Free Space Propagation
In free space, all electromagnetic waves (radio, light, X-rays, etc.) obey the inverse-square law which states that the power density of an electromagnetic wave is proportional to the inverse of the square of the distance from a point source or:
Doubling the distance from a transmitter means that the power density of the radiated wave at that new location is reduced to one-quarter of its previous value.
The power density per surface unit is proportional to the product of the electric and magnetic field strengths. Thus, doubling the propagation path distance from the transmitter reduces each of their received field strengths over a free-space path by one-half.
Read more about this topic: Radio Propagation
Famous quotes containing the words free and/or space:
“Do we call this the land of the free? What is it to be free from King George and continue the slaves of King Prejudice? What is it to be born free and not to live free? What is the value of any political freedom, but as a means to moral freedom? Is it a freedom to be slaves, or a freedom to be free, of which we boast?”
—Henry David Thoreau (1817–1862)
“Here were poor streets where faded gentility essayed with scanty space and shipwrecked means to make its last feeble stand, but tax-gatherer and creditor came there as elsewhere, and the poverty that yet faintly struggled was hardly less squalid and manifest than that which had long ago submitted and given up the game.”
—Charles Dickens (1812–1870)