Amateur Radio: When listening on Longwave Frequencies using Lower Sideband, I hear a consistent tone on frequencies such as 198 kHz, 243 kHz, 342 kHz, 387 kHz and 486 kHz. What is this tone? The tone is consistent, and the signal is strong. I live in Northwestern Mississippi, so am in the middle of the country. I am guessing that these signals are fairly local. Thank you for your help!