BALTIMORE, AUG. 28 -- Two former executives of an antenna manufacturing firm pleaded guilty today to falsifying documents concealing production of substandard radar-jamming devices designed for the Air Force F-16 fighter plane.

The antennas had been installed in some F-16s in the mid-1980s "for development and evaluation purposes" but were removed after investigators learned of their questionable value in 1988, according to a federal court source.

The F-16 is a key, all-weather fighter plane considered to be central to the Air Force fleet.

Pleading guilty in federal court were David Rider, 48, a former vice president of Nurad Inc., a Baltimore-based maker of military and civilian antennas, and Bruce Kopp, 35, a former Nurad project engineer.

Rider, of Forest Hill in Harford County, faces up to 10 years in prison and $500,000 in fines after pleading guilty to two counts of making false statements. Kopp, of suburban Baltimore, pleaded guilty to one count of making false statements and faces up to five years in prison and $250,000 in fines.

Both men are free on personal bond, pending sentencing by U.S. District Judge Walter E. Black Jr. No sentencing date has been set.

Rider and Kopp were fired from Nurad and are unemployed, according to defense attorney Joshua R. Treem. Neither defendant would comment after today's guilty pleas.

According to a statement of facts read in court by Assistant U.S. Attorney Martin S. Himeles Jr., Nurad won a $4 million subcontract in 1985 to make 700 special antennas as part of an Airborne Self Protection Jammer system for the F-16, manufactured by General Dynamics. The system was designed to jam the radars of hostile aircraft and divert missiles aimed at the F-16s.

Himeles said the antennas developed by Nurad under the direction of Rider and Kopp failed to meet several performance requirements, including a sufficient axial ratio, or ability of the antenna to perform when the plane is flying at various angles and speeds.

The antennas also had inadequate radar coverage over a broad area and insufficient radar-jamming power, according to Himeles.

To conceal these shortcomings, he said, Rider and Kopp ordered technicians at Nurad to alter testing procedures to produce documents that made the antennas appear to meet specifications.

In addition, the two men submitted paperwork to General Dynamics certifying that the antennas met all specifications, Himeles said.

Nurad delivered 468 antennas to General Dynamics before investigators, acting on a tip, suspended shipments in November 1988. At the time, the system was still in the developmental stage. It later was canceled by the Air Force, but for reasons unrelated to the Nurad antennas, Himeles said.