Do you have two FM stations in your area that are separated by 600 kc.? That will definitely do exactly what you describe. We had it on our repeater.
I caught in one of your posts that your transmitter needs to be on for the problem to appear, so that's intermod causing your interference, not just a random carrier coming from a router or whatever device. The problem here was caused by an FM station on 89.9 about a mile away, and another one on 89.3 roughly six miles away, plus our transmitter on 147.06. A+B-C=D 147.06 + 89.9 - 89.3 = 147.66. The thing to watch for with FM broadcast intermod is the wide bandwidth of the intermod product. There was no interference until BOTH stations were quiet -- no modulation. Obviously, the instances of both being quiet simultaneously are quite random in length and occurrence, depending on the program material of each. I tracked the location of the mixing with the aid of a spectrum analyzer, which turned out to be safety cables threaded through the turnbuckles. Laryn K8TVZ
