I've had severe radio interference from Mexico while I was about 600 miles from the border and less than 50 from the US station's 5KW transmitter, and I have no idea how much further from the border the Mexican station was. Handheld radios from a business illegally operating on the wrong frequency rendered radio comms at a job site I was working at completely unusable, and those transmitters are less than 1 watt. Your received power is miniscule compared to what even small transmitters are throwing out, so it really doesn't take much to degrade the signal to the point of uselessness, and those are with properly built transmitters and receivers on the wrong channels, forget some amateur station that's blasting noise all over the spectrum.