ROANOKE, Va. (AP) _ Using a transmitter and manuals anyone can buy through the mail, a man dubbed the Roanoke Phantom posed as an air traffic controller and gave pilots bogus instructions.

The airwave piracy went on for six weeks before Federal Aviation Administration agents using radio receivers and antennas traced most of the signals to an old Buick that cruised the outskirts of Roanoke Regional Airport.

Rodney E. Bocook, a 27-year-old unemployed janitor, was charged with communicating information that endangered the safety of aircraft in flight. He could get up to 22 years in prison. He was ordered held without bail.

None of the phony transmissions caused any accidents or close calls, though some pilots followed some of the instructions, FAA spokesman Paul Steucke said.

Bocook was arested Sept. 22. In his living room, federal agents found an aviation transmitter that can be bought from mail-order companies for about $500, directories on airport navigation, aeronautical frequencies and radio transmissions, two radio scanners and tape recorders.

''It was a very dangerous situation,'' said Don Poff, an FAA official who headed a task force investigating the case. ''The controllers were under a great deal of stress at times. Everything was under control, but they were trying to separate traffic and trying to deal with this person.''

The Roanoke Phantom, as he was called by controllers, told pilots of commercial and private planes to break off their landings at the last minute or change altitudes. He also transmitted a phony distress call about an ultralight aircraft.

''If an air traffic controller is working five aircraft and hears it, his attention is immediately drawn to mayday,'' Roanoke air traffic manager John Hinkle said. ''If it's a bogus mayday the distraction could be catastrophic. Two airplanes could run into each other.''

A trial date in federal court will be set after Bocook undergoes a psychiatric evaluation.

''You just don't think that too many people are going to do what he's accused of doing if they are operating with the full benefit of their senses,'' defense attorney Tony Anderson said.

Only five people in the past 10 years have communicated false information to aircraft in flight in the United States, Steucke said. None of those incidents put passengers in direct danger, he said.

''Most people after they do it for a short time either realize the danger or realize we're narrowing in on them. Some quit and never came back on,'' Steucke said. ''This particular individual seemed to carry it farther and longer.''

Steucke said technology will eliminate the danger in the future because controllers will be able to direct aircraft through the skies without uttering a word to pilots. Controllers will communicate with pilots via display screens and printers.

The nation's 31 busiest airports are using a voiceless system for pre- departure clearance now, FAA spokesman Fraser Jones said. By 1997, the remaining airports will begin using the technology, he said.