To quote myself…
In Slack, @Thomas mentioned that the reach of nodes at ground level will be much less, as the earth’s surface will be in the first Fresnell-zone, limiting the maximum distance. (I hope I paraphrased/translated this correctly.)
In another post, @Ideetron also explained that at least 6 meter high is good:
Why the height of >6 meters you ask?
This has to do with the Frenell Zone: Fresnel zone - WikipediaCalculate here: http://www.wirelessconnections.net/calcs/FresnelZone.asp
Not my cup of tea, but it sounds convincing.