Fresnel Zone: Key for Long Range WiFi Links

Fresnel Zone: Key for Long Range WiFi Links

Fresnel Zone:

The wireless or Wi-Fi Signal Travels in a shape like an Ellipse and the Fresnel Zone is the lower part of the Ellipse.

The Fresnel zone is the elliptical shape created between a transmitter and a corresponding receiver. The radius of the Fresnel zone is relative to the physical distance between the receiver and the transmitter and also to the frequency of RF transmitted.

Fresnel Zone becomes more important if a factor in long-range point to point links of bridges and backhauls.

Backhaul Fresnel zone with mountain in the Fresnel Zone:

Radio transmissions can either reach the receiver directly through a clear line of sight, making the strongest reception. Or can be reflected from spatial objects such as buildings and land feature either vertically or horizontally and assume a defected path. Reflected waves, depending on the angle of reflection travel further to reach the receiver and arrive phase-shifted at the antenna.

Fresnel Zone is an electromagnetic phenomenon, in which light waves or radio signals get diffracted or bent from solid objects near their path. The radio waves reflecting off the objects may arrive out of phase with the signals that traveled directly to the receiving antenna thus reducing the power of the received signal. Therefore you have to have more than visual line-of-sight: You need for the Fresnel zone to be clear or the obstructions will block part of your throughput - you will have lower rate of through-put if there is an obstruction in this zone.

The Fresnel zone is made up of three main zones. The first zone is closest to the line of sight and is the strongest. The second zone is between the first and the weakest third zone. Ideally, the first zone should be at least 60% free of abstraction for effective transmission.

Below are examples of the correct and incorrect placement of the antennas/radios on the towers:

Fresnel zone examples

Receiver’s antenna receives and combines both the reflected radio waves and the main wave. This can lead to constructive or destructive interference depending on the phase shift angle difference. If the two signals are phase-shifted by 180 degrees, the signals cancel out. Measurements of the Fresnel zone can be used to determine the expected phase shifts at the receiver.

Directional antennas are especially susceptible to the Fresnel zone effects. If destructive interference occurs in the forward direction of the antenna little or no reception will occur. With omni-directional antennas, the antenna will still radiate in different directions and pick up fair reception if destructive interference occurs in only one direction.

Ground clearing the Fresnel zone effectively improve the transmission throughput. With the rule of thumb, the acceptable obstacle clearance ratio is 60%.

Posted by George Hardesty on 16th Jul 2022

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