where L is the path loss in decibels, λ (or lambda) is the wavelength and d is the transmitter-receiver distance in the same units as the wavelength. Note the power density in space has no dependency on λ; The variable λ exists in the formula to account for the effective capture area of the isotropic receiving antenna.
And the area of each disk of the antenna may be had using π times r2.
Enjoy!
Powerline Lies... 