Radiation Systems Inc., a Sterling-based company that has been doing booming business in making antennae for radar and communications systems, said yesterday its profit rose 70 percent in the fiscal year ended June 30. The increase in profit for the fiscal fourth quarter was even steeper -- 267 percent.

Much of that growth, the company said, has come from sales of radar and navigation antennae and land-based microwave antennae used by the military, air traffic controllers and satellite earth stations.

Radiation Systems also said its sales have taken off in the satellite communications field because of its work for Whittle Communications, the Knoxville, Tenn.-based company that created a controversial television network providing educational programming -- including commercials -- for schools.

For the fiscal year, Radiation Systems earned a record $5.64 million ($1.04 a share), compared with a profit of $3.3 million (63 cents) in fiscal 1989. Its revenue rose 55 percent in the quarter, totaling $85.2 million, compared with $54.9 million a year ago.

The company reported a fourth-quarter profit of $2.1 million (39 cents), compared with fiscal 1989 fourth-quarter earnings of $577,000 (11 cents). Revenue increased 78 percent, to $30.9 million from $17.3 million.

Its backlog of unfilled orders totaled $108 million on June 30, compared with $98.4 million at the same point in 1989.