The Pentagon's research chief said yesterday that the United States has developed a new family of weapons lethal enough eventually to offset the Soviet advantage in armor along the NATO front.

The key part of the new weaponry is a detector system that can find tanks on the battlefield by the way nature's radio waves bounce off them.

Combining this sensor with advanced guidance systems for missiles, bombs and artillery shells promises to "change the face of battle," Pentagon research director William J. Perry said in an interview to the same extent the airplane and radar changed World War II tactics.

"What I'm giving you is not a fantasy," said Perry in discussing the millimeter wave detection system. "The question is not whether. The question is when." He said the weapon could be deployed as early as the late 1980s and "would make a profound difference" before 1990.

The research director said "millimeter wave components didn't even exist five years ago" but have now been tested, fulfilling the hopes of the Pentagon's technical experts.

The tiny antenna for detecting the distinctive pattern of radio waves bouncing off metal is put in the nose of a missile, bomb or artillery shell and harnessed to a computer the size of a package of cigarettes.

After being fired somewhere near the target, the antenna-computer combination inside the missile or bomb takes over and guides the whole business smack into the metal -- such as a tank on the ground or a ship on the sea.

"A single footsoldier can stand up against a tank" with this weapon, said Perry. In World War II, a soldier had to be within 100 yards of a tank to destroy it. With the new weapon, he can stand a mile away and not worry about his aim.

"He just has to aim it in the direction of the tank or cloud of smoke surrounding it," Perry said. The bazooka-like missile "flies over the tank, then this little wave sensor searches for it, finds it and destroys it."

Perry conceded that the enemy could set up metal decoys for the weapon, but this still would not save those tanks on the battlefield. The Pentagon has developed an "assault breaker" designed to stop a whole column of tanks. It consists of a cluster of missiles, each guided by an internal precision millimeter wave system.

Although the Pentagon is still pursuing other guidance systems for "smart weapons," such as heat seekers and laser beams, the millimeter wave is particularly attractive to Perry because it is not affected by weather or darkness and is self-contained.

Because of the U.S. lead in miniaturizing the components for the millimeter wave, Perry estimated that the United States is five years ahead of the Soviet Union on precision-guided weaponry.

"If we exploit it properly," said Perry, "it should offset the disadvantages" NATO allies in Europe "are confronted with by the 2 or 3 to 1 disparity in armored vehicles" deployed against them.

It makes more sense to concentrate on deploying these smart weapons than to match the Soviet "tank by tank." If the weapons are strongly pursued and deployed along the NATO line, the research chief continued, "they would make a profound operational difference before 1990."

Perry said "the combination of precision guided munitions and precision all-weather location devices," like the millimeter wave, "which now allow you to use indirect fire systems to make direct hits, just has to change the face of battle."

In predicting an impact as great as the airplace and radar made during World War II, Perry predicted "the tank is going to decrease in importance."

He said just as armies had to deploy their infantries differently once the machine gun of World War I showed the folly of mass charges, so will armies have to use their tanks differently to combat smart weapons.

"They take the tank off its pedestal," he said. "They probably won't be the queen of the battlefield; just one more weapon."

Perry already has been meeting with U.S. leaders to discuss changes in armor design and deployment to respond to the smart weapons' making their way toward the battlefield.

He predicted that the days of one heavy tank fighting another are coming to an end. The U.S. Army, he noted, already is moving to make some of its armored divisions lighter and faster and thus easier to transport to distant trouble spots.

Given the great promise of smart weapons, Perry said, the Pentagon may ultimately decide to buy fewer of the heavy XM1 main battle tanks just put into production.