A group of teen-agers plotting on the air to burglarize a CB radio specialty store in Rockville was nabbed in the act last week by a team of Montgomery "County Mounties" and citizens with "ears" who had been copying their transmissions loud and clear.

As a result of the radio ring's indiscreet broadcast on channel 35, county police foiled a break-in at Scanner City at 421 Stonestreet Ave. in Rockville and recovered more than $8,000 worth of portable radio equipment stolen recently from police cars in Wheaton.

"Why they got on the air and talked about it, I'll never know," said CBer Margo Maryn, one of four citizens who helped police pinpoint the chatty burglars. "The whole world was listening."

As a result of the broadcast on the CB channel, police arrested 18-year-old Walter Barrett of Wheaton and four juveniles ranging in age from 15 to 17. They were charged with attempted burglary, theft and receiving stolen property. Police said the case is not closed and more arrests in the radio ring are expected.

The teen-agers apparently had developed a sophisticated communications system in which one of them served as a base dispatcher in Rockville. Employing a mixture of Morse code and initials for place names, the young dispatcher directed two "mobile units" on how to bypass alarms at the store.

"We haven't run into anything this sophisticated," said Wheaton District police officer Bruce Blair. "Even though they were juveniles they had a fairly sophisticated organizational structure. It's unusual."

Clark Johnson, co-owner of Scanner City, which recently was burglarized for $60,000, said: "If they would have put that much effort into being honest they would be good kids. They could have made some money. All this effort to be dummies . . ."

The trail that led to a successful stakeout at Scanner City on Friday night began with a tip from an anonymous CBer who had tuned in on a conversation in which someone mentioned that he had some portable police radios for sale.

Aware that several radios had been stolen last month from cruisers in Wheaton, police dispatchers began listening to transmissions on their own frequencies with more cars than usual, hoping they could track down the thieves.

"The juveniles were impressing their friends that they could talk on a police frequency," Blair said. But, he added, they also slyly used the radios to keep tabs on police activity and call in false emergencies when they wanted to divert attention.

Police officers decided to join forces with the CB experts of REACT, an international group of volunteers that listens for emergencies on CB channel 9. Sometimes, just for fun, Margo Maryn and other REACT CBers practiced pinpointing signals with special finders. This allowed them to isolate in the static a voice selling stolen goods.

With police officers, three REACT volunteers fanned out in a large triangle in the Rockville area and began scanning for their garrulous quarry.

"It was fun," Maryn said, "It was a challenge."

Friday night the three teams -- which were now communicating with each other on special frquencies because they knew their subjects might hear them on a CB radio on regular police channels -- heard the teen-agers sending messages in Morse code and talking about "S.C." It was after 11 p.m.

"We knew there was going to be a crime at midnight," Maryn said. "They were talking about sneakers and crowbars. One guy asked a woman if she had bail money. She said anyone who was out at that time of night didn't deserve bail money. It was absolutely unreal."

As the posse converged, they groped for the meaning of "S.C." until suddenly it was obvious -- Scanner City.

For some reason the already well-equipped group was headed for more apparatus. Police speculate they may have been searching for batteries for their stolen radio.

Unmarked police cars converged near the store and officers slid down in their seats, hiding, as they waited for events to unfold. Around midnight a small foreign sedan with an aerial pulled up. A man got out. Police listening on Channel 35, heard a voice giving instructions on how to circumvent the alarm.

Apparently, his instructions were wrong. Bells erupted and the man fled on foot, getting 300 yards before two officers chased him down. A juvenile in a truck was arrested nearby.