Antenna Spreadsheet Questions

[cyee]

I'm not an RF engineer and the following calc results from the spreadsheet seem odd to me ... if someone could explain why:

1) If the antenna cable is extended (higher losses) why does the tower height requirement reduce?

2) If the speed is reduced, why does the tower height increase?

These characteristics appear contrary to what would be expected.

[beth-martin]

The way it was explained to me is that the tower height is based on the maximum distance that can be obtained with the defined cables and data rate. If you increase the cable length, the maximum distance that the link will work goes down (less EIRP) and therefore the Fresnel zone (http://www.cisco.com/univercd/cc/td/doc/product/wireless/bbfw/ptop/p2pspg02/spg02ch2.htm#xtocid827118) and earth bulge are less, hence a lower tower requirement. Likewise if the datarate reduces, the receiver threshold get better, and the maximum link distance increases, therefore a higher tower is needed.