Live from Lake Wobegon; WILL NPR'S 'PRAIRIE HOME COMPANION' BRING RADIO OF AGE AGAIN?

He has become public radio's No. 1 fund-raiser, the hero of a growing national cult, and he single-handedly made this little town (pronounced ''woebegone'') and its ''residents'' famous.

Yet Garrison Keillor spends a disproportionate amount of his time trying to avoid the limelight and trying to convince people that he is only having fun.

The 39-year-old Minnesotan hosts National Public Radio's increasingly popular ''A Prairie Home Companion.'' Athenaeum has just released a collection of his humorous New Yorker and Atlantic pieces entitled ''Happy to Be Here.'' Can the cover of People be far behind?

Not if Keillor can help it.

After all, he is also the man who composed the stirring ''shy rights'' manifesto to President Carter (''It is clear to me that if we don't get some action on this, it could be a darned quiet summer. . . . Whatever you decide will be OK by me'') and then in typical ''shy person'' fashion, didn't mail it. He also steadfastly declines to speculate that there is any significance to ''A Prairie Home Companion'': It is just two hours of fun.

In fact, when he brought the show to the East for the first time last fall, Keillor chided reporters for ''wanting to know a demographic profile of the audience and whether the show represents a search for the simple verities of the past.'' He good-naturedly twitted the press for ''taking too many humanities courses in college.''

With all due respect for Mr. Keillor's rights as a shy person, if someone can make people of all ages, urban and rural, liberal and conservative, switch off the TV for two hours and huddle around the radio for a celebration of small-town life and courtesy toward others, it's. . . . well, nothing to be shy about. It's news.

''A Prairie Home Companion'' is produced by Minnesota Public Radio's National Program Service and airs live each Saturday night on most National Public Radio member stations. In 1980 it won broadcasting's most distinguished award, the George Foster Peabody Award, but even its fans admit it is an unlikely candidate for success in the slick, video-oriented '80s.

The show, done before a capacity audience of 630 at St. Paul's down-at-the-heels World Theater, steams along at a pace midway between that of a snail and turtle. It is a combination of eclectic musical acts, Keillor's tales of Lake Wobegon, and an inordinate number of commercials.

Commercials, on noncommercial public radio?

Yes, and truth to tell, some of those products are just not in keeping with the highbrow image of public radio. For instance, there's the designer cat collection (in colors to match your decor) at Bertha's Kitty Boutique, Jack's Toast Shop (with a toaster on every table), and the Deep Valley Bed from the linchpin of Jack's Lake Wobegon empire, Jack's Auto Repair and School of Thought (''all tracks lead to Jack's, where the flashing lights show you the way to complete satisfaction''). There are several hard sells each night for the show's prime sponsor, Powdermilk Biscuits.

The latter, made with ''wheat grown by bachelor Norwegian farmers,'' come in ''the big blue box with the biscuit on the front,'' or in brown bags ''with the stains that indicate freshness.'' The claims made for the product are so extreme that they cry out for an FTC investigation: Keillor says Powdermilk biscuits are not only ''tasty and expeditious,'' but that they also ''give shy persons the strength to do what needs to be done.'' (The show has also been underwritten since its inception by a real-life grant from Cargill, the agricultural conglomerate.)

The second component of each week's show is a constantly changing assortment of little-known musical groups, which Keillor and producer Margaret Moos select. One week Phillip Brunelle and Vern Sutton of the Minnesota Opera may be joined by Stevie Beck, crowned by Keillor as the ''queen of the autoharp.'' Next it may be gospel singer Lisa Neustadt teamed with Scottish folk-singer Jean Redpath, followed by the Klezmer Conservatory Orchestra, enrapturing the audience with obscure Yiddish klezmer music.

In what has become a tradition, Keillor sets aside one night a year for the ''mouth-off,'' a bizarre competition among musicians whose only instruments are their mouths. One element of continuity is jazz played each week by the versatile house band, the Butch Thompson Trio.

The eclectic music is dictated by the show's low budget. As Keillor said, ''Our guests represent an impoverished cottage industry,'' recording for little-known purist labels such as Folk-Legacy and Philo. ''Because of the show's low budget, we have to wait for them to come by,'' he said. Happily, Ms. Neustadt said that her frequent appearances on the show have increased her audience, and that her hard-to-find records sell particularly well in the Twin Cities.

The third ingredient in the ''Prairie Home'' mix seems most appealing to listeners: Keillor's weekly monologues about what happened last week in his ''hometown.''

A cynic would say Lake Wobegon is just a figment of Keillor's imagination. Frequent listeners will have none of that: His richly detailed profiles of the personalities and activities of ''residents'' such as Senator Thorvaldsen, the wealthy Ingquvist clan, and even expatriates such as Barbara Ann Bunsen (who now lives on a llama and walnut ranch in Wisconsin), have made them friends of the family.

Even when Keillor shows someone's foibles, he also underlines their essential goodness. As William Whitworth, who has bought several of Keillor's essays since he became editor of The Atlantic last year, says: ''His humor is particularly Midwestern: satirical but never mean or savage. He's a very funny man.''

According to his staff, Keillor usually just outlines the monologues, then speaks without notes. The listener would never know it. The richness of detail and his ability to wrap up seemingly forgotten digressions at the conclusion invariably hush the audience. They bend forward to listen intently, and then loudly applaud when Keillor closes the monologue with ''And that's the news from Lake Wobegon, Minnesota, where the women are strong, the men are good-looking, and all the children are above average.''

Admittedly, these components do not make for overnight success. In fact, when Keillor launched the show in July 1974 the live audience totaled around 20. The inspiration for ''Prairie Home'' came when The New Yorker sent Keillor to Nashville for an article about the granddaddy of live radio, ''The Grand Ole Opry.'' ''Its liveness appealed to me then -- it charged the show with an excitement above and beyond the music, the old magic of radio as a connection to distant places,'' he said. Two months later ''Prairie Home'' was on the air.

It is impossible to describe the offbeat appeal of the show and a disservice to Keillor to try to paraphrase his writing. There's hard evidence, however, that the public is recognizing that it's a special show with a special message. The number of stations carrying it since NPR's satellite service began in May 1980 increased to 118 by its first anniversary, 150 by fall, and 175 by Christmas. It has also become the centerpiece of most stations' fund raising. In urbane, urban Boston a fund-raising appeal taped by Keillor resulted in a record high in donations. WKAR in East Lansing, Mich., went from a normal $38 a minute in pledges to $135 during the show.

Keillor says he aims the show at his contemporaries, those in their mid-30s to 40s, ''who grew up late, at a time of terrible violence. Now they are resolutely trying to build their lives, settle down, and raise families.'' The middle-class family life the show exalts ''really is an accomplishment.''

Whatever Keillor's aims, the actual listenership seems to defy the categorization: urban and rural, low- and upper-income alike. ''We cherish the fact that our audience includes a great variety of people who disagree about everything, but enjoy the show,'' he said.

That's reflected in the fan mail, postmarked from New York as well as from North Dakota. As one fan said, ''This city of Washington, I suspect, can use a good bit of Lake Wobegon. It would help us over some of the rough spots.'' The writer: Supreme Court Justice Harry A. Blackmun.

The show does have its critics. Jack, the Lake Wobegon tycoon, has canceled his sponsorship to protest the show's success. Keillor is forced to read letters from Jack criticizing him for going big-time and asking rhetorically, ''What happened to the tall, moody kid who used to stuff potatoes in the stovepipe?''

Keillor is quick to debunk what he sees as a false aura of sweetness and light about the Lake Wobegonese that attracts so many. He pointed out that the ''residents'' appear so shy and reserved that it seems to others they are aloof, and ''there's a failure of generosity on their part.''

As for the pangs of nostalgia many urbanites feel every Saturday night, Keillor said to forget them. ''You couldn't live there: All the people there know the same things and the same people. Their purpose is not to know new things, or encounter challenges, but to weave these ties so they can express themselves to others subliminally.''

After debunking any attempts to ascribe significance to his show, Keillor was asked which of the Lake Wobegon residents he felt closest to. His choice was Senator Thorvaldsen, who was given the name not by the voters, but by his parents, ''because it had a certain ring to it. He grew into the name and became an imposing figure,'' the 6 ft., 4 in. Keillor says, ''even though he didn't do much work. He will give you a big hello, and make a big fuss over you.''