24 Fresnel Zones and Deployment

24.1 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. It does not predict reliability by itself. A release decision still needs a path profile, a link budget, an appropriate propagation model, and field measurements.

24.2 Learning Objectives

By the end of this chapter, you will be able to:

• calculate (r_1) at the midpoint or at a specific obstacle;
• explain why wavelength and obstacle position matter;
• distinguish a 60% geometry screen from a reliability claim;
• build an antenna-height decision from a complete path profile;
• define evidence for validating the installed link.

24.3 First Fresnel Zone Geometry

Imagine an ellipsoid whose two focal points are the antenna phase centres. Points on the first Fresnel ellipsoid have a path via that point that is one-half wavelength longer than the direct path. A cross-section through the ellipsoid forms the first Fresnel zone.

For wavelength (lambda), the radius at one location along the path is

where (d_1) and (d_2) use the same distance unit and the result follows that unit.

With distance in kilometres and frequency in gigahertz, a convenient form is

At the midpoint of a total path (D), (d_1=d_2=D/2), so

for (D) in kilometres, (f) in gigahertz, and radius in metres.

24.3.1 Two geometry checks

Path	Midpoint (r_1)	(0.6r_1) screening distance
100 m at 2.4 GHz	1.77 m	1.06 m
5 km at 0.915 GHz	20.2 m	12.1 m

The table says how large the geometric screen is at the midpoint. It does not say that a 2–3 m pole guarantees the first link, that a 15 m mast guarantees the second, or that either link will meet a delivery target. Antenna phase-centre elevations, terrain, obstacles, Earth curvature, clutter, antennas, interference, and receiver performance still matter.

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.
• Calculate at every important obstacle; the midpoint is not automatically the limiting location.

24.4 Try It: Fresnel Geometry Calculator

This calculator produces geometry only. “Clear” means the selected (0.6r_1) screen is clear at that point—not that the link is reliable.

24.5 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.

That supports this workflow:

1. calculate the first-zone radius along the path;
2. draw the (0.6r_1) screening envelope around the direct path;
3. find terrain, vegetation, structures, or Earth curvature that enter it;
4. use an appropriate diffraction or propagation method for those obstructions;
5. include the predicted loss in the link budget;
6. validate the installed link under representative conditions.

It does not support these shortcuts:

• “60% clear guarantees a reliable link”;
• “40% clear always costs a fixed number of decibels”;
• “every outdoor sensor needs a minimum pole height”;
• “a material always contributes one fixed loss”;
• “real range is a fixed percentage of free-space range.”

Reliability and availability are system targets. They depend on link margin, antenna patterns and orientation, polarization, terrain and clutter, interference, weather and seasonal change, implementation behavior, and the metric being protected.

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.
• Keep the link budget and propagation assumptions beside the geometry.

24.6 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.

For each sampled position (x), record:

• ground elevation and relevant clutter or obstacle height;
• the straight line between the two antenna phase centres;
• first-zone radius (r_1(x)) and the selected (0.6r_1(x)) screen;
• Earth-curvature treatment and the effective Earth radius used, where relevant;
• the vertical clearance between the path line and the obstacle top.

The limiting point is the smallest normalized clearance, not necessarily the tallest obstacle or the midpoint.

24.7 Worked Path-Profile Decision

Consider a hypothetical 5 km path at 915 MHz with an obstacle 2 km from the transmitter and 3 km from the receiver.

so the 60% screen is about 11.9 m at that location.

Suppose the direct path line is 18 m above the common datum there and the surveyed obstacle top is 9 m. The geometric clearance is 9 m, or

The obstacle enters the 60% screen. The correct conclusion is not “the link loses 15 dB” or “raise one antenna by exactly 2.9 m.” The next steps are:

1. confirm the vertical datum and obstacle survey;
2. include Earth curvature and the project’s refractivity assumption if relevant;
3. calculate diffraction with a method suited to the path geometry;
4. test candidate height changes at both ends;
5. update cable loss, antenna pattern, and structural constraints;
6. measure the installed link and compare it with the target.

The example teaches the decision boundary: the screen found a path that needs modelling and evidence.

Checkpoint: Path Profile

• Put terrain, obstacles, path line, and Fresnel radii on one datum.
• Find the minimum normalized clearance along the full path.
• Treat antenna height as a candidate to test, not a universal lookup-table answer.

24.8 Knowledge Check: Frequency and Radius

24.9 Knowledge Check: Interpret 60%

24.10 Field Validation

Geometry and prediction produce a candidate design. Field evidence decides whether the installed link meets its target.

Record the test conditions:

• endpoint coordinates and antenna phase-centre heights;
• antenna model, gain pattern, polarization, orientation, and cable loss;
• transmit configuration and receiver configuration;
• terrain/profile source, obstruction state, and season;
• interference conditions and channel settings;
• firmware, retry, and rate-adaptation settings.

Record both radio and application outcomes:

Choose the fade and availability margin from the required service target and the applicable propagation evidence. Do not insert a universal 10–20 dB margin without explaining the target and model that produced it.

If prediction and measurement disagree, investigate rather than hiding the discrepancy in a larger arbitrary margin. Check antenna orientation, connectors, cable loss, datum errors, vegetation, unexpected reflectors, interference, receiver settings, and the propagation model’s scope.

24.11 Knowledge Check: Antenna Height

Work this check from geometry to acceptance evidence. Frequency and distance determine a Fresnel-zone radius, but they do not determine a mast height by themselves. The path review still needs endpoint elevations, antenna phase-centre heights, the full terrain and obstacle profile, the chosen curvature treatment, and the clearance target. Only after that geometry is combined with the link budget and fade margin can the team propose a height and state what field survey or link test will confirm it.

Checkpoint: Field Evidence

• Keep the reviewed path profile and link budget with the installation record.
• Measure delivery, retry, latency, RSSI, and SNR under representative weak conditions.
• Reopen the model when field evidence disagrees with prediction.

24.12 Common Reasoning Traps

24.13 Fresnel Release Record

24.14 Summary

• The first Fresnel zone describes radio-path geometry around the direct line.
• 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.
• Release evidence combines geometry, propagation modelling, link budget, installation details, and measurements.

24.15 Engineering Reference

• Recommendation ITU-R P.526-16: Propagation by diffraction—Fresnel ellipsoids and zones, the 60% diffraction-zone boundary, and diffraction methods for real obstacles and terrain.

24.16 What’s Next

24.17 Key Takeaway

Use Fresnel geometry to find where a path deserves closer analysis. Reliability comes only after the path profile, diffraction model, link budget, installation, and field measurements agree.