Link Budget Calculations
A link budget calculation estimates whether a radio path between two nodes will work reliably before you deploy hardware. It's the single most useful tool for avoiding wasted installation trips and surprised failures.
The link budget equation
Received Power (dBm) = TX Power (dBm)
+ TX Antenna Gain (dBi)
− TX Cable Loss (dB)
− Free Space Path Loss (dB)
− Obstruction Loss (dB)
+ RX Antenna Gain (dBi)
− RX Cable Loss (dB)
Link Margin (dB) = Received Power (dBm) − Receiver Sensitivity (dBm)
A positive link margin means the link should work. A margin of 10 dB or more is considered reliable. Below 3 dB is borderline and not recommended for permanent infrastructure.
Key values for LoRa at 915 MHz
Receiver sensitivity by MeshCore preset
| Preset equivalent (SF / BW) | Receiver sensitivity |
|---|---|
| USA/Canada (SF7 / 62.5 kHz) | ~−125 dBm |
| Long Fast (SF11 / 250 kHz) | ~−137 dBm |
| Long Slow (SF12 / 125 kHz) | ~−141 dBm |
| Medium Slow (SF10 / 250 kHz) | ~−134 dBm |
Lower sensitivity number = can receive weaker signals = more range potential. Long Slow gives the best sensitivity but at the cost of extremely low data rate.
Free Space Path Loss at 915 MHz
FSPL (dB) = 20×log10(d) + 20×log10(f) + 20log10(4π/c)
In practical terms for 915 MHz:
| Distance | Free Space Path Loss |
|---|---|
| 1 km (0.62 mi) | 91.6 dB |
| 5 km (3.1 mi) | 105.6 dB |
| 10 km (6.2 mi) | 111.6 dB |
| 20 km (12.4 mi) | 117.6 dB |
| 50 km (31 mi) | 125.6 dB |
Note: Free space path loss assumes clear line of sight with no obstructions. Real-world losses are always higher.
Worked example: Rooftop repeater to ground-level node
Scenario: 5 km path, rooftop repeater at 30m height, portable node at 2m height.
| Parameter | Value |
|---|---|
| TX Power (repeater) | 27 dBm |
| TX Antenna Gain | +5 dBi |
| TX Cable Loss (1m LMR-200) | −0.1 dB |
| Free Space Path Loss (5 km, 915 MHz) | −105.6 dB |
| Obstruction/Fresnel loss estimate | −10 dB (mixed urban) |
| RX Antenna Gain (portable node, 2 dBi) | +2 dBi |
| RX Cable Loss (none for portable) | 0 dB |
| Received Power | 27 + 5 − 0.1 − 105.6 − 10 + 2 = −81.7 dBm |
| Receiver Sensitivity (USA/Canada SF7) | −125 dBm |
| Link Margin | −81.7 − (−125) = +43.3 dB |
A 43 dB margin is very comfortable - this link will work reliably even with additional obstruction losses not captured in the estimate.
Fresnel zone clearance
Even in "clear" line-of-sight paths, the Fresnel zone must be 60% clear of obstructions for reliable communication. The first Fresnel zone radius at the midpoint of a path:
r = 8.66 × sqrt(d / f_GHz) meters
Where d = path length in km, f = frequency in GHz
For 915 MHz, 10 km path:
r = 8.66 × sqrt(10 / 0.915) = 8.66 × 3.30 = 28.6 meters
Any obstruction within 28.6m of the direct path midpoint will partially block the signal.
This is why hilltop-to-hilltop links work so well: the terrain clears the Fresnel zone naturally. For rooftop-to-rooftop links in cities, trees and building facades at path midpoints can add 10 - 20 dB of loss even when the antennas themselves have direct line of sight.
When to use a link budget
- Before installing a repeater at a new site, calculate whether it can reach your intended coverage area
- When planning a point-to-point relay link between two specific nodes
- When a deployed link is underperforming - work backwards from measured RSSI to identify where the losses are
- When comparing two candidate repeater sites - small differences in height can produce large differences in link budget
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