Fresnel Zone

Last Updated on May 10, 2026 by John Berry

The concept of Fresnel zones is important in understanding and calculating path loss due to foreground and mid-path obstructions. It is also important in understanding how reflections occur over specific path geometries. Fresnel zones get their name from the French physicist Augustin-Jean Fresnel, who developed the mathematics to enable understanding of how waves interfere. He showed how concentric zones of light or radio waves constructively and destructively interfere.

This page describes his two core ideas and how they affect radio amateurs. Reflections are important to EME operators, for example, when seeking signal strength enhancements at Moon rise and Moon set. And obstructions are important, for example, when choosing contest sites and maximising the chance of DX. The effects apply at all frequencies and are most relevant between about 10MHz and 10GHz. Whilst most relevant in space wave communications, the principles also apply when signals travel via the ionosphere.

Defining Fresnel zones

Waves spread out from a transmitter antenna. If a reflecting surface exists at the right place in the path toward the receiver, these spreading waves can interfere with one another. The result is Fresnel’s ‘concentric zones’. 

Using a physical model, you can imagine the Fresnel zone as a rugby-football-shaped lozenge extending between transmit and receive antennas. The first zone is the innermost and has the most significant volume.

Technically, the boundary of this zone is where reflected waves arrive 180 degrees out of phase with the direct wave. That occurs because the path travelled is different. Reflection from an obstacle also causes an inherent 180-degree phase shift. These two shifts cancel each other out. This results in the reflected wave arriving in phase with the direct wave.

The signals add. The result is constructive interference and a stronger signal. Conversely, reflections from the second Fresnel zone cause destructive interference. So in summary, reflections from odd order zones realise signal reinforcement at the receive antenna, but reflection from even order zones cause signal reduction.

The first Fresnel zone also describes the area within which obstructions cause a decibel loss over free space.

Any object intruding into this rugby-football-shaped volume results in diffraction loss, even if the direct line between the transmit and receive antennas is unobstructed.

You must maintain substantial clearance within most of this volume to avoid signal degradation. Radio amateurs are typically more constrained and suffer significant obstruction. Losses over free space for amateur radio links can be anything from a few decibels to many tens of decibels. Despite high loss, communication is often still possible.

Clearance and path loss

The relationship between clearance and path loss is shown in the graph below. This data is based on the foundational work of Kenneth Bullington (1957). The graph displays two distinct areas: the interference zone and the obstruction zone. In the interference zone, signal strength varies based on zone numbers as discussed above. Reflections from odd-numbered zones reinforce the signal. Reflections from even-numbered zones weaken it.

A key concept is the ‘0.6 FFZ clearance rule’. If you clear 60% of the first Fresnel zone radius, the path loss equals the so-called free space loss. This is the standard engineering target for reliable radio links. It acts as a point of departure for calculating diffraction loss ‘over free space’.

R is the reflection coefficient – 0 for very dry ground, 1 for salty, wet ground. Reality is normally somewhere between the two. Bullington’s methods for calculating loss have been replaced today with much more exact and accurate methods described in Rec. ITU-R.526-15 referenced in the bibliography.

Calculating the radius

To plan your station, it’s useful to calculate the radius of the first Fresnel zone for each frequency band on which you intend operating. The radius (R) in metres at any point is given by:

Where:

• R: The radius of the first Fresnel zone in metres (m).
• d₁ and d_2: The distances from the point to each antenna in kilometres (km).
• f: The operating frequency in gigahertz (GHz).
• D: The total path length between antennas in kilometres (d₁ + d₂).

To use megahertz (MHz), divide the frequency by 1000 first. Alternatively, change the constant to 547.7. For 60% clearance, multiply the final result by 0.6.

Here’s a Fresnel zone calculator using the above formula with frequency in megahertz.

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Practical paths

Consider then the geometry of a 500km space wave path – a tropospheric DX path. The radius of 0.6 of the 1st Fresnel zone at 144MHz at 10 metres out from the transmitter is about 3m.  So you need to make sure that for a 10 metre antenna height, say, the lower 3 metres is clear of obstruction. You need to be sure that everything in front of your station is no more than 7 metres high. Then repeat the calculation at say 30 metres.

The same principle can be employed for placing contest stations. Don’t throw away dBs in foreground loss for the sake of a little calculation. But as I noted above, a path of length 500km is going to be heavily obstructed anyway under normal conditions, thereby reducing the effect of any obstruction out at 300m. And short of moving house, there’s little either radio amateurs can do about the mid-path.

Here’s a diagram of a typical Fresnel zone radius and the nature of the zone in space as it relates to the station foregrounds and mid path..

On this simplified path profile between two amateur stations, there are two obstructions into the 0.6First Fresnel Zone. There’s a broad rounded obstruction mid path (highlighted by the big dashed circle) and a narrow, sharp obstruction in the foreground (a hill perhaps, within the first 1km) of the left-hand station. The mid-path obstruction could exhibit say 40dB diffraction loss but with Earth bulge impinging on a long path, this might be higher. This mid-path is, of course, unavoidable.

A foreground obstruction obscuring the bottom half of the 0.6 First Fresnel Zone might exhibit 6dB, and 12dB if the whole of the ellipse is intruded. This foreground obstruction loss is, to a large extent, often avoidable. Just as a reference, 12dB loss negates the gain afforded by a decent sized Yagi!

So, what does this analysis tell us?

Antenna height critical

Simply, make sure that your foreground is as clear as you can make it at all azimuths round your station.

Often the single variable here is antenna height. Increasing the antenna height to avoid foreground obstruction is much better than increasing transmitter power or antenna gain. But remember, the 0.6FFZ must be clear – not just the optical line of sight.

This analysis was at 144MHz. At 50MHz and 10m out from the antenna, the 0.6FFZ radius is about 5m. So even higher antennas are needed at lower frequencies in order to clear the foreground and avoid foreground obstruction loss.