The cable TV companies that have won franchises in Fairfax and Montgomery countsystems, when they are completed, at no additional charge.

"There's a trend of cable operators going into the SMATV business in order to preempt the market that they thought they had in the first place," said Paul Kagan, publisher of a leading cable industry newsletter. "Cable operators are being advised by their bankers that it's time to go get 'em."

Under the plans announced this week, Media General pledged to offer 10 channels of programming, including two movie channels, to large multiple dwellings in Fairfax County at a subscriber rate of $29.95 per month. Complexes that sign up for what is being called "Early Bird" service will then be plugged into the county cable system automatically when it is completed within the next four years, according to James L. Dillon, vice president of Media General.

"This will preserve the opportunity for us to sell them cable service later," said Dillon.

Michael Pohl, vice president of corporate relations for Tribune-United, said his firm is planning to offer interim service in Montgomery County complexes where it is "economically feasible." Details for the service, including the programming and price, have yet to be worked out, Pohl said.

Spokesmen for Super TV and Marquee said yesterday they were not worried about the cable firms' plans. Figures recently compiled by Super TV show the company offering one channel of pay TV service via scrambled UHF signals to some 54,000 subscribers around the area at rates up to $26 monthly. Marquee transmits Home Box Office to 42,600 subscribers via microwave at rates that range up to $16.95 monthly.

"It seems to me that they are afraid that they're going to lose their [potential] subscribers," said Ed Yoe, marketing vice president for Marquee. "But I got news for you. There's room for everybody out there."

"It's not a significant concern," said Thomas C. Thompson, president of Super TV, who predicted that his company would continue the rapid growth it has seen over the past year. "That's not really our market. We don't really go after the large complexes the way we do single-family homes."

Media General's Dillon said yesterday that the company's interim pay-TV plans would not slow construction of the county's cable system. The system is scheduled to be delivering cable service to some county residents by this fall and to be completed by 1987. "This will have absolutely no adverse effect," Dillon said. "We have promised the county a schedule of completion for the work, and we will stick to it."