Fresnel zones and Radio Line
Set display of Fresnel zones on LOS Parameters form
Computation of Fresnel zones for radio line of sight requires two parameters:
- frequency in Mhz affects size of Fresnel zones.
- k factor, or effective earth radius multiple, which is set as
part of the vertical earth curvature model. A
standard radio atmosphere (standard refraction) has a k
factor of 1.333; this value can be used for gross
planning of radio link systems. The value of k will vary
with altitude, time of day, season, weather
conditions, latitude, and proximity to the coast. Values
for various conditions have been tabulated; often the
tabulations give delta N, the mean refractivity gradient
in the first km of the atmosphere. If k > 1, the wave
refracts toward the earth; if k < 1, the wave refracts
upward toward space. Worst case k values will be about
0.4; k=1.33 represents the ideal case. K affects the amount of curvature on the
Interpretation of the Fresnel zones:
- Radio line of sight requires no intrusions into the first
Fresnel zone within the first and last1 km next to the
receiving and transmitting antennas.
- Want no intrusions more than 40% into the first Fresnel
zone at any point. (optimal clearance requires 60% of the
first Fresnel zone).
- At least grazing line of sight must exist during adverse
refraction (when k =1 or k= 0.667).
The Fresnel ellipsoid is defined as the loci of all points for
which of the sum of the distances from the two antennas is
greater by half a wavelength than the direct distance.
The first Fresnel zone is shown in maroon on the profile. The
inner 20% of the first Fresnel zone is shown in purple, and must
be free from all obstructions.
You can get a graph showing
intrusions into the first Fresnel zone.
Last revision 7/7/2015