Analysis of a microstrip Rotman lens using domain decomposition method

A Rotman lens is a cost effective RF beamformer based on geometrical optics which provides simultaneous wide scanning angle feed structure for phased array antennas. In a beamforming network, both radiation beam pattern and impulse response are significant performance indicators. However, use of geometrical optics in evaluating the broadband microstrip type Rotman lens to obtain spatial and time information may be inaccurate due to internal reflection, mutual coupling between ports, fringing field and higher order modes in the feed taper are ignored. Therefore, using numerical methods to simulate the lens gives a more comprehensive analysis. Customized numerical codes for simulating such non-repetitive electrically large Rotman lens are implemented. In this paper, we utilize a more efficient numerical approach - domain decomposition method (DDM) to facilitate the design and analysis of a microstrip Rotman lens. The rest of the paper is organized as follows: the validation of domain decomposition method on prototype microstrip Rotman lens at design frequency, f_c, is accomplished first, then compares the simulated S-parameters with the measurement results. Also, the beam pattern and the corresponding impulse response can be calculated through the simulated S parameters. Furthermore, an adaptive frequency sweep technique is introduced in order to obtain sufficient frequency response data to avoid aliasing effect while reconstructing impulse response. Finally, after simulating the Rotman lens using adaptive frequency sweep DDM, the comparison on multiple beam patterns and impulse response between simulation and measurement is obtained.