A blonde woman, smiling and holding a blue balloon, sat in front of a beach-and-sea backdrop as five television cameras of various sizes pointed at her. Her image popped up on five elevated monitors, a multiple presence that soon attracted a crowd.

It doesn't matter what she and her five TV images were selling - the message has been the medium this week in Washington at the more than 200 broadcast and technology exhibits at the 55th National Association of Broadcasters convention. The convention, which ends today, has brought together 6,000 radio and television executives, 4,000 industry representatives and assorted politicians for four days of seminars and sales pitches.

The basement halls of the Washington Hilton, Shoreham Americana and Sheraton Park are crammed with sophisticated computer systems and all the other latest technological rages that can put a show on the air.

And a model, often blonde and often female, is frequently employed to pose next to or in front of the machinery.

If sexy basement sales pitches caused no flap among the virtually all-male conventioneers, sexism in television programming was one of a myriad of subjects raised in various panel discussions upstairs in the three hotels.

After one titled "Now what's new in TV . . . if anything?". Mary Tyler Moore was mobbed by autograph seekers. She had been seated in the audience while her husband, Grant Tinker, served as a panelist.

"C'mon, fellas, you're too old for this," she told one passel of admirers, holding out pens and paper.

Exhibit visitors carried plastic shopping bags loaded with giveaway information. There were contests to enter; one company, for example, living up to its boast. "We custom blend our service," offered the chance to win 50 pounds of coffee.

The halls were filled with cameras, monitors, tapping devices, blinking control panels. An audio and video editing recorder was said to sell for $100,000, $300,000 for the three-recorder set.

Displays included screening rooms, editing rooms, tape rooms - and curtained-off rooms in which business could be conducted. Many items already had been marked sold.

The functions of many of the products on sale surpassed easy understanding: an "FM-K series of FM transmitters, featuring the MS-15 maximum signal solid state exciter, with digitally synthesized modulation and overshoot compensation."

To see all the displays and attend the radio and television sessions and workshops, the broadcasters had to walk from hotel to hotel or use charter buses to the Washington Hilton from the other two hotels.

"At least the weather is good," said one exhibitor after expounding on the need for a local convention center that could handle such a large assembly.

One manufacturer displayed only a sign in his assigned space in the Shoreham."Sorry," it said. "Exhibit lost in transit. See you next year. Minnesota Magnetics."