Q: What is ‘fade margin’?
With any radio link installation, it is desirable to have signals strong enough to maintain communications regardless of weather or environmental extremes. Every radio link will have a threshold signal strength below which the wanted signals are too far buried in the noise of the radio channel to be received clearly. Ideally, radio links should be engineered to be working with signal levels at least 100 times (20dB) stronger than the absolute minimum workable signal. In this way, even with severe weather extremes or environmental factors such as summer leaf growth on trees that can attenuate signals, 20dB of ‘fade margin’ will ensure that a working link is maintained even if signal strengths are temporarily affected.
Fade margin of a link is therefore the amount that the radio signals are above the bare minimum threshold. An installer will temporarily fit coaxial attenuators into the coaxial cable feed to one of the antennas, adding extra attenuation until a point is reached where the link is just failing. The amount of attenuation that the installer fitted gives the ‘fade margin’ for the radio link, e.g 20dB + 6dB. Obviously the attenuators are removed once this test is done.
For long distance radio hops or where there are dense obstructions, 20dB fade margin may not be achievable. Sometimes as little as 10dB has to be accepted. A signal with 10dB fade margin is ten times stronger than the threshold signal level. On some higher radio frequencies having just 10dB fade margin may mean that some extreme weather phenomena could cause temporary radio link failure.