Wondering what side lobes are and if they are harmful to your WISP network? In this short video you will learn about Side Lobes, Front to Back (FtB) ratio and side lobe level (SLL).

The radiation diagram of an antenna tells us how strongly antenna radiates in any direction. The main lobe has the highest gain and most antennas have only one main lobe.

All the lobes other than the main lobe are called side lobes. The size of sidelobes depends on the type, size, and construction of an antenna. It’s important to understand that through side lobes, antenna transmits but also receives the signal from unwanted directions.

Requirements on the sidelobes vary depending on the application, but generally, we can say side lobes are a no-no. For point to point applications, we want to deliver as much energy as possible to a single point, and any side lobe means part of it is wasted. For area coverage applications - like sector antenna - we want to avoid all side lobes outside the main lobe.

Antenna side lobes are especially harmful in fixed wireless access networking. Any signal radiated to unwanted direction is noise to any other radio in the area. And vice versa, any signal received from unwanted direction is noise to your own radio.

Side lobes increase the noise level and therefore should be avoided at all costs to ensure sustainable use of the spectrum. Whether the spectrum is licensed or unlicensed, does not matter, however, the point of no return is achieved quicker in the unlicensed bands due to uncontrollable amount of devices deployed.

Side lobes are often judged by so-called Side Lobe Level or Front to Back ratio. Side Lobe Level is the difference between the gain of the main lobe and the gain of the strongest sidelobe. Front to Back ratio is the difference between the gain of the main lobe and the back lobe. Most antenna manufacturers often misuse Front to Back ratio as the measure of sidelobe suppression, while it only gives information about the gain of the back lobe. It says nothing about the rest of the side lobes antenna might have.

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0:00 Introduction
0:14 Radiation Diagram
0:24 Side Lobes
0:42 Requirements
1:07 Avoid Side Lobes
1:27 Side Lobe Level (SLL)
1:32 Front to Back Ratio
1:47 Examples

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