A federal judge here today rebuffed an EL Paso, Tex., widow seeking to sue the Army, the CIA and FBI for allegedly conspiring to kill her husband, an Army intelligence double agent who was electrocuted in a motel room near Fort George G. Meade in April 1976.

U.S. District Judge Edward S. Northrop invoked a legal principle known as the Feres Doctrine to rule that the military and civilian superiors of Ralph J. Sigler, the agent, cannot be sued for damages because Sigler's "alleged injuries were incurred incident to service."

In a two-year legal battle, Sigler's widow, Ilse Sigler, and her daughter, Karin Meras, sought $100,000 in damages as a result of Sigler's mysterious death on April 13, 1976, a month before his 48th birthday and three months before his planned retirement.

Sigler, whose job was to sell misleading information to Soviets, was found dead at a motel in Jessup, Md. Electrical wiring, stripped of its insulation, was wrapped around his upper arms and plugged into a wall socket.

Maryland police and Army authorities concluded that Sigler committed suicide.

For 10 years he had dealt with the Russians, identifying their agents in this country and abroad by selling them military secrets, often fabricated, about American radar and missile systems.

Then, during a debriefing he flunked a lie detector test about his activities with the Russians. He was sent to Fort Meade, where he was interrogated for nine days, according to Mrs. Sigler.

Then, he was found dead. Army psychiatrists concluded in an official report that he "was not mentally responsible at the time of the act which caused his death."

But Mrs. Sigler believed that Army intelligence had done "something wrong," that her husband knew too much about it and that Army personnel murdered him for this reason. She said she hoped her suit would shed light on the matter.

According to James Kenkel, Mrs. Sigler's attorney, the doctrine invoked by the judge applies solely because Sigler "was on active duty in the military on military orders and ordered to Fort Meade."

Kenkel said he plans an appeal and called the Feres Doctrine "a throw-back to the days when the king could do no wrong. I think that's too harsh in this day and age.

"Look at all the recent abuses," Kenkel continued. "We have big Watergates and little Watergates and we've learned that national security is a shield for a whole host of wrongdoings. A complete shield is wonderful. You can do anything."

Richard R. Beauchemin, one of the defense attorneys, said, "Our hearts and sympathies go out to the individual and rightly so. But sometimes you just have to stand back and say wait a minute, the whole community could go down the drain.

"My God, we'd never be able to function without it [the Feres Doctrine]," said Beauchemin. "My comments, I guess, would be called waving the flag. But these are necessary requirements for our country being able to operate in an international sphere."

The judge also denied a request by Mrs. Sigler that the Army return materials it confiscated from her house in what she called "an illegal search and seizure. He said the materials, which he inspected in his chambers, could jeopardize national security.

But Northrup did not dismiss the entire case. He said Mrs. Sigler and her daughter could still sue for damages possibly suffered as a result of the search and seizure if the government would not be jeopardizing national security by naming the agents who took the materials.

But Kenkel claimed that the heart of the suit "has been gutted with the ruling on Feres . . . We can't even question how he [Sigler] died."