WHEN THE Beach Boys take the field at RFK Stadium this afternoon, following a Team America game with the Fort Lauderdale Strikers, the 25,000 to 35,000 fans in attendance will be part of the newest wrinkle in the expanding field of sports/entertainment.

Soccer, the gangly stepchild of modern American pro sports, is trying to sell itself as wholesome family entertainment; rock 'n' roll, the genre most suited to stadiums, has long viewed itself as athletics with a beat. Pax Productions, the San-Diego-based marketing firm running the operation at RFK, started putting sports and music together in 1980 and has since become the premier sports/entertainment corporation in the world.

The break between today's game and concert won't be much longer than a halftime intermission, and with hundreds of clowns clowning, cheerleaders cheering and stagehands racing around constructing an elaborate stage in less than 25 minutes, it may actually be more entertaining than the game (or, for that matter, the concert). Specially outfitted John Deere tractors are to deliver 16,000 pounds of equipment to midfield without damaging the turf; 80,000 watts of sound will be aimed at the audience. The ringmaster guiding these proceedings with loud airhorns and an ultraspecific 20-page script will be Chip "Woodstock" Monck.

The finished performance area, 143 feet deep by 250 feet wide, with more than 35,000 square feet of painted muslin surrounding the stage, will feature scenes of ocean waves, beach and palm trees scrims over the sound carts. And if they follow the form of previous concerts, the Beach Boys will hit the midfield beach in an ancient "Woody" station wagon before launching into "Good Vibrations."

"This whole process cross-markets well," says Monck. "It introduces sports people to music and obviously introduces music people to sports, which is exactly what it needs, because a lot of stadiums, concert halls and arenas are not necessarily full." WHILE PAX doesn't see itself as a concert promoter (though it will do 40 shows this year) it has provided a profitable twist in the no longer recession-proof entertainment industry. The concept is simple enough: Providing double value with a single ticket price is a cost-effective audience expander for pro sports franchises. Pax's prices are low--$7 for the Team America/Beach Boys event, $4 for those under 15, a reflection of its family target audience. "If we start jacking up the prices," says Pax president Fred Moore, "then we're in the concert business."

In most cities the concerts have resulted in sellouts, some of them for games that would have been considered weak draws without the tie-in. In Cleveland a Sunday game attracted 7,900, while a Monday game/concert drew 47,000. In San Francisco 22 percent of the 53,000 who turned out for a double-header had never been to a Giants game, exactly the audience the Giants wanted to reach. In all cases, the attendance has at least doubled (as it has here, where Team America has been averaging 11,222 in three homes games).

"Our main job here is really to promote the sport, and in this case, Team America," Moore says. "Soccer is having trouble developing a core of fans in America. Presenting the Beach Boys will expose the sport to that younger age bracket, but with groups like the Beach Boys and Chicago, you're also getting that 35-and-above demographic who grew up with them. In Washington we want people to expose themselves to Bob Lifton's Team America, let them come out and see what soccer's about . . . and get exposed to the Beach Boys as a bonus."

What if everyone showed up for the concert and skipped the game? Research has shown that doesn't happen; in previous soccer promotions, 90 percent of the crowd is in before the half, "so we know they'll be exposed to at least 45 minutes of actual soccer," Moore points out. "People like to get their value, so they'll go out and watch these games."

Pax is most active in baseball, promoting 23 shows with nine franchises; it also works with a half dozen North American Soccer League teams, but has so far limited its involvement with the United States Football League ("We're still trying to get a balance on what they're doing"). Among the performers Pax works with frequently are Charlie Daniels, the Oak Ridge Boys, Merle Haggard, Chicago, Jefferson Starship, America and Pablo Cruise. "We couldn't use a Van Halen or an Ozzy Osbourne in our type of show," says a Pax official. "It wouldn't fit." One of the few black acts, Lou Rawls, is tentatively set for another Team America promotion later this summer. MONCK, best known for staging the Monterey, Woodstock and Altamont festivals, the concert for Bangladesh and tours for Neil Young, Bette Midler and the Rolling Stones, is the Pax man, whose production duties include on-site rehearsals (sometimes at 5 in the morning). He travels with his own crew heads and all sound, staging, scenery and electronics packed into four massive trucks. "We pick up local labor in every city--including local cheerleaders, by writing to all the high schools and colleges and hoping there isn't a game on the same date. We encourage the cheerleaders to wear their school colors, and we feed them and make them comfortable for the rigorous schedule we put them through."

Monck is a combination Busby Berkeley-George Patton, supervising the entire operation right on field with a script, binoculars and air horns ("boat-in-distress signals with different tones") taped to his legs. "Sooner or later I lose something if it's not taped to me," he says.

As soon as the last soccer kick is made, clowns will spike the field with bright orange traffic cones and 100-foot sections of yellow cord, pitching sight lines to guide the large crews waiting for the signal to charge. The rope lengths have knots every 10 feet. "What happens is there's a head and tail on each line and those girls literally do nothing more than direct the other cheerleaders in what to do and where to go. We rehearse with the heads and tails of each line in the morning and everybody runs through it once on the site. That's enough. It's amazing, the energy that's required as well as what they give out even in a partially filled stadium." Nine rows of 10 cheerleaders will lay down more sight lines and occupy the crowd's attention while the stage is tractored in. The self-contained set-up is quite elaborate: Pax developed a special system of hydraulics, flotation tires, and sound and staging carts. "We don't damage any turf, much less than the ball game itself, in actuality," Moore says.

The whole set-up is geared to consume between 17 to 22 minutes, as it has been since the first major promotion in Pax's hometown of San Diego. "Ballard Smith owner of the Padres was the one who made the break in baseball. He'd seen us do it on a smaller scale, but that showed him that the attendance would increase. Plus we're able to convert a stadium into a theater in less than 20 minutes. They'd done concerts after baseball games in the '60s and even in the early '70s, but it always took 90 minutes to two hours to set up, and you can't hold an audience that way."

Drawing on Monck's expertise, Moore saw the solution in "making a show out of it--bringing out a large set, rather than just guys bringing out lumber and pulling cable. We're able to keep the fans watching our show and then enjoying 90 minutes of concert."

There are 92 stadium-size venues, and Pax has played 17 of them so far.

Although no figures have been made public, it's obvious that everybody--promoter, band, franchise--makes a little money, particularly since cross-merchandising is a vital part of the process: there are posters, T-shirts, visors with sponsor logos. There's usually a promotional tie-in with a local television or radio station (WRQX in the Team America/Beach Boys event). "Without cross-merchandising it couldn't work," Moore insists. "Without cooperative advertising and corporations willing to support these endeavors, we couldn't do it; there's no way we could bring to the public a seven-dollar ticket."

Maybe Team America will win today. The Beach Boys, Pax, WRQX and the folks who purchased tickets already have.