Radio station WMAL has offered stereo broadcasts to its listeners since July 1982, the first AM station in the Washington area to do so. But until last week, no one could hear the stereo transmissions.

Now WMAL's listeners, along with those of more than 200 AM stations across the country, can hear AM stereo with just the flick of a switch and an $89.95 investment.

The Sony Corp. last week sent out its first shipment of portable AM stereo receivers to retailers, featuring a two-position mode switch that picks any of the four AM stereo broadcasting systems available to stations.

Sansui plans to send out a car stereo unit next month that will retail for $519 and a home unit for $390. The home unit senses and automatically switches to the AM stereo system that the broadcaster is using.

Some Washington-area department stores already have received the Sony units, and company officials say by the end of August up to 50 area retailers will be selling the radio.

AM radio programmers hope the new stereo broadcasts will improve AM's listening quality to rival FM stereo, which soared in popularity in the early 1970s because of its better fidelity and a trend toward specialized programming.

"Will there be a trend toward AM stereo ? I think so. I hope so," said J. B. McPherson, chief engineer of WMAL.

WMAL invested $100,000 to $150,000 in a more sophistical system developed by Kahn Communications Inc.

McPherson said the station offered AM stereo even though listeners could not hear the stereo because "we were just interested with working with the state of the art and being an industry leader."

Former owners and operators of the recently closed WHFS-FM in Bethesda hope to introduce another AM stereo station into the Washington market, which the National Association of Broadcasters says is one of the top 10 in the United States.

Cardinal Broadcasting Associates, owned by some of WHFS' former owners and operators, bought WEAM-AM in May and, if the purchase is approved by the Federal Communications Commission, will operate it from a new 11-acre facility in Falls Church. The firm plans to change WEAM's call letters to WHFS.

"The biggest problem with the AM market in this country is FM came along with a product that gave them better fidelity," said David Einstein, program director for WHFS. "FM has as good as any sound product at the moment. But we want to provide a product that is not on FM--programming and good quality sound music with the state of the art AM broadcasting."

WHFS's 102.3 FM frequency was sold last winter to WTOP-AM, and the station stopped broadcasting in mid-July.

Michael Rau, a staff engineer and lobbyist for the NAB, said an AM stereo transmission system costs $10,000 to $25,000, compared with the average cost for an FM system of $20,000 to $25,000.

All of the AM stereo systems "are very similar," Rau said. "I don't believe the public would be able to tell the difference" between them.

The arrival of the receivers on the market culminates a six-year struggle between the FCC and radio broadcasters and manufacturers to determine an acceptable transmission system.

In June 1977, the FCC announced it would pick one transmission system for the industry from the five then available. It tested systems made by Belar Electronics Corp., Magnavox Corp. and Motorola Corp. A system by the Harris Corp. was not included because it had no transmitting equipment, and Kahn Communications Inc. elected not to participate.

The Hazeltine Corp. later joined with Kahn to develop and to promote the Kahn system, which WMAL has installed.

The FCC in March 1982 decided the marketplace should pick what will become the standard system. The commission permitted AM stations to start broadcasting in stereo on April 26, which many stations did, even though there was no available stereo receiving unit.

The FCC decision came after it stated it could not determine a superior system because uniform test procedures were not used, judgments were subject to variance depending on the analyst and results of its testing showed no major differences in quality.

An FCC report said normal market development will occur as some systems fail and others gain supporters.

"We feel each broadcaster should have the right to select the type of equipment," said John Reiser, the FCC's assistant chief of the technical and international branch of the mass media bureau.

"Also, the FCC did not make a dn because the broadcasters and manufacturers could not agree on which factors were more important than the oth manufacturers said the FCC did not take into consideration the cost of receivers, and broadcasters said "we d of the components or the audiences at the fringe at the service areas."

That decision upset the broadcast sticking to the political philosophy of the marketplace letting the industry, not the government, choose amon NAB's Rau said. "This is not good in technical areas. When you have an industry that deals with the entire pos very, very important."

Broadcasters said the success of AM stereo depends on the use of a single transmitple systems become popular, a single system will never become popular, and the consumer will spend more money ," Rau said.

And, he said, "A multiple system cannot receive the broadcasts as well as a single system. Wthe FCC."

The birth of AM stereo contrasts sharply with the evolution of FM stereo. In March 1959, the FCC announced a study to determine an FM stereo transmitting system. Two years' later, the FCC chose a system proposed by Zenith and General Electric from the seven that were introduced.

AM stereo offers several advantages over FM stereo: It is not as susceptible to "picket fencing," the annoying jumps in reception that occurs in rapid intervals; the signal is easier t when the radio is near such things as bridges and tunnels; and, in many cases, the signal travels farther thals of comparable power levels.

Most AM stations that had been offering stereo mostly for promotional reasonnd in the Midwest.

"It's a natural for places like Seattle, San Francisco and Los Angeles because they're so hilly," said Jon Strom, radio products marketing manager for Sony. "And the Midwest is so wide open that the signal can cover hundreds of miles. Also, the East Coast has always been more conservative."

Now, however, some stations in the East, including WMAL and the proposed new WHFS, are taking the plunge.