Radar beamwidth narrowing improves detection range and system accuracy. However, an antenna cannot concentrate all radiated power into a single beam. In practice, radar power is distributed among several components, commonly referred to as the main lobe and sidelobes.
Basic concepts
The main lobe is the region around the direction of maximum radiation, typically defined as the angular area within 3 dB of the main beam peak; it is the radar's primary working direction. Sidelobes are smaller radiation lobes surrounding the main beam and generally represent undesired radiation directions. Backlobes (or rear lobes) are sidelobes oriented opposite the main lobe.
The half-power beamwidth (the -3 dB beamwidth) is the angular interval where the radiation pattern amplitude falls to 50% (or -3 dB) of the main lobe peak. The first-null beamwidth is the angle between the first nulls on either side of the main lobe, also known as the main lobe subtended angle; it is another useful parameter.
Problems caused by sidelobes
Although sidelobe power is much lower than the main beam, energy received through sidelobes can still degrade radar detection performance and cause several issues.
When the main beam is near the horizon, a significant portion of sidelobes may point toward the ground. Even with lower power, sidelobe returns from the ground can produce strong clutter because the ground is much closer to the radar than an airborne target; sidelobe echo strength can be comparable to the target echo. Increasing transmitted power does not solve sidelobe clutter. Doppler processing can mitigate some clutter, but sidelobe clutter still causes many detection problems.
Because sidelobes may still point at potential receivers while the main beam points elsewhere, strong sidelobes make a radar easier to detect. High-power sidelobes can alert targets or electronic support measures/receiver warning systems (ESM/RWR) before the main beam intercepts them, giving adversaries more time to analyze the radar's transmitted signal and geolocate the radar. Adversaries can also inject deceptive or spoofing signals through sidelobes. Reducing sidelobes is therefore important but challenging, requiring careful design, precise fabrication, and calibrated measurement.
Normal radar sidelobe levels are typically 13 to 30 dB below the maximum main beam gain. Although there is no strict definition of ultra-low sidelobes, they are generally considered to be sidelobe levels more than 40 dB below the peak gain.
Ultra-low sidelobes
To detect a radar from its sidelobes, electronic support or radar warning receivers need sufficient sensitivity to receive sidelobe signals. Sidelobe intercepts receive lower signal power compared with main-lobe intercepts; the power drop equals the radar's average sidelobe isolation. Radar signals also attenuate with the square of distance, so a 20 dB sidelobe isolation reduces the detection range of a jammer or receiver to about one tenth, greatly lowering the likelihood of detecting the radar signal. In addition, lower sidelobes reduce the jamming-to-signal ratio (J/S) at the radar when jamming from outside the main beam, which also reduces effective jamming range. Ultra-low sidelobes therefore make it more difficult for electronic support systems to detect a radar and harder to deliver jamming power sufficient to affect the radar.
