I've tested LoRa from Hill<->Hill and Flat Land<->Flat Land and 30-60km worked with plenty of SNR left.

On 868MHz with standard 5dBi omnis.

Thesis: as long you are using one of the more robust LoRa settings it always will work as long you have LoS or at least only lightly obstructed LoS.

If you have line of sight, a simple flashlight would work for communication

A robust communication needs to work in valleys

Blinking a flashlight 1200 times per second is a bit exhausting.

What's the data transmission rate with the balloon, put of curiosity?

Hard limit is 3.5kbps with 148dB link budget (Medium Fast preset in Meshtastic [1]). In practice - a packet worth of hundred bytes every half a second or so.

[1] https://meshtastic.org/docs/overview/radio-settings/

Is that the cap of the whole system or can multiple users use it with individual caps of 3.5kbps? That's somehow both faster and slower than I expected.

If one node kept transmitting non-stop, it could push data out at 3.5kbps and no one else in that range can transmit at the same time.

However, in EU there is a legal limit of 1% duty cycle on 868MHz band and collision avoidance mechanism, meaning on average you can send a packet (up to 255 bytes) once a minute.

Thanks for the info, that's really interesting!