A Uniplanar $3\times 3$ Nolen Matrix Beamformer with Beam Squint Reduction

In this paper, a uniplanar 3×3 Nolen matrix is proposed to improve the beam squinting for spatial FFT beamformer. The proposed multiple beam feeding network has three input ports and three output ports, and the signal is divided into three equal magnitudes and features unique progressive phase shifts at its output ports for each input ports excitation. To reduce the variation of progressive phase delay across the frequency band, a group delay compensation approach is proposed to minimize the changes of phase response across the wide operating bandwidth. To prove the concept, a 3 ×3 Nolen matrix with group delay compensation operating at 5.8 GHz is designed, and it shows that the beam squint is reduced for three radiation angels.

I. Introduction

Multiple simultaneous radiation beams are widely applied in wireless communication, imaging and radar to increase the capacity of the network. For RF beamforming, the most common approach is feeding the antenna elements with a phase delay network, and the series feeding, parallel feeding, and matrix feeding are three common topologies investigated so far. Comparing to other two feedings, the matrix feeding supports multiple beams, and the Butler, Blass, and Nolen matrixes have been studied in various way. Typically, the Butler matrix generates radiation beams, and the Blass and Nolen matrixes are more flexible for arbitrary number of beams. From the circuit topologies, the Blass matrix requires resistive loads in the network, which causes power loss and degrades the overall efficiency. Thus, the modified topology Nolen matrix [1] is invented to remove the resistive loads and cut the half of components from original Blass matrix. Therefore, the Nolen matrix features low power loss, a smaller number of components, and without additional crossovers for arbitrary number of multibeam. In [2], a microstrip line Nolen matrix is presented, and a 2-D Nolen matrix is proposed in [3] for multiple beam applications. As a spatial FFT beamformer, the previous proposed Nolen matrixes suffer from the beam squint problem for system having relatively higher fractional bandwidth, making it difficult for high data rate applications. To be specific, the beam direction tilts, and the boresight gain is reduced when the frequency is changed. The main reason for this beam squint in Nolen matrix feeding phased array is from the variation in phase progression response across the interest of the band. Fig. 1.

The proposed nolen matrix with beam squint reduction