Why height beats wattage every time
“Raise it before you turn it up” is the most repeated piece of advice in mesh networking, and unlike most rules of thumb it falls straight out of the geometry. This note works through why, with the arithmetic laid out so you can check it against your own site rather than take it on faith.
This is a reference explainer built from published RF fundamentals, not a write-up of a test we ran. Every figure below comes from the standard formulas, which are given so you can recompute them. When we publish measurements, they will be labelled as measurements.
LoRa is patient; terrain is not
LoRa’s chirp spread-spectrum modulation is genuinely excellent at pulling a signal out of the noise floor — that is the entire point of the scheme, and it is why a few hundred milliwatts can travel so far. But sensitivity only helps against weak signal. It does nothing against a blocked one. A hill, a tree line, or the curve of the earth does not care how loud you are.
The horizon is a hard limit
The distance to the radio horizon depends only on antenna height. Accounting for the way the atmosphere bends radio waves slightly around the earth (the standard 4/3-earth model), it is:
| Antenna height | Radio horizon |
|---|---|
| 1.5 m — on a bench, or held | 5.1 km |
| 5 m — short mast or painter’s pole | 9.2 km |
| 10 m — house roofline | 13.0 km |
| 20 m — tall building, silo, tower leg | 18.4 km |
The usable path is the sum of both ends. Two nodes at bench height can see about 10 km of horizon between them; lift one onto a 5 m pole and it becomes 14 km; put both on 20 m structures and it is 37 km. No change to the radio in any of those cases.
d ≈ 4.12 × √h — d in kilometres, h in metres,
per antenna. Because distance goes as the square root of height, quadrupling your
height exactly doubles your horizon.
Why power can’t buy the same thing
Here is the comparison that settles the argument. In free space, doubling your range costs +6 dB — four times the transmit power. Doubling your range by height also costs a factor of four, but of height: 1.5 m to 6 m. One of those is a painter’s pole; the other runs you into the 30 dBm regulatory ceiling in short order.
And that comparison is the optimistic one, because it assumes a clear path. If the earth or a ridge is in the way, extra power buys you nothing at all: the horizon is geometry, and geometry does not respond to watts.
Leave room around the line, not just along it
A clear sightline is necessary but not sufficient. Radio needs clearance around the straight line too — the first Fresnel zone, an ellipse between the two antennas. Obstructing it costs you signal even when you can see the far end perfectly well. Aim to keep at least 60% of it clear.
| Link length | Zone radius at midpoint (915 MHz) | 60% clearance |
|---|---|---|
| 1 km | 9.1 m | 5.5 m |
| 5 km | 20.2 m | 12.1 m |
| 10 km | 28.6 m | 17.2 m |
| 20 km | 40.5 m | 24.3 m |
This is the part that surprises people: a 10 km link wants roughly 17 m of clearance at its midpoint, not a metre or two. It is also why a link across an open field behaves so differently from one that grazes a treetop halfway along, even though both “have line of sight”.
r ≈ 17.32 × √(d / 4f) — r in metres, d in
kilometres, f in GHz (0.915 for the US band). This gives the radius at the midpoint, where the
ellipse is widest.
Practical takeaways
- Mount relays as high as you safely can — rooftops, masts, and tall structures earn their keep.
- Keep the antenna vertical and the feedline short; a bad connector undoes a good location.
- Plan hops around what each node can see, not how far apart they are on a map.
- When a link won't hold, raise it before you turn it up.
If you're planning a network, the antennas & line-of-sight manual goes deeper on Fresnel zones and hop planning, and the Solar Relay v2 build is a node designed to go up a pole and stay there.