Cancellation of Beam Squint with Frequency in Serial Beamforming Network-Fed Linear Array Antennas

This communication investigates the capability of serial beamforming networks to cancel beam squint with frequency in linear array applications. Such a property could be of interest for broadband applications with multiple narrow beams. The theoretical aspects are described, as well as the impact on the beamforming network design. It is found that the main limitation to achieving beam-squint cancellation in serial beamforming networks comes from the insertion phase of the different directional couplers needed. A specific design at S band with a single-beam four-element linear array is detailed. Experimental validation in microstrip technology is presented, confirming the cancellation of beam squint with frequency over a 9% bandwidth. The design of the elementary directional couplers is the main limitation in terms of frequency bandwidth. The applicability of such a design procedure to serial multiple beamforming networks, namely Blass and Nolen matrices, is discussed.

1. Introduction

Serial beamforming networks associated with linear array antennas are known to introduce phase dispersion due to unbalanced electrical path lengths. This results in frequency-controlled beam steering, sometimes used in radar applications [1]. This is a typical characteristic of traveling-wave and leakywave antennas. On the other hand, wideband beamforming networks associated with linear array antennas, such as parallel feeding networks, introduce a beam squint. This is due to the fact that the distance between radiating elements normalized to the wavelength varies with the frequency. Consequently, all beams deviate toward the boresight of the antenna as the frequency increases. A combination of these two phenomena could be explored to cancel the beam squint with frequency in serial multiple beamforming network designs, such as Blass [2] and Nolen [3] matrices. To do so, the beamforming network has to produce a true-time delay, compensating the time delay observed in radiation when the beam is tilted. This characteristic is naturally achieved in quasi-optic beamformers, as the wave is distributed over the antenna's aperture in the radiation mode. Well-known solutions with linear array antenna apertures include the Ruze lens [4], the Rotman lens [5], and, more generally, all bootlace lenses. Other quasi-optic solutions - such as reflector antennas, pillbox antennas [6], and the Luneberg lens [7], [8], which are characterized by a continuous antenna aperture - also display the same property. In guided-wave beamformers, this property requires more attention to achieve, particularly in the case of serial beamforming networks.