Calculate free space path loss, received signal strength, link margin, and maximum range for drone radio links.
Free Space Path Loss
At 1000m, 2400MHz
Received Signal Power
Excellent link — significant range reserve
Link Margin
Excellent link — significant range reserve
Theoretical Max Range
Without environment losses
Faded Rx Power
After Open / LOS fade margin (0dB)
Fresnel Zone Radius
Keep obstacles below this height at midpoint
The Friis transmission equation is the mathematical foundation of every radio link analysis. It describes the power received by an antenna given the transmitted power, antenna gains, wavelength, and distance between transmitter and receiver in free space:
P_r = P_t + G_t + G_r − FSPL (all values in dBm / dBi / dB)
Where P_r is received power, P_t is transmitted power, G_t and G_r are transmit and receive antenna gains, and FSPL is the free space path loss.
The equation reveals that received power depends on four factors: how much power the transmitter outputs, how efficiently each antenna focuses that power, and how much energy disperses during propagation. Working in decibels converts multiplication into addition, making the calculations straightforward.
The key insight of the Friis equation is that doubling the distance reduces received power by 6dB — a consequence of the inverse square law. Doubling frequency also increases path loss by 6dB. These relationships make antenna gain and receiver sensitivity critical levers for extending range, since quadrupling TX power only adds 6dB — the same as doubling distance removes.
Free space path loss (FSPL) describes the reduction in signal power as a radio wave spreads out from a point source. It is calculated as:
FSPL (dB) = 20·log₁₀(d) + 20·log₁₀(f) − 147.55
Where d is distance in metres and f is frequency in Hz. The constant 147.55 arises from combining 20·log₁₀(4π/c) where c is the speed of light.
Critically, FSPL is not caused by absorption or atmospheric resistance — it is a purely geometric effect. As a radio wave propagates, the same power is spread over a sphere whose surface area grows with distance squared. Higher-frequency signals have shorter wavelengths and smaller effective antenna apertures, so they capture less of the available energy per unit area at the receiving antenna.
For drone radio systems, this means frequency is a fundamental performance parameter. A 900MHz system experiences approximately 8.5dB less path loss than a 2.4GHz system at the same distance — translating to roughly 2.7× greater range at equal link budget. This is why long-range drone systems such as TBS Crossfire and ExpressLRS 900 operate in the 900MHz band despite narrower bandwidth availability.
| Frequency | FSPL at 1km | FSPL at 5km | vs 2.4GHz at 1km |
|---|---|---|---|
| 433MHz | 85.2dB | 99.2dB | −14.9dB |
| 868MHz | 91.2dB | 105.2dB | −8.9dB |
| 915MHz | 91.7dB | 105.7dB | −8.4dB |
| 2.4GHz | 100.1dB | 114.1dB | — |
| 5.8GHz | 107.7dB | 121.7dB | +7.6dB |
Link margin is the difference between the received signal power and the receiver's minimum sensitivity threshold. It represents the available headroom before signal loss causes dropouts:
Link Margin (dB) = P_received − P_sensitivity
A link margin of 0dB means the signal is right at the sensitivity threshold — any small perturbation causes loss of link. The minimum recommended margin for reliable drone operation is 10dB. This buffer covers:
Multipath fading
Reflections from the ground, structures, and vegetation create interference patterns that can cause 10–20dB signal nulls at specific positions. A healthy link margin ensures these nulls are survivable.
Antenna orientation
Omnidirectional antennas have radiation pattern nulls at certain angles. As the aircraft manoeuvres, the received signal can drop 5–10dB when flying through a pattern null.
Interference and noise
Other RF systems on the same frequency increase the effective noise floor. Wi-Fi, other drone systems, and industrial equipment all degrade link margin in populated areas.
Vegetation and rain
Flying through or near trees causes 3–10dB attenuation. Rain adds 0.01–0.02dB/km at 2.4GHz — negligible for short links but meaningful for longer ranges.
For critical operations — autonomous missions, BVLOS, long-range fixed-wing — target 20dB or more. For local freestyle FPV within visual line of sight, 10–15dB is generally sufficient.
Many pilots assume that visual line-of-sight between transmitter and receiver is sufficient for a clean link. In practice, radio signals require a clear Fresnel zone — an elliptical region around the direct path that must be free of obstructions.
The first Fresnel zone radius at the midpoint of the path is:
r₁ = 0.5 × √(λ × d)
Where λ is wavelength in metres and d is the total path distance in metres. This is the simplified formula valid at the midpoint; the full formula accounts for the position along the path.
If more than 40% of the first Fresnel zone is obstructed, the signal experiences significant diffraction loss — often 6dB or more. At 2.4GHz flying 1km away, the first Fresnel zone radius at the midpoint is approximately 5.6m. Flying lower than this height above terrain creates a partial obstruction that reduces effective link margin.
Lower-frequency systems have larger Fresnel zones due to their longer wavelengths. A 433MHz system at 5km has a midpoint Fresnel radius of approximately 65m — requiring the aircraft to fly well above terrain for clean propagation.
Different drone radio systems make different trade-offs between frequency, power, and receiver sensitivity. The table below compares the major systems used in the FPV and UAV ecosystem.
| System | Freq | Power | Sensitivity | Typical Range |
|---|---|---|---|---|
| ExpressLRS 2.4GHz | 2.4GHz | 100–250mW | −108dBm | 5–15km LOS |
| ExpressLRS 900MHz | 900MHz | 100–500mW | −112dBm | 10–40km LOS |
| TBS Crossfire | 868MHz | 100mW–2W | −130dBm | 40–150km LOS |
| DJI O3 | 2.4GHz | 700mW peak | −100dBm | 6–12km LOS |
| Analog 5.8GHz Video | 5.8GHz | 25–200mW | −85dBm | 0.5–2km LOS |
| SiK Telemetry | 915MHz | 100mW | −121dBm | 5–20km LOS |
Range figures assume open LOS conditions. Real-world range varies significantly with antenna type, terrain, and local RF environment. Always verify compliance with local frequency regulations before operating.
Antenna placement
Mount the aircraft receiver antenna vertically when the transmitter is below the aircraft — this aligns polarisation for maximum gain. Avoid mounting antennas inside carbon fibre frames, which act as Faraday cages and attenuate 5–20dB. Route antennas outside the frame with clear sky view.
Prop wash interference
Spinning carbon fibre propellers can attenuate and scatter RF signals. Mount receiver antennas on arm ends or below the frame rather than directly above propellers. This is particularly relevant for 5.8GHz systems where the prop diameter is a significant fraction of the wavelength.
Polarisation matching
Mismatched polarisation between TX and RX antennas causes a 3dB loss for 45° difference, and up to 20dB loss for 90° cross-polarisation. Use circularly polarised (RHCP/LHCP) antennas on video transmitters for polarisation independence. Match RHCP to RHCP or LHCP to LHCP — mixing hands adds ~20dB loss.
Diversity and redundancy
Many modern receivers use spatial diversity — two antennas oriented at 90° to each other, switching to the stronger signal. This effectively eliminates pattern nulls by ensuring at least one antenna always has good orientation. Higher-end systems use true diversity with two complete RF chains.