Coherent backscatter observations with the Millstone Hill UHF radar (MHR) are used to investigate spatial/temporal variations in the ionospheric sub-auroral polarization stream (SAPS) electric field. For the 440 MHz MHR, coherent amplitude is on average linearly proportional to electric field strength. The use of both main-beam and sidelobe returns and the great sensitivity of the MHR system permits observations spanning 3¿ of the SAPS region with 1-sec temporal and 10-km spatial resolution. For a moderately disturbed event on May 25, 2000, the SAPS channel moved steadily equatorward. Large-scale (30 mV/m peak to peak) wave-like oscillations in the electric field magnitude (200s--300s periodicity) were seen to propagate across the SAPS channel throughout the hour-long event. It is suggested that such localized electric field intensifications, which exhibit many of the characteristics of the narrow SAID features described in the literature, arise as wavelike perturbations within the SAPS channel.