RF Engineering
What Is Free Space Path Loss (FSPL)? Formula, Examples and the 6 dB Rule
Free space path loss, or FSPL, is the signal a radio link loses simply because the wave spreads out as it travels, before any obstruction, rain or terrain is added. It is the floor of every link budget and the first number a planner reaches for. This guide explains what FSPL is, the formula in every useful form, why it depends on frequency even though empty space absorbs nothing, a worked 5 GHz example, the 6 dB rule for doubling distance or frequency, and where FSPL stops and real world path loss begins.