We have developed a unique multiple-channel subband system to detect and record broadband electromagnetic transient events in carrier-dominated radio environments. Between May and September of 1994 we used this system to detect and record over six thousand transient radio frequency events in the band from 3 to 30 MHz. Approximately 500 of the events have been characterized as paired bursts of radio noise with individual burst durations of 4 to 10 μs and separations between the bursts of 5 to 160 μs. The paired transients are typically 5 to 40 dB brighter than the background electromagnetic spectrum between carrier signals. We have termed these events subionospheric pulse pairs (SIPPs) and presently have no explanation as to their source. Our observations of SIPPs resemble observations of transionospheric pulse pairs (TIPPs) recorded by the Blackbeard instrument on the ALEXIS satellite; the source of TIPP events is also unknown. Most SIPPs do not exhibit dispersion, implying groundwave (line-of-sight) propagation; but seven of the pairs exhibit dispersion characteristic of ionospheric skywave propagation. For our experimental configuration the maximum line-of-sight observation range for radio propagation extends to 300 km for a source at an altitude of 5 km and 520 km for a source at 15 km. Refraction from the ionosphere may allow HF skywave radiation to propagate around the globe. Information about the times and locations of sources, in conjunction with regional weather maps and computer models of ionospheric propagation, suggests that some SIPP events originate in the vicinity of large-scale thunderstorm complexes. ¿ American Geophysical Union 1996