Experiments were performed at the Kennedy Space Center, Florida, during Trip 78 and later repeated at Wallops Island, Virginia, to identify sources of the strongest radio frequency radiation from lighting. This was done by monitoring the fast electric field changes associated with strong radiation in the HF to VHF range, using a field change system triggered on the output of an RF detector. The electric field changes associated with the strongest RF radiation were short duration (10--20 μs) bipolar pulses having an initial negative half cycle followed by a positive overshoot. The shape of these pulses was independent of the frequency of the RF radiation used to trigger the field change system. These fast pulses consistently were associated with more RF radiation than were return strokes and would appear to be a cloud discharge with characteristics suggestive of a very fast K change.