Fresnel Zone

More information here on wikipedia about Fresnel zone.

But here is scenario…

It’s early in the morning on a cool, clear day. The top of a proposed Air-Stream site is just visible through your binoculars and everything seems good

You go out and purchase and test the necessary wireless equipment, mounting an antenna on the rooftop, then fire it up, and… then nothing but problems.

The link does not seem to cooperate at all. It fades in and out, and the radio signal levels are low when there is any link at all. You check all the connections, twist and turn the antenna, and use your best language to get it to work… no improvement.

On first analysis it appears that some component in the wireless unit is malfunctioning, but the test worked properly and the units performed as expected.

Standing on the roof, you grab your mobile and call another Air-Stream member. They listen to your story, and then they ask a question:

“Any chance that another signal is interfering with your signal?”

I checked, and can pick out a few other SSIDs but nothing on the same channel “Good”, you reply

“Now, do you have clear Radio Line of Sight?” “Of course”, is your response “I have some binoculars, and can see the other site from where I’m standing??”

They pauses and then ask “That’s good to know, but what about proper Fresnel zone clearance?”

Fresnel zone clearance

At that moment, you begin to suspect that there are other factors other than a visual sight involved with establishing proper, usable Radio Line of Sight.

So what is going on here?

For two radios, or wireless units, to connect to each other, the radio signal must reach both units with adequate strength, and in a usable form.

For a wireless network spanning many kilometres these factors collectively are known as “Radio Line of Sight”, and become very important indeed.

Visual and Radio line of sight: are they different?

Light and radio are both forms of electromagnetic radiation but operating at very different frequencies. Just because you can establish visual line of sight, or see a distant building with binoculars on a clear day, does not mean you can establish a wireless link between them.

What is the Fresnel effect?

In the early 19th century French physicist, Augustin Fresnel (pronounced “Freh-Nel”) made an important observation about the behaviour of light. Fresnel noted that a ray of light passing near a solid object is subject to diffraction, or bending.

This diffraction caused the intensity of the original light beam to increase or decrease depending on how near the object was to the beam. This characteristic of electromagnetic radiation is known as the Fresnel effect.

Light and radio waves are subject to the same laws of physics, including the Fresnel effect. If an object like a mountain ridge or building is close to the radio signal path, it can affect the quality and strength of the signal.

Radio waves diffracted by such objects can affect the strength of the received signal. This happens even though the obstacle does not directly obscure the direct visual path.

This area, known as the “Fresnel zone”, and must be kept clear of all obstructions. That means the earth curvature, depth of the Fresnel one, and height of objects in the radio path must be added together to get the antenna mounting height. It’s usually adequate to use less than the full depth of the Fresnel zone to calculate clearance.

How much Fresnel zone clearance do I need?

60% of the Fresnel zone (F) is the generally accepted portion which must be kept clear. This assumes that there are no buildings or other obstructions in the way. If such obstructions exist, their height must be added to the total antenna mounting height.

To see the distant end with binoculars, we need only elevate our eye high enough to clear the Earth’s curvature. Remember, you saw the top of the distant site. The problem was that you only confirmed visual line of sight, not radio line of sight.

When it came time for the radio signal to pass from site A to site B, the lack of adequate Fresnel zone clearance caused signal diffraction, and degradation of the radio signal.

So what do you do?

  • Use an antenna with a more narrow lobe pattern, usually a higher gain antenna will achieve this

  • Raise the antenna mounting point on site A and/or site B

  • Build a new structure, e.g. a tower tall enough to provide adequate clearance

  • Increase the height of the existing antenna mounting point by installing a taller mast with stabilizing guy wires

  • Locate a different mounting point, e.g. building or tower, for the antenna

  • Remove the obstacle (such as taking a chainsaw to that tree)