User manual

Appendix A: Path Evaluation Information A-3
Moseley SL9003Q 602-12016 Revision G
There are in addition, of course, the second, third, fourth, etc. Fresnel zones, and these may be
easily computed, at the same point along the microwave path, by multiplying the first Fresnel
zone radius by the square root of the desired Fresnel zone number. All odd numbered Fresnel
zones are additive, and all even numbered Fresnel zones are canceling.
A.1.4 K Factors
The matter of establishing antenna elevations to provide minimum fading would be relatively
simple was it not for atmospheric effects. The antennas could easily be placed at elevations
somewhere between free space loss and first Fresnel zone clearance over the predominant
surface or obstruction, reflective or not, and the transmission would be expected to remain
stable. Unfortunately, the effective terrain clearance changes, due to changes in the air
dielectric with consequent changes in refractive bending.
As described earlier, the radio beam is almost never a precisely straight line. Under a given set
of meteorological conditions, the microwave ray may be represented conveniently by a straight
line instead of a curved line if the ray is drawn on a fictitious earth representation of radius K
times that of earth's actual radius. The K factor in propagation is thus the ratio of effective earth
radius to actual earth radius. The K factor depends on the rate of change of refractive index
with height and is given as:
K = 157/(157+dN/dh)
Where,
N is the radio refractivity of air.
dN/dh is the gradient of N per kilometer.
The radio refractivity of air for frequencies up to 30 GHz is given as:
N = (77.6P/T) + (3.73 x 105 )(e/T2)
Where,
P = total atmospheric pressure in millibars.
T = absolute temperature in degrees Kelvin.
e = partial pressure of water vapor in millibars.
The P/T term is frequently referred to as the "dry" term and the e/T2 term as the "wet" term.