Link Fade Margin

Value to Enter:

Enter any additional fade margin you would like to apply to the calculation. A very widely used, "typical" link fade margin is 10 dB and this value becomes the default if no other value is entered. You may want to use a fade margin of 12 dB to be conservative and a fade margin of 6 dB to be aggressive. You could also considering Increasing your fade margin by and additional 3 dB in a highly reflective environment and by another 3 dB to 6 dB in an electrically noisy environment. Hence, a automotive repair shop, with many power tools and arc welding rigs might require a Link Fade Margin margin as high as 21 dB (12+3+6)

Significance of This Value:

The Link Fade Margin is applied to link budget calculations as a way to compensate for normal and ordinary deviations in link characteristics. It comprises both "Fade Margin" (caused by multipath reflections) as well as other factors that may effectively attenuate the transmitted signal. There are a number of factors that may cause the theoretical calculations for a link to vary from the measured signal characteristics. The Link Fade Margin allows the designer to input additional loss that will be incorporated into calculated results.

Background and Technical Perspective:

Dynamic Environmental Conditions

Theoretical RF engineering formulas use constants of proportionality that are based on average conditions and the real world has a dynamic range of condition measurements. Real-world conditions vary. Environmental conditions change from clear sky, to high humidity, to rain; and from hot to cold. Birds fly through elevated wireless links and people walk in the signal volume of ground-based antennas. The link fade margin is used as a "margin of error" to accommodate the practical expectations of a wireless link. It should be noted that Terrain Roughness can introduce additional fading, beyond any standard fade guidelines.

Rayleigh Fading

When signal energy leaves the transmitter it spreads out, forming a generally spherical wavefront. This wavefront propagates outwards and strikes various objects along its way. Some paths are generally straight from the transmitter to the receiver, others bounce off walls or objects and are longer. The difference in path lengths causes the composite signal to vary in strength. When the paths are of such a length as to cause received signals to arrive out of phase the signal strength goes down. This is called multipath fading, also called Rayleigh Fading. The design of a wireless system must consider Rayleigh Fading and must attempt to minimize the depth of any given fade event (the degree of signal reduction), the duration of the fade, and whether the fade causes the received signal to drop below the required receiver sensitivity. A system should be designed to operate at a low outage probability.

Foliage Attenuation

When trees are present, and the 60% radius of the 1st Fresnel Zone is penetrated by the tree line, the Link Fade Margin can be used to adjust for the foliage attenuation.