Wideband RF beamforming: the Rotman lens vs. photonic beamforming

The discovery of the Rotman lens (Rotman, W. and Turner, R.F., 1963) was an enabling technology that greatly increased the capabilities of multibeam antennas. The original intent of the inventor (WR) was somewhat less ambitious. He wanted to study various interesting configurations of TTD (true time delay) beamforming, and has been somewhat amazed and fascinated to see the evolution of his original idea into its present forms. The paper compares a relatively new TTD technology which, in the near future, for certain usages, may supersede or supplement the Rotman lens - all with the guidance and the blessing of Walt Rotman. Wideband RF phased array beamformers have increasingly stringent requirements as to their performance, given the desire to detect and classify targets with increasing accuracy, and to eliminate false alarms caused by clutter. Instantaneous bandwidths are growing accordingly. Traditional error analyses of phased arrays, which concentrated on spatial patterns, must be supplemented to take into account also pulse distortion created by the transfer function of the beamformer. While these phenomena have been well known for UWB antennas, they have rarely been analyzed for large phased arrays with highly complex feed networks. The paper provides a method to analyze the performance of TTD beamformers and, as an example, shows measured results of a novel photonic beamformer in transmission with a comparison to the traditional Rotman lens.