Lot of fed back

Make sure the antenna has a good clean ground. I know it sounds silly, but here's what's happening: You're getting RF feedback. If you don't have a good ground at the antenna, some of the RF travels back down the coax and makes the whole radio "hot". Some of that RF couples back into the microphone input, and causes feedback.

It's also possible that if your amp is really big, you're getting RF feedback directly from the antenna itself due to the really strong field.

You could try moving the antenna further away from the radio, or if that's not practical, some snap on ferrite chokes (available at radio shack) around the mic cable near where it enters the radio will probably solve the problem.