Design of frequency-invariant robust beam patterns by the oversteering of end-fire arrays

• Filter-and-sum beamforming for broadband linear arrays is addressed.
• Oversteering is exploited to generate end-fire frequency-invariant beam patterns.
• Optimum constrained directivity is achieved at the lower bound of the frequency band.
• A better frequency invariance than in traditional broadband beamforming is achieved.

The end-fire steering of a data-independent beamformer is well suited to achieving superdirective performance by a linear array whose aperture is shorter than the wavelength. Here, we focus on frequency-invariant beam patterns obtained by filter-and-sum beamformers that are robust against errors and fluctuations. We demonstrate that the oversteering technique applied to a weakly directive beam pattern can considerably increase the directivity, providing a frequency invariance that is better than those of traditional methods. The performance is evaluated with respect to the maximum constrained directivity that a given array can provide at the lower bound of the frequency band.