Fresnel Zones and Multipath Effects

See why a visually clear radio path can still fail when the Fresnel zone is blocked or reflections arrive out of phase

Wireless Propagation Fresnel Clearance Interactive Workbench

Fresnel Zone and Multipath Workbench

Move an obstacle through a radio path, check the 60% first-zone clearance rule, and compare the direct signal with a reflected signal that can add or cancel at the receiver.

868 MHz Frequency and wavelength

9.3 m Midpoint first-zone radius

43% Clearance at obstacle

Mixed Multipath result

What

A Fresnel zone is the rounded space around the line of sight where radio energy still matters.

Why

An object can hurt the link even when it does not fully block the visual line between antennas.

Try First

Choose Fence near midpoint, then press Step Position. The radius changes as the object moves.

Notice

The zone is widest near the middle. Lower frequencies and longer links need more vertical clearance.

Scenario

Preset Fence near midpoint Clear farm link Long LoRaWAN hop Indoor multipath 5 GHz backhaul

Link Controls

Frequency 433 MHz868 MHz5800 MHz Link distance 0.05 km1.00 km5 km TX antenna height 1 m8.0 m30 m RX antenna height 1 m8.0 m30 m Obstacle position near TX50%near RX Obstacle height 0 m4.0 m30 m

This teaching model assumes a single obstacle and a single ground reflection. Real links also need fade margin, terrain data, antenna patterns, polarization, and local interference checks.

Shared wireless path

Fence near midpoint: 868 MHz, 1.00 km, obstacle at 50%.

3. Clearance check

Fresnel clearance view

Multipath reflection view

Obstacle point500 m from TX

Required gap5.6 m below line

Available gap4.0 m below line

Path difference0.00 m

Fresnel zone is intruded 60% of the first Fresnel zone should stay clear for a dependable outdoor planning target.

Lower frequency, larger zone For the same distance, lower frequency means longer wavelength and a wider Fresnel zone.

Multipath can fade Reflected copies can arrive in phase, out of phase, or somewhere in between.

Current explanation

The obstacle cuts into the recommended clearance region. The line of sight is not enough. Compare the available vertical gap with 60% of the first Fresnel-zone radius at the obstacle position.

1. Direct path

Start with the line between transmitter and receiver.

2. First zone

The zone is widest near the middle and shrinks at both antennas.

3. Clearance check

Keep at least 60% of the first zone free from obstacles.

4. Reflections

Multipath adds phase-shifted copies at the receiver.

Learning Support

Minimum Background

A radio wave does not travel as a thin line. Some useful energy travels near the line, so near-path objects matter.

Mental Model

Think of the link as a long rugby-ball-shaped space around the direct path. The middle needs the most room.

Design Meaning

If the zone is blocked, raising antennas can help more than changing protocol settings or transmit power.

Quick Reference

Radius Formula

First-zone radius at a point: r1 = sqrt(lambda * d1 * d2 / (d1 + d2)). Distances use meters.

Midpoint Shortcut

At midpoint: r1 = sqrt(lambda * D / 4). For 868 MHz over 1 km, r1 is about 9.3 m.

60% Rule

A common planning target is to keep 60% of the first Fresnel zone clear, not necessarily the full zone.

Frequency Effect

Lower frequency has longer wavelength and wider Fresnel zones. Higher frequency has smaller zones but often less penetration.

Multipath Phase

Path difference divided by wavelength gives phase turns. Near half a turn can cancel the direct signal.

Model Limits

This is a planning visual. Real deployments should also include link budget, fade margin, antenna gain, and terrain survey.

Guided Practice

Move the Obstacle

Press Step Position. The same obstacle becomes more serious near the midpoint because the zone is wider there.

Change Frequency

Lower the frequency to 433 MHz. The zone expands, so the same link needs more room.

Raise Antennas

Increase TX and RX height until the available gap reaches the 60% clearance target.

Compare Long Link

Select Long LoRaWAN hop. Notice how distance grows the midpoint zone even when frequency is similar.

Watch Phase

Move the reflection point. Small path changes can shift the reflected signal from helpful to harmful.

Decision Rule

If line of sight is clear but the zone is not, change mounting height or path before increasing transmit power.

Link Budget Calculator Combine clearance with antenna gain, loss, and fade margin. Path Loss Models Compare distance, materials, and attenuation models. Signal Propagation Simulator Explore propagation effects in a broader interactive view.