 Hi, I'm Thomas, and in today's Inside Virus episode we'll talk about the Fresnel zones. Fresnel zone is a pill-shaped box between a transmitting and receiving antenna. It was named after the 19th century French physicist Augustine Jean Trunel. The number of Fresnel zones is infinite, but for the practical use we only care 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 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 first. So when vertically polarized wave reflects from a horizontal surface, it flips its face. Additionally, compared to the line of sight wave, the wave reflected from the first Fresnel zone border is half a wavelength delay, where the wavelength is the distance over which a wave repeats its shape. So the total shift at the receiver antenna equals to one whole wavelength. The direct and the reflected waves are in phase at the receiver antenna and the total received signal is therefore stronger. When the wave reflects from the second Fresnel zone border, the situation is quite the opposite. The total received signal is weakened. So for a vertically polarized antenna system, the resulting signal is stronger when the wave reflects from the odd Fresnel zones and weaker when it reflects from the even Fresnel zones. For a horizontally polarized system, the situation is opposite. The odd Fresnel zone reflections weaken the signal and even ones strengthen it. In practice, you can see the influence of the reflections from the Fresnel zone as changing signal level with the changing position 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 a link might not perform well, maybe a complex task, the knowledge of the Fresnel zone influence might 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 IRF waves reflect from all kinds of surfaces, such as lakes, roofs or earth surfaces, 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 zone becomes, potentially making the link more sensitive to the vertical antenna position adjustment. For more interesting videos on different topics from the IRF world, check out some of our previous videos and don't forget to subscribe to our channel.