I originally posted this as a discussion but given the new guidelines for posting, I thought I would post it as blog. Also anybody who saw this before will find that I discovered a few important things (like the minimum SNR for zigbee). The spreadsheet that I created I left this as an editable Google Docs sheet so please don't save as you play with it.
With this spreadsheet you will find that in some cases it would be possible to transmit 10km, with the right antennas further. This assumes an XBee PRO 2.4 GHz.
There are two important caveats you should know:
1) I am not an engineer, I only play one on TV - seriously though, I am in marketing, and I haven't derived an equation in a decade - so check my work if you feel like it. That said, I do work at a WiFi company, so at least I know how RF is supposed to work.
2) Digi is not forthcoming with the radio properties of their XBee module, this isn't surprising; with radios it is rare to publish this sort of information. I found the best information in IEEE research papers outlining Zigbee. the XBee could be worse or better than stated.
Anyhow, enjoy the spreadsheet.http://spreadsheets.google.com/ccc?key=0AqpOpoOZx7jhdFdsR1FlcGJ1ZEh...