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.

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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.
The first Fresnel zone is a geometry tool for finding that sensitive space.

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.
Points on the first Fresnel ellipsoid have a path via that point that is one-half wavelength longer than the direct path.

Major section

Checkpoint: Geometry

Longer wavelength means a larger Fresnel zone for the same path.

The radius is zero at each endpoint and largest near the midpoint.
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.

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.
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.
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.

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.
Longer wavelength means a larger Fresnel zone for the same path.
(0.6r_1) is a screening boundary associated with diffraction—not a promise of packet delivery.
so the 60% screen is about 11.9 m at that location.

Retrieval practice

Recall check 1 of 5

Packet Pete says: answer from memory, then check your reasoning.

Q1For the same path length, why check a lower-frequency link at each obstacle rather than only at an endpoint?

AAntenna brand fixes Fresnel radius at obstacles independent of geometry.

BHigher frequency enlarges a constant-radius zone along the whole path.

CLonger wavelength enlarges the zone, widest near the path midpoint.

DEndpoint clearance alone matters because midpoint radius is zero.

Show answer

Answer: C Frequency and obstacle position both affect first-zone radius.

Retrieval practice

Recall check 2 of 5

Packet Pete says: answer from memory, then check your reasoning.

Q2A path clears 60% of its first Fresnel zone; what does that result establish?

AA diffraction-geometry screen, not assured packet delivery.

BA precise fade margin across every weather and site condition.

CProof that obstacles along the radio path are all harmless.

DA guarantee of complete packet delivery on the path.

Show answer

Answer: A Availability still needs budget, model and field evidence.

Q3A radio path has wavelength 0.12 m. An obstacle is 100 m from each antenna. What is the first Fresnel-zone radius there?

AAbout 6 m.

BAbout 2.45 m.

CAbout 0.12 m.

DAbout 4.90 m.

Show answer

Answer: B Substitute both obstacle distances into the radius formula and take the square root.

Retrieval practice

Recall check 3 of 5

Packet Pete says: answer from memory, then check your reasoning.

Q4A surveyed obstacle enters the 60% first-Fresnel screening envelope. What is the strongest justified conclusion?

AThe screen assigns exactly 20 dB loss without path-specific diffraction analysis or installed tests.

BThe obstruction proves the link can never function under any conditions.

CThe path needs diffraction analysis and validation; the screen alone cannot set loss.

DA one-metre antenna raise as a guarantee of enough clearance and link margin.

Show answer

Answer: C The screen identifies a path where negligible diffraction should not simply be assumed.

Retrieval practice

Recall check 4 of 5

Packet Pete says: answer from memory, then check your reasoning.

Q5After a path profile predicts acceptable clearance, what decides whether the installed link meets its target?

ACopy the other site's mast height because an equal link distance produces an equal clearance profile.

BAccept the path prediction because a modeled clear link establishes installed performance.

CMeasure the installed link under representative acceptance conditions.

DAccept catalogue imagery alone without path or received-signal evidence.

Show answer

Answer: C Field evidence checks the actual link against its target.

Retrieval practice

Recall check 5 of 5

Packet Pete says: answer from memory, then check your reasoning.

Q6A team asks for the required antenna height for a 5 km link and provides only frequency and distance. What information is still essential?

AUse one standard mast height for a 5 km path because distance fixes the needed clearance above the route.

BNumber of sensors behind the gateway alone as a clearance-height rule.

CEndpoint elevations, antenna heights, terrain and obstacles plus the link target.

DVendor free-space range alone as proof of installed clearance and margin.

Show answer

Answer: C 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.

C · Frequency and obstacle position both affect first-zone radius.
A · Availability still needs budget, model and field evidence.
B · Substitute both obstacle distances into the radius formula and take the square root.
C · The screen identifies a path where negligible diffraction should not simply be assumed.
C · Field evidence checks the actual link against its target.
C · Distance and frequency give Fresnel radius, but required height depends on the path line, obstacles, curvature treatment, link budget, and acceptance target.