Core Networking · Study deck
Fresnel Zones and Deployment
Two antennas can see each other while the radio path is still vulnerable to diffraction.
Packet Pete is your guide for this deck.
After studying this chapter
Learning objectives
You will be able to:
- Explain: Paths slightly longer than the direct path can arrive with different phase, so terrain or an object near that line can change the received field.
- Explain: The zone is widest near the midpoint, but an off-centre ridge, tree line, roof edge, or curvature term can produce the smallest normalized clearance.
- Explain: Recommendation ITU-R P.526-16 defines the diffraction zone as beginning where path clearance equals 60% of the first Fresnel-zone radius.
- Explain: ITU-R P.526 uses clearance equal to 60% of the first-zone radius as a boundary for the diffraction zone.
Major section
Start With the Invisible Space Around the Link
Two antennas can see each other while the radio path is still vulnerable to diffraction.
- Radio energy does not travel only along a pencil-thin sight line.
- Paths slightly longer than the direct path can arrive with different phase, so terrain or an object near that line can change the received field.
Major section
In 60 Seconds
The zone is widest near the path midpoint.
- Lower frequencies have longer wavelengths and therefore larger Fresnel radii for the same geometry.
- ITU-R P.526 uses clearance equal to 60% of the first-zone radius as a boundary for the diffraction zone.
Major section
First Fresnel Zone Geometry
Points on the first Fresnel ellipsoid have a path via that point that is one-half wavelength longer than the direct path.
- where (d_1) and (d_2) use the same distance unit and the result follows that unit.
- The table says how large the geometric screen is at the midpoint.
Major section
What the 60% Screen Means
Recommendation ITU-R P.526-16 defines the diffraction zone as beginning where path clearance equals 60% of the first Fresnel-zone radius.
- It also provides diffraction methods for actual terrain and obstacle geometries.
- Reliability and availability are system targets.
Major section
Checkpoint: Screen, Then Model
(0.6r_1) is a screening boundary associated with diffraction—not a promise of packet delivery.
- An obstruction inside the screen triggers more analysis; it does not justify an invented fixed loss.
Major section
Build the Path Profile
Antenna height is a property of the entire path, not “obstacle height plus midpoint radius.” Build a profile in a common vertical datum.
- The limiting point is the smallest normalized clearance, not necessarily the tallest obstacle or the midpoint.
Major section
Worked Path-Profile Decision
so the 60% screen is about 11.9 m at that location.
- The example teaches the decision boundary: the screen found a path that needs modelling and evidence.
Major section
Common Reasoning Traps
A clear pencil-thin sight line can still have terrain or clutter inside the first-zone screen.
- The zone is widest near the midpoint, but an off-centre ridge, tree line, roof edge, or curvature term can produce the smallest normalized clearance.
- Penetration loss varies with frequency, material composition, thickness, moisture, incidence angle, openings, and construction.
Major section
Summary
Radius depends on wavelength and the distances from the point to both antennas.
- The 60% value is a useful ITU diffraction screening boundary, not a reliability guarantee.
- Antenna height must come from the complete path profile and link target, not a universal minimum.
Deck summary
Key takeaways
Two antennas can see each other while the radio path is still vulnerable to diffraction.
- The zone is widest near the path midpoint.
- Points on the first Fresnel ellipsoid have a path via that point that is one-half wavelength longer than the direct path.
- Recommendation ITU-R P.526-16 defines the diffraction zone as beginning where path clearance equals 60% of the first Fresnel-zone radius.
- (0.6r_1) is a screening boundary associated with diffraction—not a promise of packet delivery.
Retrieval practice
Recall check 1 of 3
Packet Pete says: answer from memory, then check your reasoning.
Q1Two links have the same endpoints and path length. Link A uses a lower frequency than Link B. Which statement about the first Fresnel zone is correct?
Show answer
Answer: A Radius scales with the square root of wavelength, so lower frequency produces a larger zone for the same geometry.
Retrieval practice
Recall check 2 of 3
Packet Pete says: answer from memory, then check your reasoning.
Q2A surveyed obstacle enters the 60% first-Fresnel screening envelope. What is the strongest justified conclusion?
Show answer
Answer: A The screen identifies a path where negligible diffraction should not simply be assumed.
Retrieval practice
Recall check 3 of 3
Packet Pete says: answer from memory, then check your reasoning.
Q3A team asks for the required antenna height for a 5 km link and provides only frequency and distance. What information is still essential?
Show answer
Answer: A Distance and frequency give Fresnel radius, but required height depends on the path line, obstacles, curvature treatment, link budget, and acceptance target.
Print reference
Answers
Answer key.
- A · Radius scales with the square root of wavelength, so lower frequency produces a larger zone for the same geometry.
- A · The screen identifies a path where negligible diffraction should not simply be assumed.
- A · Distance and frequency give Fresnel radius, but required height depends on the path line, obstacles, curvature treatment, link budget, and acceptance target.