User manual

A-2 Appendix A: Path Evaluation Information
Moseley SL9003Q 602-12016 Revision G
Primary effects, caused by an obstacle that blocks the direct path, depend on whether it is
totally or partially blocking, whether the blocking is in the vertical or the horizontal plane, and the
shape and nature of the obstacle.
The most serious of the secondary effects is reflection from surfaces in or near the path, such
as the ground or structures. For shallow angle microwave reflections, there will be a 180° (half
wavelength) phase shift at the reflection point. Additionally, reflected energy travels farther and
arrives later, directly increasing the phase delay. The difference in distance traveled by the
direct waves and the reflected waves, expressed in wavelengths of the carrier frequency, is
added to the half wavelength delay caused by reflection. Upon arrival at the receiving antenna,
the reflected signal is likely to be out of phase with the direct signal, and may tend to add to or
cancel the direct signal. The extent of direct signal cancellation (or augmentation) by a reflected
signal depends on the relative powers of the direct and the reflected signals, and on the phase
angle between them.
Maximum augmentation will occur when the signals are exactly in phase. This will be the case
when the total phase delay is equal to one wavelength (or equal to any integer multiple of the
carrier wavelength); this will also be the case when the distance traveled by the reflected signal
is longer than the direct path by an odd number multiple of one-half wavelength. Maximum
cancellation will occur when the signals are exactly out of phase, or when the phase delay is an
odd multiple of one-half wavelength, which will occur when the reflected waves travel an integer
multiple of the carrier wavelength farther than the direct waves. Note that the first cancellation
maximum on a shallow angle reflective path will occur when the phase delay is one and one-
half wavelengths, caused by a path one wavelength longer than the direct path.
The direct radio path, in the simplest case, follows a geometrically straight line from transmitting
antenna to receiving antenna. However, geometry shows that there exist an infinite number of
points from which a reflected ray reaching the receiving antenna will be out of phase with the
direct rays by exactly one wavelength. In ideal conditions, these points form an ellipsoid of
revolution, with the transmitting and receiving antennas at the foci. This ellipsoid is defined as
the first Fresnel zone. Any waves reflected from a surface that coincides with a point on the first
Fresnel zone, and received by the receiving antenna, will be exactly in phase with the direct
rays. This zone should not be violated by intruding obstructions, except by specific design
amounts. The first Fresnel zone, or more accurately the first Fresnel zone radius, is defined as
the perpendicular distance from the direct ray line to the ellipsoidal surface at a given point
along the microwave path. It is calculated as follows:
F1 = 2280 × [(d1×d2) / (f × (d1+d2))]½ feet
Where,
d1 and d2 = distances in statute miles from a given point on a microwave path to the ends of the
path (or path segment).
f = frequency in MHz.
F1 = first Fresnel zone radius in feet.