Appendix
5-6
602-95555-01 Rev A
NXE1-20 Digital Radio
h
= (2
d
1
x
d
2
)/3
K
Where,
h
= earth bulge in feet from the flat-earth reference.
d
1
= distance in miles (statute) from a given end of the microwave path to
an arbitrary point along the path.
d
2
= distance in miles (statute) from the opposite end of the microwave
path to the same arbitrary point along the path.
K
= K-factor considered.
Three K values are of particular interest in this connection:
1. Minimum value to be expected over the path. This determines the degree of "earth
bulging" and directly affects the requirements for antenna height. It also establishes
the lower end of the clearance range over which reflective path analysis must be
made, in the case of paths where reflections are expected.
2. Maximum value to be expected over the path. This leads to greater than normal
clearance and is of significance primarily on reflective paths, where it establishes the
upper end of the clearance range over which reflective analysis must be made.
3. Median or "normal" value to be expected over the path. Clearance under this
condition should be at least sufficient to give free space propagation on non-reflective
paths. Additionally, on paths with significant reflections, the clearance under normal
conditions should not fall at or near an even Fresnel zone.
For most applications the following criteria are considered acceptable:
K = 1.33 and CF = 1.0 F
1
K = 1.0 and CF = 0.6 F
1
K = 0.67 and CF = 0.3 F
1
Where CF is the Fresnel zone clearance and F
1
is the first Fresnel zone radius.
5.1.1.5
Path Profiles
Using ground elevation information obtained from the topographical map, a path profile should be
prepared using either true earth or 4/3 earth's radius graph paper. To obtain a clear path, all
obstacles in the path of the rays must be cleared by a distance of 0.6 of the first Fresnel zone
radius. Be sure to include recently erected structures, such as buildings, towers, water tanks,
and so forth, that may not appear on the map. Draw a straight line on the path profile clearing
any obstacle in the path by the distance determined above. This line will then indicate the
required antenna and/or tower height necessary at each end. If it is impossible to provide the
necessary clearance for a clear path, a minimum clearance of 30 feet should be provided. Any
path with less than 0.6 first Fresnel zone clearance, but more than 30 feet can generally be
considered a grazing path.
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