LoRa airtime.
See how long each packet ties up the channel, and how many your duty cycle allows per hour. Pick a Meshtastic preset or set your own.
Sensitivity at these settings is about -132 dBm. Every step up in spreading factor roughly doubles airtime and buys about 2.5 dB of range.
The maths, plainly.
The formula
Time on air is the preamble plus the payload symbols, each lasting 2^SF ÷ bandwidth. Low-data-rate optimisation switches on when a symbol exceeds 16 ms. This follows Semtech's AN1200.13.
Why it matters
In most of Europe you may transmit for only 1% or 10% of each hour. On slow presets that is a few dozen messages, and every relay in the mesh spends the same budget.
What to change first
Shorter messages and a faster preset free the most airtime. Drop the spreading factor before you cut power: range falls less than you might think.