Early warning radar antennas operated by the Air Force picked up the signals last Sept. 22 of what many scientists suspect was an atomic explosion in the atmosphere near South Africa.

Sources in the Carter administration said the radar dishes may have witnessed the echoes of the shock waves from a nuclear explosion as they moved rapidly through the layers of clouds in the upper atmosphere. If an atomic explosion had taken place in the atmosphere, it could have transmitted shock waves through the upper atmosphere that would be carried halfway around the world.

"What we have is a very tenuous piece of data," one source said. "It could be that thunderstorms and atmospheric disturbances that had nothing to do with an atomic explosion were giving us the radar signals we got."

The trouble is, the source said, the radar echoes were picked up by the Air Force at about the same time on Sept. 22 that a Vela satellite saw the double pulse of light in the atmosphere that is the unmistakable signature of a nuclear explosion. The twin light spike occurs when the fireball briefly disappears as the shock wave makes it opaque, then reappears when the shock wave dissipates.

Carter administration has not revealed the face that radar echoes were also picked up about the same time as the Vela sightings, in part because the radar signals were so ambiguous.

Apparently, fierce electrical storms can also account for radar echoes of the kind the Air Force picked up the night of Sept. 22, when the suspected nuclear explosion took place. For this reason alone, the Carter administration has kept silent about the radar echoes being confirmation that a nuclear explosion indeed took place.

The missing link in the puzzle is confirming evidence beyond the observation of the Vela satellite. What scientists have really looked for is the radioactive fallout from an explosion, which should have shown up in the rainwater in the Southern Hemisphere, but has not shown up in quantities that would be convincing evidence of a nuclear test.

The White House Office of Science Policy is in the midst of preparing a report on the South African event that may be made public this week. A draft of the report is understood to have been circulated among members of the National Security Council late last week for comment.