Had that exact same scenario when I was chief for a 50kw AM in Mississippi. A local ham operator complained about interference on 3.540 MHz when using a long wire antenna. He was less than 3 miles from the AM site. He was never obnoxious about it, and once I finally convinced him it was front end overload in his HF rig, he stopped complaining.In the complaining DX'er cases I dealt with, they usually were using long wire antennas well inside the 50mVm field of the AM station. When they switched to a loop antenna, the harmonics disappeared.