What Is a Link Budget?
A link budget is a running tally of all signal gains and losses from transmitter to receiver. For a UAV telemetry link, it answers the critical question: "Given my hardware, at what distance will the signal reliably get through?" The margin left over after subtracting all losses is called the fade margin — and you want it to be at least 10–20 dB for a stable link.
The link budget equation is simple: EIRP – Path Loss – Noise Floor + Receiver Gain = Link Margin. Every component of your RF chain contributes positively or negatively to this equation.
SX1276 Key RF Parameters
Before running numbers, understand what the SX1276 actually delivers at the silicon level:
| Parameter | Value | Notes |
|---|---|---|
| Max TX Power | +20 dBm (PA_BOOST) | 100 mW; requires PA_BOOST pin |
| Receiver Sensitivity | –137 dBm (SF12/BW125) | Best case, slowest data rate |
| Receiver Sensitivity | –118 dBm (SF7/BW500) | Faster, for low-latency telemetry |
| Frequency Range | 137–1020 MHz | Sub-GHz; 433/868/915 MHz typical |
| Supply Current (TX) | 120 mA at +20 dBm | PA_BOOST mode |
| Supply Current (RX) | 10.3 mA | Continuous receive |
Free-Space Path Loss Calculator
The dominant loss in air-to-ground UAV links is Free-Space Path Loss (FSPL). The formula is:
FSPL(dB) = 20·log₁₀(d) + 20·log₁₀(f) + 20·log₁₀(4π/c)
Where d is distance in meters and f is frequency in Hz. In practical units:
FSPL(dB) ≈ 20·log₁₀(d_km) + 20·log₁₀(f_MHz) + 32.44
| Distance | 433 MHz | 868 MHz | 915 MHz |
|---|---|---|---|
| 1 km | 85.2 dB | 91.5 dB | 91.9 dB |
| 5 km | 99.2 dB | 105.5 dB | 105.9 dB |
| 10 km | 105.2 dB | 111.5 dB | 111.9 dB |
| 20 km | 111.2 dB | 117.5 dB | 117.9 dB |
| 30 km | 114.7 dB | 121.0 dB | 121.4 dB |
Worked Example: 10 km at 433 MHz
Let's calculate the link margin for a typical long-range UAV setup using two SX1276 modules, 5 dBi antennas, and SF10/BW125 kHz (sensitivity ≈ –131 dBm):
- TX Power (PA_BOOST): +20 dBm
- TX antenna gain: +5 dBi (ground station dipole)
- EIRP: +25 dBm
- FSPL @ 10 km, 433 MHz: –105.2 dB
- Cable/connector losses: –1.5 dB
- RX antenna gain (drone): +2 dBi (whip)
- Received power: 25 – 105.2 – 1.5 + 2 = –79.7 dBm
- Sensitivity (SF10): –131 dBm
- Link margin: 131 – 79.7 = 51.3 dB ✅
51 dB of margin is excellent. Even with Fresnel zone obstruction and multipath fading, this link will be bulletproof at 10 km.
Fresnel Zone and Why It Matters
The Fresnel zone is an ellipsoid around the direct line-of-sight path. Obstructions within the first Fresnel zone cause diffraction loss. For a 10 km link at 433 MHz, the first Fresnel zone radius at the midpoint is approximately:
r₁ = 17.3 × √(d_km / (4 × f_GHz)) ≈ 17.3 × √(10 / 1.732) ≈ 41.5 m
This means you need approximately 41 meters of clearance above obstacles at the midpoint of a 10 km link. At drone altitudes of 100+ meters, this is generally achievable.
Spreading Factor vs. Data Rate Trade-Off
| SF | BW (kHz) | Sensitivity | Data Rate | Best Use Case |
|---|---|---|---|---|
| SF7 | 500 | –118 dBm | ~21 kbps | Short range, low latency |
| SF9 | 125 | –127 dBm | ~1.8 kbps | Mid-range telemetry |
| SF10 | 125 | –129 dBm | ~1.0 kbps | Long-range MAVLink |
| SF12 | 125 | –137 dBm | ~0.3 kbps | Maximum range, slow updates |
For most UAV telemetry (MAVLink at 10 Hz update rate), SF9 or SF10 at 125 kHz bandwidth provides the best balance of range and responsiveness. The SX1276 and its upgraded sibling SX1262 both support these modes natively.
Antenna Selection Guide
Antenna choice has a larger impact on effective range than almost any other hardware decision. For UAV telemetry:
- Ground station: 5–9 dBi omnidirectional collinear or patch (directional for >15 km)
- Drone: 2 dBi whip or cloverleaf for circular polarization (better in multipath environments)
- Cable: Use LMR-200 or better; avoid RG-58 at 433 MHz (0.85 dB/m loss)
- Connector: SMA or RP-SMA; add 0.3 dB per connector in your budget
SX1276 vs SX1262: Should You Upgrade?
The SX1262 offers +3 dBm higher output power (+22 dBm vs +20 dBm) and improved sensitivity at high spreading factors. However, the SX1276 remains the dominant choice due to:
- Wider ecosystem support (ExpressLRS, RFD900, LoRa32)
- Lower cost at volume
- Pin-compatible modules widely available
- Proven field reliability in thousands of drone deployments
For new designs targeting >20 km with minimal antenna size, the SX1262 is worth the BOM premium. For standard 5–15 km telemetry, SX1276 is the right choice.
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Submit RFQ Now →Frequently Asked Questions
A minimum of 20 dB fade margin is recommended for reliable operation. Many commercial systems target 30–40 dB to handle unexpected obstructions, multipath fading, and RF interference. With SX1276 at 433 MHz and good antennas, achieving 50+ dB margin at 10 km is realistic.
Yes, with optimal configuration. At SF12/BW125 on 433 MHz with +20 dBm TX, 8 dBi ground antenna, and clear line of sight, 30 km links have been documented. However, regulatory TX power limits and licensing requirements vary by country — always check local RF regulations.
The SX1276 covers 137–1020 MHz (including 915 MHz), while the SX1278 is optimized for 137–525 MHz (433 MHz only). For multi-band designs targeting both 433 and 915 MHz, the SX1276 is the correct choice.
Use 433 MHz for LoRa telemetry and 5.8 GHz for video to maximize frequency separation. If using 868 MHz for telemetry, keep the antennas physically separated by at least 30 cm from 5.8 GHz video transmitters.
Yes. ExpressLRS supports SX1276 and SX1278 in 433 MHz and 868/915 MHz configurations. SX1262 support was added in later firmware versions for improved range and efficiency.