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This paper describes findings from a research program to investigate the main beam and sidelobe impulse responses of large phased array antennas and reflector antennas typically used for radar applications. Particular emphasis is given to the way that the impulse response changes from the main beam to the sidelobe region of the antenna. Antennas are designed to radiate a plane wave with a given polarization and frequency in a specified main beam direction. However, the time-domain impulse response will vary according to the actual implementation of the antenna design. Reflectors are designed to collimate a radiated beam by creating equal path delays and tend to have few parts. This produces a relatively simple and short time-domain impulse response. A phased array, on the other hand, generally collimates the beam by adjusting phases instead of path delays. In addition, there are more parts, which introduce multiple reflections. This gives the phased array a more complicated and longer impulse response than a reflector. These time-domain effects affect the transmit and received waveforms and therefore the advanced signal processing algorithms such as adaptive digital beamforming (ADBF) and space-time adaptive processing (STAP). Georgia Tech Research Institute (GTRI) has instituted an internal research program to investigate the time-domain phenomenology of phased array antennas and develop computer simulation tools and measurement capability for characterizing these phenomena. Some comparisons of a reflector antenna and a series corporate-fed phased array are presented.