PathAlign-R
Spectracom Corporation
2200 / 2240 / 2241 PathAlign-R Manual
5-44
5.1.4 Fade Margin
Fade margin is the depth of fade, expressed in dB that a microwave receiver can tolerate while
still maintaining acceptable circuit quality [4].
5.2
Fresnel Loss
The primary component to path loss is the free-space signal loss from the transmitting antenna
to the receiving antenna. But additional path loss may also exist from multi-path reflections
(sometimes called Fresnel reflective loss) due to reflective surfaces such as water near the
direct wave, and intervening obstacles such as buildings, mountain peaks, etc., in the Fresnel
zone.
5.2.1 Fresnel Zone
Fresnel (frä nel'), named after Jean Augustin Fresnel, 1788-1827, French physicist. The Fresnel
zone is an elliptically shaped conical zone of power that propagates from the transmitting
antenna to the receiving antenna due to cancellation of some part of the wavefront by other
parts that travel different distances. If the total path distance between the transmitting antenna,
mountain peak, and receiving antenna is one wavelength greater than the direct distance
between antennas, then the clearance is said to be two Fresnel zones [4].
5.2.2 Fresnel Boundaries
The outer boundary of the first Fresnel zone is defined as the additional path length of all paths,
which are one-half wavelength of the frequency transmitted longer than the direct line-of-sight
path between antennas. If the total path distance is one wavelength longer than the direct path,
then the outer boundary is said to be two Fresnel zones. There are an infinite number of Fresnel
zones located coaxially around the center of the direct wave path. Odd number Fresnel zones
reinforce the direct wave path and even order number Fresnel zones cancel the direct wave
path.
5.2.3 Clearance
For reliability, point-to-point links are designed to have at least 0.6 of the first Fresnel zone
clearance from any obstruction from all sides (top, bottom, left and right of the first Fresnel
zone).
5.2.4 Refraction
The earth's curvature, as well as atmospheric conditions (temperature, pressure, and water
vapor), can refract or bend electromagnetic waves either up, away from, or down toward the
earth's surface. This bending can change frequently, hour to hour, day to night, season to
season, and weather pattern to weather pattern. Refractivity is usually greatest close to the
earth's surface and becomes smaller the higher above the surface you go. To compensate for
this effect, a refractivity gradient, or 'K' factor, is used when designing point-to-point
communication links. The 'K' factor is the ratio of the effective Earth radius to the actual Earth
radius.
