Inside Wireless: Fresnel Zones
Channel: RF elements s.r.o.
Date: 2020-08-18

The longer the RF link distance, the more obvious the Fresnel zone influence can become. What is Fresnel zone and what is it good for? Watch this short video for answers.

Fresnel zone is a pill shaped region between transmitting and receiving antenna. The term was named after 19th century French physicist Augustin-Jean Fresnel. The number of Fresnel zones is infinite but for practical use, we care most about the first few ones.

For good operation of an RF link, the line of sight should be clear and at least 60% of the first Fresnel zone radius in both azimuth and elevation plane should be free of obstructions. If an object is at the border of the first Fresnel zone, it can cause a decrease of the received signal level, also known as fading. 

Let’s take a vertically polarized antenna system. The delay of the wave reflected from the first Fresnel zone border is half a wavelength, where the wavelength is the distance over which the wave repeats its shape. When reflecting from a horizontal surface, vertically polarized wave flips its phase, so the total shift equals to one whole wavelength.
 
The direct and the reflected waves are therefore in phase at the receiver antenna and the total received signal gets stronger. When the wave reflects from the second Fresnel zone border, the situation is opposite - the total received signal is weakened. 

So for vertically polarized antenna system, the total signal strength is higher when the wave reflects from the odd Fresnel zone and weaker when it reflects from the even Fresnel zone. 

For horizontally polarized system, the situation is opposite - the even Fresnel zones reflections strengthen the signal and odd ones weaken it.

In practice, you can see the influence of the Fresnel zone reflections as changing signal level when changing the position of one of the antennas or, when you switch the polarizations - you can see a significant change in the signal level even if the antennas have balanced chains.

While figuring out why your link might not perform well can be a complex task, the knowledge of Fresnel zone effect can help you rule out some of the possible problems. Try to move the position of an antenna on one side of the link up and down to see if the signal improves. 

Since RF waves reflect from all kinds of surfaces such as lakes, roofs, or earth surface as well as the walls of the buildings, the general rule of thumb is to be aware that the longer the link distance, the bigger the Fresnel zones become potentially making the link more sensitive to the vertical antenna position adjustment.

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0:00 Intro
0:41 V-polarized antennas
1:27 H-polarized antennas
1:51 Practical tips

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