What do you give an airport that has everything but enough passengers for its 20th birthday?

A big promotional birthday bash, replete with an old-fashioned barbecue and pig roast, an Irish fiddler, a Bluemont concert series, the Lowdown Cloggers and scores of exhibits highlighting the latest in aviation technology.

That's what is planned for Dulles International Airport this weekend at Dulles Expo '82.

While Expo visitors may be wishing Dulles many happy returns, a lot of people will be wishing it many more departures as well. And, as nearly everyone associated with Dulles agrees, it is going to take a lot more than a whiz-bang birthday party to do that.

"We don't have any problem selling people on the idea that we've got a beautiful airport, but we've still got a long way to go in getting them out here," said Dexter P. Davis, airport manager at Dulles.

Dulles straddles the Loudoun-Fairfax county line, and getting people out to Dulles has been a major concern of civic leaders in both counties. Better business at Dulles, they say, means better business for their home counties.

One group that has been working for nearly 16 years to boost business at Dulles is the Committee for Dulles. Last year, in a survey of 190 business in Loudoun, Fairfax and the Maryland suburbs, the committee found that 65 percent of the firms that now use Dulles less that 20 percent of the time would increase that to 90 percent if service to other cities were improved.

Dulles boosters also say that rapidly changing demographics in Loudoun, Fairfax and Prince William indicate a vast untapped source of business for airlines. The major change, supporters note, is a significant westward shift in the area's population, income, employment and retail sales.

Because of the changing patterns, notes Carrington Williams, head of the Washington Dulles Task Force, "We're working with businesses in Tysons and elsewhere, getting them to go to the airlines and say, 'Look, we need better service and we're not getting it. It costs us money to send employes and equipment back and forth to National where they have to wade through all that traffic. Look at what our needs are, as well as those people in downtown Washington.' "

The nonprofit task force, formed last month as an offshoot of the Dulles Policy Task Force, is a private group of business and civic leaders formed to promote Dulles. Williams, a former Virginia legislator, said the group hopes to more than double the number of Dulles passengers to 6 million by 1985.

Although Dulles is a starkly handsome, passenger-oriented facility, it has been viewed as the ugly stepsister to the more convenient National Airport. National, on the edge of Crystal City in Arlington, is only a five- to 10-minute ride from downtown Washington, while Dulles, which is 26 miles from the White House, is 45 minutes or more.

In the last calendar year, Dulles handled 2.3 million passengers versus 14.2 million at National and logged 29,177 carrier operations (departures and arrivals), in contrast to 193,493 at National, Davis said.

Passenger preference also shows up on the bottom line: Between Dulles and National, both operated by the Federal Aviation Administration, there was a $4 million profit last fiscal year --National made $12 million and Dulles lost $8 million. Most of the loss, Davis said, came because Dulles decided to waive traditional landing fees and mobile lounge charges to provide an incentive for airlines to make greater use of the airport.

In the last three years, Dulles has suffered some dramatic losses. For example, passenger use in 1980 was down 26 percent from 1979 and carrier operations were down 35 percent.

But, Davis says optimistically, "I think we're seeing the end of a three-year downward trend that's been all bad. We're starting to turn around and go up again."

In the first three months of 1982, the number of passengers has increased by about 14 percent and carrier operations by nearly 12 percent compared with the same months in 1981.

Dulles, which is used by 11 major airlines and four commuter lines, had a net gain of one airline in the past year. It actually added three--Western and the commuter lines, Ransome and Christman Air Systems--but lost Braniff to bankruptcy and the Soviet liner, Aeroflot, as part of U.S. sanctions imposed following the crackdown in Poland. (National has at least 18 major airlines and 13 commuters.)

The problem, as Davis and others outline it, is the typical chicken-or-egg riddle: Dulles needs more airlines and more regularly scheduled flights to the vast middle portion of the country before it can attract more passengers. The airlines don't want to move in or schedule more flights until there are more passengers.

"We have excellent service to California, very good service to Europe, super service to Atlanta and Texas," Davis said. "But once you get past that, it's hard to get anywhere from Dulles. We need a better service pattern for the convenience of customers.

"There are an awful lot of people now demanding and needing service at Dulles, but the carriers simply haven't reacted to the changes in the area--but I suppose it's difficult to react when times are terribly bad."

Recently, some of the obstacles to improving business at Dulles have been cleared.

Last year, Congress approved a policy limiting to 16 million the annual number of passengers at National and the number of flights permitted. Congress also imposed a strict nightly curfew on operations at National. In addition, more stringent noise control rules will be implemented in 1986, requiring almost all existing aircraft at National to switch to Dulles or Baltimore-Washington International. Congress also barred non-stop flights of more than 1,000 miles to and from National.

One major problem for Dulles has been ground transportation. Part of that problem will be eased late next year, when work to connect Dulles Access Road to I-66 is completed, cutting trips between Dulles and downtown to 20 to 25 minutes, Dulles manager Davis said.

Right now, Davis says, the airport could easily handle 5 million to 6 million passengers a year without any major investment, other than expanding the 3,400-space parking lot. But it could not handle that number easily, he adds, if 45 percent of its passengers continue to come and go between 4 and 7 p.m. as they currently do.

In anticipation of a better spread of traffic in the future and because of current demands, Davis said, there has been a concerted effort to improve ground transportation.

The airport recently opened a new baggage claim device to speed up service for travelers; it also purchased and refurbished five slightly used buses as part of a long-range plan to increase the bus fleet to 14 for trips to National and downtown.

The airport is planning to install a computerized ground-transportation information system that would allow passengers to get a printout of various means of transportation and costs to their destination, Davis said.

In addition, Davis said, officials are studying the possibility of opening a satellite transportation terminal at a Metro stop in Northern Virginia. An aggressive advertising campaign boasting of the new amenities is expected to get under way soon, he said.

"We want to make everything more convenient for travelers," Davis said, "and to tell airlines there's money to be made at Dulles . . . and there's money to be made today."