach tens of kilometres line-of-sight at legal power, while 2.4 GHz offers more bandwidth over shorter range. Many professional systems pair a robust sub-GHz C2 link with a separate high-band video link.
How far can a drone telemetry link reach legally in Europe?
At 868 MHz the EU limit is typically 25 mW ERP (500 mW in some sub-bands with restrictions), yet well-designed links achieve 10–40+ km line-of-sight thanks to receiver sensitivity and antenna gain. Range claims assume unobstructed Fresnel zones — terrain and buildings change everything.
Why does my drone lose link at long range even with good radios?
Usually geometry, not radios: the first Fresnel zone clips terrain or buildings, antennas are cross-polarised or masked by the airframe in certain attitudes, or ground-station antenna height is too low. Raising the ground antenna a few metres often outperforms doubling transmit power.