HACKENSACK, N.J. (AP) _ A man who was arrested with a loaded handgun during President Reagan's campaign trip here last year made ''a dumb mistake,'' said a Superior Court judge who fined him $200 and placed him on two years probation.

Judge Alfred D. Schiaffo said Bernard W. Kordelski, 57, formerly employed as an armed security guard, had no intention of harming the president.

Kordelski pleaded guilty on April 15 to a charge of possessing an unregistered .357 Magnum revolver. He could have been sentenced to 180 days in jail, said Bergen County Assistant Prosecutor David Repetto.

Kordelski was arrested on Oct. 26 in a parking lot about 41/2 hours before Reagan's helicopter was scheduled to land there.