Free-Space Path Loss (Wireless Bridge)

Free-space path loss on a point-to-point wireless bridge.

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Details, formula, and sources

Free-space path loss FSPL(dB) = 32.44 + 20 log10(d_km) + 20 log10(f_MHz) from the Friis equation (the 32.44 constant is for km and MHz); the loss climbs 6 dB every time distance or frequency doubles. A 2.4 GHz link over 1 km loses 100.0 dB, and at 5.8 GHz about 107.7 dB. Received power Pr = Pt + Gt + Gr - FSPL, so a 20 dBm radio with 12 dBi antennas each end over that 1 km 2.4 GHz path delivers -56 dBm. Ideal free space only: cable/connector loss, rain and atmospheric attenuation, obstruction and Fresnel-zone diffraction, and multipath are separate, and a real link keeps a fade margin over this (compare Pr to the receiver sensitivity). Per Friis / ITU-R P.525. A planning estimate; the path survey and radio spec govern.

FSPL(dB) = 32.44 + 20 log10(d_km) + 20 log10(f_MHz); received power Pr = Pt + Gt + Gr - FSPL (all dB). The 32.44 constant is for distance in km and frequency in MHz.

The Friis transmission equation / free-space path loss, per ITU-R P.525, by name; first-principles RF.

The Friis / FSPL relation is public first-principles physics and ITU-R P.525 is a free ITU download; the distance, frequency, power, and antenna gains are the user's link inputs.

Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

Field names used by the API: distance_km, frequency_mhz, tx_power_dbm, tx_gain_dbi, rx_gain_dbi, fspl_db, rx_power_dbm

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