Why LoRa is so sensitive
LoRa trades speed for sensitivity. With a 125 kHz channel, the datasheet of the SX1276 chip gives a sensitivity of about -123 dBm at spreading factor 7 and about -136 dBm at spreading factor 12 (-118 dBm at SF6). That is 13 dB better, equivalent to 20 times less power, or about 4.5 times the range in free space, in exchange for a data rate many times lower. Regulations limit transmit power and duty cycle and differ by region. In the European 868 MHz band, for example, most sub-bands allow 25 mW ERP with a 1% duty cycle, while the 869.4 to 869.65 MHz sub-band allows 500 mW ERP with a 10% duty cycle. Other regions, such as the Americas, have different limits: always check the rules of your country.
From budget to distance
Take 14 dBm of power and 2 dBi antennas at both ends: with a sensitivity of -136 dBm the allowed path loss is about 154 dB. In free space that would reach well over 1000 km, which shows that the budget is not the limit. At 868 MHz the loss is 111.3 dB over 10 km. What stops the signal is the radio horizon, hills and buildings, plus the absorption of vegetation and walls. A node on a roof sees much further than one in a pocket.
Planning a gateway or a mesh node
Put the antenna as high as possible with a clear view, use a good outdoor antenna and short, low-loss cable, and avoid metal boxes around it. To see where a node can be heard, load 868 MHz and one of the LoRa modes on the coverage map: the map shows the zones that stay above the sensitivity of the chosen spreading factor, with terrain from real elevation data.
