Robert Van Kampen addresses this question in his book The Sign. Once the 7th trumpet sounds, the rule of Antichrist will be ended. Yes he will still be on the scene, but he will be at the mercy of the bowl judgments without being able to do anything about them. The only other action that is mentioned after the 7th trumpet is the gathering together of the nations at Armageddon. But this is an event that God has decreed not Antichrist – it is part of the judgment poured out at the 6th bowl.