The Navy's F14 "Tomcat" fighter was impressive in passing its first test in actual combat yesterday, but its Libyan opponent, the Soviet-built SU22, is far from the Soviet Union's best warplane, according to U.S. military experts.

Military spokesmen expressed surprise that the SU22 had been sent on the mission because Libya has much better Soviet fighters, including late-model Mig23s.

Both SU22s were downed by "Sidewinder" missiles fired from the Navy F14s after one of the Libyan planes fired a Soviet "Atoll" missile at the U.S. jets, the Navy said.

Both "Sidewinder" and "Atoll" are heat-seeking, air-launched missiles with essentially similar characteristics. They home in on heat sources, such as jet engine exhaust (or the sun, if improperly aimed).

Navy Lt. Bob Baker, now at the Patuxent Naval Air Station's test pilot school, has been a pilot since 1976 and has substantial F14 experience.

In the situation described yesterday, Baker said, "it is probable" that the F14 pilots would see the Libyan missile as it was fired "because of the smoke the missile makes when it leaves the aircraft." The missile also can be detected by radar.

The F14 pilot's first reaction, Baker said, would be to take evasive action, a procedure practiced repeatedly. "Obviously, the sooner you pick it up the better position you're in" to avoid the missile, Baker said. The "Atoll" missile has a range of about three to four miles before exhausting itself in flight.

Then, Baker said, the F14 would maneuver for position -- best would be behind the SU22 but a head-on shot would be possible -- and the pilot would "point his plane's nose at the opposing aircraft, get a tone in his headset that tells him the seeker head of the missile is looking at a heat source and pull a trigger." The trigger is on the control column in the front cockpit of the two-man F14.

Yesterday's encounter lasted about one minute, the Navy said. Decisions in the cockpits had to be reached in fractions of seconds as the jets flew possibly as fast as twice the speed of sound.

The F14, designed to be flown from a carrier, is a highly sophisticated twin-engine jet with a "variable geometry wing" whose angle to the fuselage can be adjusted from 20 degrees to 68 degrees. It is armed with three types of missiles and a 20-mm. cannon.

The SU22 has a crew of one, two engines and a variable geometry wing. But it has limited performance characteristics and comparatively little electronic gear. It carries missiles and two 30-mm. cannons