I was curious about the Xbee range in free space. I put together a spreadsheet with a derivation of the Friis Equation (somebody check my work if they get a chance). I know very little about 802.15.4, so I cannot speak in an educated manner about target link budgets, interference avoidance, packet detection, guardbands etc. Some things you should know:
- Digi claims that the sensitivity of the xbee is -100dBm, I suspect that it is not quite that sensitive
- Digi does not provide an SNR requirement, so I would recommend 3dbm for the lowest datarate (this is similar to 802.11b)
- Noise in the 2.4 spectrum outdoors is very high especially in places with tall buildings. XM Radio puts 40kW terrestrial repeaters in cities with tall buildings in an adjacent spectrum. I have not taken noise into account, but if you want to be conservative change the sensitivity to -92 dBm
- I used 0db gain for the antennas because I assume that the antenna gain of a simple 3dBi omni is negated by polarity mismatch - however I do have some ideas about how to reliably put a 3dbi Omni on the plane with a fixed 6 dbi "uptilt" omni (or 6dbi patch) on the ground. This would have the effect of more than doubling the range.