edVirtus Reference

What Are Sidelobes?

Why Do Antennas Radiate Energy Outside Their Main Beam?

Sidelobes are smaller regions of radiation that occur away from the main beam of an antenna. While the main lobe directs most of the transmitted or received energy toward the intended direction, sidelobes radiate or receive smaller amounts of energy at other angles. Sidelobes are an unavoidable consequence of the way real antennas distribute electromagnetic energy and are present, to some extent, in almost every practical antenna.

The basic principle is that an antenna cannot concentrate all of its transmitted power into a single, infinitely narrow beam. Because electromagnetic waves generated across the antenna combine by constructive and destructive interference, small secondary peaks appear in the radiation pattern alongside the main beam. These secondary peaks are the sidelobes, and their strength depends on the antenna's design, size, and method of excitation.

A useful analogy is the beam from a handheld flashlight. Most of the light is concentrated in the bright central beam, but some weaker light spreads to the sides, illuminating areas outside the main direction. Similarly, an antenna radiates most of its energy in the main lobe while smaller amounts appear in the sidelobes.

In transmitting systems, sidelobes represent energy that is radiated in unwanted directions. This reduces the amount of power delivered into the main beam and can increase the likelihood of interference with other communication systems. In receiving systems, sidelobes allow signals from unintended directions to enter the antenna, potentially increasing interference, noise, or the reception of unwanted transmissions.

The strength of sidelobes is usually expressed relative to the main beam. A sidelobe level of –20 dB, for example, indicates that the sidelobe radiates only one-hundredth of the power density of the main lobe. Antenna designers generally seek to minimise sidelobe levels while maintaining adequate gain and beamwidth, although reducing sidelobes often requires a compromise with other aspects of antenna performance.

It is important to distinguish sidelobes from the back lobe. Sidelobes occur at angles away from the main beam, whereas the back lobe is the radiation directed approximately opposite the main beam. Both represent unwanted radiation, but they arise from different characteristics of the antenna and are often specified separately.

Today, controlling sidelobes is an important objective in radar, satellite communications, radio astronomy, and cellular networks. Low sidelobe antennas improve spectrum efficiency, reduce interference between neighbouring systems, and increase resistance to unwanted signals. Although sidelobes can never be eliminated completely, careful antenna design and signal processing techniques can reduce them significantly, allowing communication systems to operate more efficiently and reliably.