The crew of Challenger today turned to repair-work again when the most important experiment aboard the space shuttle was threatened with at least partial and perhaps total loss.

The problem arose when a large dish antenna in Challenger's cargo bay began to swing uncontrollably, making it almost impossible to contact a tracking and data relay satellite (TDRS) thousands of miles away. The astronauts then discovered that one of the wings of an imaging radar antenna experiment also in the cargo bay had come loose and needed to be fixed quickly if the experiment was to work at all.

Late this afternoon, astronaut Sally K. Ride attempted to lock the loose antenna with the shuttle's 50-foot-long robot arm.

On the third try she was able to force the arm against the loose antenna panel to lock it up.

In reference to the last shuttle flight when a crew member used the robot arm to knock some ice off the fuselage, Ride said: "It looks like the 'ice buster' strikes again."

Replied astronaut David Hilmers from the Mission Control Center in Houston: "Every handyman ought to have one of these in his tool box."

The antenna Ride repaired is for a new type of radar that sends thousands of pulses per second toward a target on Earth with such precision that scientists can use the echoes to form photograph-like images of any feature on Earth. So powerful is the radar that it can penetrate vegetation and dry or sandy soil. Among the radar's experiments planned for this mission were observations of almost every major desert on Earth.

Though Ride was able to fix the imaging radar antenna, the large dish antenna's swinging already had ruined some experiments.

The imaging radar collects so much information so fast that it must use the dish antenna to send the data to the TDRS, which passes it on to Earth. The dish antenna and the relay satellite are the only way the shuttle can forward the information with sufficient speed.

Scientists had hoped to receive about 40 hours of data from the imaging radar experiment on this mission but now say they will be satisfied with 25 hours. Even that depends on whether the astronauts can fix the antenna so it can lock onto the TDRS in space.

"The antenna just does its own thing up there," Flight Director T. Cleon Lacefield said at Houston's Johnson Space Center. " . . , it's useless unless we can fix it."

The repair began just before 5 p.m. when Cmdr. Robert L. Crippen and other crew members moved the swinging antenna to point toward and above the shuttle's tail.

"We asked for 45 degrees," astronaut Hilmers said from Houston. "You gave us 45.87 . . . . Not bad."

The crew still had to make another repair before it could restore the antenna to service. Crippen, Ride and Kathryn D. Sullivan removed four equipment lockers in the shuttle cabin to get at the electronic box that controls the antenna's movements. Ride and Sullivan literally pulled the plug on the antenna, locking it in the position where it was placed this afternoon.

Sunday morning the astronauts will be asked to maneuver the 100-ton spaceliner to lock the antenna onto the TDRS. If that works, it will be the first time an orbiting spacecraft has been used as its own radio antenna to link itself to Earth.

The task is considered so difficult that flight directors tonight said they were considering a delay of Tuesday's spacewalk by astronauts David Leestma and Sullivan for one or two days to give the crew time to choose procedures to link their antenna with the relay satellite's.

"I hope we find TDRS tomorrow. It's not going to be easy," Crippen said.