How this calculator works
Geometric spreading reduces the power intercepted by an antenna as distance and frequency increase for fixed antenna gains. The isotropic attenuation is the basic free-space term. Subtracting transmit and receive gains gives the net link loss used here. The Friis model assumes unobstructed far-field propagation, matched polarization and aligned antennas; it excludes rain, atmospheric absorption, cable losses, reflection and diffraction.
Formula
Lfs = 20 log10(4 pi d f/c). Llink = Lfs - Gt - Gr, with gains in dBi, loss in dB, d in metres, f in Hz and c = 299792458 m/s.
Worked example
A 1 km link at 1 GHz has isotropic free-space attenuation 92.44778 dB. Transmitter gain 3 dBi and receiver gain 2 dBi reduce the net link loss to 87.44778 dB.
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Frequently asked questions
Why does frequency change the loss?
With fixed gain, effective receiving aperture decreases as wavelength decreases. Fixed physical apertures imply frequency-dependent gains.
Is the result valid beside an antenna?
The relation requires far-field separation. Check the antenna far-field distance before using it.
Does this predict received power?
Subtract net link loss and any additional losses from transmitted power expressed in dBm to obtain received dBm.