An Arlington judge ruled yesterday that prosecutors had failed to prove that a colorful Northern Virginia private detective attempted to illegally wiretap the telephone of an undercover policewoman who supposedly was seeking help in checking on her husband's phone calls.

"There is just no evidence that he wanted to intercept (conversations)," said Circuit Court Judge Paul D. Brown as he dismissed the felony charge against Joel L. Kaplan, the 31-year-old head of Action Investigative Services of Alexandria, a firm specializing in divorce and child custody cases.

Brown cited tape recording of conversations between Kaplan and an undercover D.C. police officer who posed as a private investigator to gain Kaplan's confidence and was the chief witness against him. During those conversations, which were played in court, Kaplan repeatedly said he did not plan to listen to the tapes but agreed to install the electronic surveillance equipment.

Kaplan still faces a charge of possession of illegal wiretap equipment, an offense to which he has pleaded innocent and which carries a maximum five-year prison term. Brown is expected to rule on that charge today after hearing closing arguments in the case, one of the first of its kind in Northern Virginia.

Under Virginia law, wiretapping is illegal without the consent of the telephone subscriber except when authorized by court order and performed by law enforcement officers.

Prosecutors claim that Kaplan boasted to undercover D.C. detective Michael E. Hubbard that he had performed more than 7,000 wiretap in the past 14 years and possessed electronic equipment valued at $175,000 which was more sophisticated than that of most federal agencies.

Hubbard and another undercover officer testified Monday that in April they watched Kaplan install an illegal wiretapping device on the telephone of of an officer who was posing as a suspicious wife seeking evidence of her husband's infidelity for use in a divorce case.

Kaplan's lawyer argued yesterday that the equipment Kaplan used is not illegal, is available in many stores and can be used legally by radio operators and others.