1---
2app_version: 0.1.0
3precision: 2
4form_title: Fresnel Zone Clearance
5form_description: Will that hill/tree/roofline actually block a point-to-point link? Enter the path distance, an obstacle's position and height, and the frequency — get the first Fresnel zone radius and how much clearance you really have.
6form_inputs: Total path distance (km)|d_total_km, Distance to obstacle (km)|d1_km, Obstacle height above the line-of-sight (m)|obstacle_height_m, Frequency (MHz)|freq_mhz
7form_outputs: First Fresnel zone radius at the obstacle (m)|r1_m, Clearance (m)|clearance_m, Clearance as % of r1|clearance_pct
8---
9# Fresnel Zone Clearance
10// "The antennas can see each other on the map — but is the signal path
11// actually clear?"
12//
13// Line-of-sight isn't enough: a chunk of the first Fresnel zone (an
14// ellipsoid around the straight line between the antennas) needs to be
15// obstacle-free too, or diffraction eats into the signal. The rule of thumb
16// used in link planning: keep at least 60% of the first Fresnel zone clear.
17
18d_total_km = 15 // km, total path length
19d1_km = 6 // km, distance from your end to the obstacle
20obstacle_height_m = 25 // m, obstacle height ABOVE the straight line-of-sight
21freq_mhz = 144 // MHz
22
23d2_km = d_total_km - d1_km // distance from the obstacle to the far end
24
25# First Fresnel zone radius at the obstacle
26speed_of_light = 299792458 // m/s
27wavelength_m = speed_of_light / (freq_mhz * 1e6)
28
29d1_m = d1_km * 1000
30d2_m = d2_km * 1000
31
32r1_m = sqrt(wavelength_m * d1_m * d2_m / (d1_m + d2_m))
33
34"Wavelength (m)": wavelength_m
35"First Fresnel zone radius here (m)": r1_m
36
37# Clearance
38// Positive clearance = the obstacle sits below the line-of-sight already;
39// clearance_m is how much of the Fresnel radius is still free above it.
40clearance_m = r1_m - obstacle_height_m
41clearance_pct = clearance_m / r1_m * 100
42
43"Clearance above the obstacle (m)": clearance_m
44"Clearance as % of first Fresnel zone": clearance_pct
45
46// Common planning thresholds.
47"60% clearance needed (m)": 0.6 * r1_m
48"100% clearance (m)": r1_m
49