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Link margin calculations (line of sight)

Knowing antenna gain, receiver sensitivity and transmit power, calculate the Link Margin

Calculate the Path Loss in dB

Path Loss (dB) = 37 dB + 20log(f) + 20log(d) with f in MHz and d in miles.

Calculate the maximum Received Signal Strength in dBm

Rss(dBm) = Transmit Power(dBm) + Receive Antenna Gain(dB) + transmit antenna gain(dB) – path loss(dB)

Estimate Fade Margin in dB

Outside operation with no obstruction, half wave antenna protruding from electronic enclosure: Fade Margin should be at least 5dB Inside building operation or compromised antenna (inside enclosure, antenna less than ½ wavelength from other conductors or operator's body: Fade Margin should be at least 40dB

Link Margin(dB) = Rss (dBm) – Receiver Sensitivity (dBm) – Fade Margin(dB)


Fade Margin is affected by many factors. In addition to type and mounting of the antennas, other factors include orientation of the antennas (cross polarization), proximity of large metal objects (causing multipath) and interfering radio signals (intentional or not) that may affect operation of the receiver itself.

ham_radio/calculate_link_margin.txt · Last modified: 2013/01/08 19:00 (external edit)
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