Transient analysis of a Rotman lens for HPEM systems

Beam-steering of ultrashort (<; 10 ns) transient voltage pulses on transmit can be achieved by using true time delay concepts for phasing of the antenna elements. A prototype beam steering device for an anti-electronics HPEM system in the frequency range from 300 MHz to 5 GHz is presented. The results of the measured prototype lens are compared to numerical simulation and to an analytical model. A ray-optical model of the Rotman-lens is used with a treatment of the port structures similar to [1]. The input and output ports and the dummy region of the parallel-plate region are the main restrictions to the usable bandwidth [2]. The design of the dummy port region and the optimization of the whole Rotman-lens is done numerically in CST Microwave Studio, where the initial model is based on the analytical design approach. The couplings of the feeding lines and the interaction of all ports together are not straightforward to implement into an analytical system model. The detailed mechanical setup of the Rotman lens is presented. Additionally the output pulses are investigated when inputting a Gaussian-like transient signal. Then time domain measures of quality (full width at half maximum, ringing, delay spread, maximum of transfer function) are investigated for these output transients and the simulation and measurement results are compared. A concluding analysis of the realizable time domain array pattern shows the radiated pulse form.