Charles There is another possibility, at least mathematically. That is A+B=C, in this case 1830 kHz. For example 610 + 1220 = 1830. This is a classic intermod product. It would require both sources to be near enough to some kind of junction to create the IM product, and the whole thing would have to be near enough to you for you to hear it. 73, Bob N6WG