He has heard teachers at Haddonfield High School grumbling about the administration. He has heard drug dealers negotiate sales. He has heard private detectives discussing an investigation. He has heard men give their credit card numbers in return for a graphic description of their sexual fantasies.

"I guess I feel like a peeping Tom," said the high school senior. "I guess you could say I find out more gossip than anyone in town."

The intimate conversations that Sullivan has overheard are not the normal banter that clutters the ham radio bands. It is as though the conversants were chatting privately, unaware of eavesdroppers - as though they were talking on the telephone.

Which is exactly what they were doing.

Welcome to 1984, when millions of Americans unwittingly are exposing their souls to their neighbors by using cordless telephones.

Consumer groups and government officials are beginning to understand a phenomenon that some amateur radio operators discovered long ago - the highly popular cordless telephones are filling the airwaves with private conversations, privy to any of the country's 400,000 licensed ham radio operators, as well as numerous other users of less sophisticated equipment.

"It's very dangerous," Sullivan said recently. "Some of this would be very damaging if it were in the wrong hands. Somebody without any values could really exploit this."

Amateur radio operators call the eavesdropping "legal wiretapping," because it is legal to listen to the conversations - but not to record them. Because of "electrical propagation," the radio signals transmitted by cordless phones are broadcast farther than the few hundred feet that phone manufacturers specify. Electrical wiring in a house can act as an antenna, boosting the telephone's signal beyond the neighborhood.

From the radio in the third-floor room of his parents' home near the center of this borough, Sullivan said, he has overheard thousands of conversations from all sections of Haddonfield's 2.2 square miles, where 12,337 people live.

Under optimal broadcasting conditions, Sullivan said, he has overheard telephone calls made from homes two miles away - from each of the surrounding communities of Cherry Hill, Barrington, Haddon Heights, Westmont and Audubon.

A representative of the telephone manufacturers, though, said the range of Sullivan's reception was "a fluke."

"It's like anything else on the radio; you can probably listen to it," said Peter Bennett, vice president of the telecommunications group of the Electronic Industries Association.

"In terms of picking them up for miles and miles, I frankly don't believe that," said Bennett. "I would doubt very much if this kid in Haddonfield is hearing thousands."

But Victor Misek, a Hudson, N.H., engineer who manufactures an antenna system for ham radio operators, said he, too, has overheard phone calls that emanated from Nashua, three miles away. "I've heard reports from some radio amateurs that have heard them as far as 10 miles away," he said.

'OBLIVIOUS'

"These people are oblivious to the fact that they're broadcasting all over the place," said Misek. "It's really quite sad, because they might be giving away personal information. . . . A person is left open to that."

The Federal Communications Commission, which governs the use of cordless phones, does not require phone manufacturers to inform consumers that their calls could be broadcast to anyone listening.

"Always assume your cordless phone is being 'tapped,' " said Sam Simon, director of the Telecommunications Research and Action Center, a Washington, D.C., consumer group. "I think consumers are basically uninformed of that."

According to industry estimates, as many as 9 million cordless phones have been produced for U.S. sales. The frequencies the government has assigned to the phones are sandwiched between the bands used by AM radio stations and those assigned to amateur radio operators - slightly higher than an AM radio dial's upper limit of 1600 kilohertz.

"The frequencies (assigned to cordless phones) are adjacent to an amateur band," said Julius P. Knapp, an electronic engineer in the FCC's radio frequency devices branch, "so anybody with an amateur radio can pick them up."

An incident in Rhode Island several months ago illustrates the potential for private phone conversations to become part of the public domain. In December, police in Woonsocket, R.I., arrested a group of drug dealers after intercepting their cordless telephone conversations. The police had been tipped off by a neighbor, who inadvertently overheard the calls on a conventional AM radio.

FASCINATED

Sullivan, an honors student and editor of the Haddonfield High School yearbook, was at first fascinated with eavesdropping after another radio amateur told him in December about the unintentional broadcasts. Gradually, he said, the realization of the potential danger overcame the novelty of aural voyeurism.

"I would hear people say they were leaving the house, or I would hear them call a baby sitter," he said. "I knew when nobody was at home. I could easily break in, if I were a burglar. And then, so often you hear people talking about their businesses. . . . There must be 250 lawyers that have offices in Haddonfield, and a lot of them use cordless phones."

He said he could identify about 40 percent of the callers he overheard from their voices - "this is a small town and I know a lot of people" - or because they would commonly reveal their names or phone numbers during the conversations.

So he began telephoning some of his neighbors who used the cordless phones, telling them that he had intercepted their calls. "When I talk to someone," he said, "I try to tell them how dangerous it is."

Misek, too, said he had intercepted phone calls that were errantly broadcast in the amateur radio bands.

One cordless phone manufacturer, he said, mistakenly produced thousands of phones that used a ham radio frequency. Misek said he would tune his transmitter to the intruding caller's frequency and interrupt the conversation. He explained the problem to callers, instructing them to return their phones to the store.

"Sometimes they're quite shocked when you do that," said Misek. "I listened in one morning to a guy on the phone calling his bank. He gave his name and account number and wanted to know the balance in his account. So I interrupted, 'Hmmm, that's an interesting account number.' And before I could explain to him that his call was being broadcast, he hung up."

Sullivan, who speaks in a rapid stream of words that he has adopted from years of radio chattering, also said he occasionally interrupted callers whom he could not identify. He considers it a public service. Sometimes, the startled callers do not.

"You get different reactions," he said. "There was one lady I talked to the other night. She was really angry. She said, 'Have you read 1984?' And I said, 'Sure, I had to read it in school.' And she said, 'This is just like Big Brother.' And then she drew an analogy that this was like Watergate. Most of the people, though, seem to appreciate the information."

One of his appreciative neighbors was Earl Cooley, who lives about a block from Sullivan.

"I'm glad he told me about this, because I briefed my family," he said. "At the time I bought the phone, I wasn't aware that this type of situation could come up. I was very surprised when Mr. Sullivan called me.

"I'll tell you," he said, "I don't use that phone now when I call about anything to do with business, or when I'm going out of the house."

Cooley said he had believed his phone calls could not be overheard. His phone, like most of those now being sold, is equipped with a device that is designed to prevent "pirating" - the unauthorized use of private telephone lines by marauders who drive around with cordless phones, hoping to pick up somebody else's dial tone. But the device does not prevent eavesdropping.

STATIC

"You are essentially using a radio, and almost none of the phones is equipped with an encryption device," said the FCC's Knapp. An encryption device, such as a voice "scrambler," cloaks the radio signals with static, thwarting most eavesdroppers.

Only one telephone manufacturer sells a cordless phone equipped with a voice scrambler, and it costs substantially more than other models.

Amateur radio operators, though, said it does not take an electronic wizard to create a device to decode the scrambled signals. "Anything the telephone company can put out, we can match it," said Sullivan.

FCC officials said that amateur radio operators would not be able to receive calls from the new generation of cordless telephones that is now being developed, because those phones will use high-frequency signals. But those signals are similar to the ones used by law enforcement radio bands. So anyone listening to a conventional police scanner, officials said, might be able to overhear calls made on the new generation of phones.

"The manufacturers," said Misek, "should be required to put out a little booklet to the (people) who buy these things, which tell them you have no privacy whatsoever.

"Everybody's baring their guts out to the community," he said. "It's colossal, really. It's ridiculous. And most of these people are oblivious to this."

Said the FCC's Knapp: "I don't know how far the commission can go to educate people."

Even so, he said cordless telephone conversations probably always will be easy prey for eavesdroppers, despite the changes in frequencies and despite the advent of devices to scramble radio signals.

"Obviously," he said, "if somebody is trying to pick these up, they can do it without a great deal of difficulty."