How is my guitar cable picking up clear radio-signals? I've recently noticed radio-signals coming through my guitar amplifier. I'm on the top floor of a building in a densely populated city, with a clear visual path to many other buildings. My guitar cable is plugged into the amp when I hear the radio-station, and the guitar is not attached to the cable. I can no longer hear the signals when I unplug the cable, so I know the cable is almost certainly the culprit. 
The core of my question is: I know that an electromagnetic wave will induce oscillations in an amplifier circuit, but I would expect that to only produce a hum. How am I able to hear clear signals via the cable? 
I was horrified to hear that infernal Kars4Kids commercial playing through my beloved Marshall Amp. I'm replacing this cable immediately.
 A: An electromagnetic wave will induce oscillations in an amplifier circuit, and if you vary the frequency of the wave, you will get more than just a steady hum.
It's also indemnified by law.
8.2 FCC: "this device may not cause harmful interference, and this device must accept any interference received, including interference that may cause undesired operation."

Interestingly, my Marshal amp also did this, but neither the Fender nor the Hartke amps ever did. The Peavey powered mixer was also subject to this, but to a much lesser extent: you could only hear it when cranked up to 11, while the Marshal needed to only be at about 2. 
I wonder if Mosfets have anything to do with it. AFAIK, the Marshal is the only one that had them.
Also on the Peavey, it only happened if you touched a lose wire to the metal case. I would suspect that using the ground lift switch might help. But IME, that's just usually the thing keeping it from being worse. There has to be some pretty dirty incoming power before lifting the ground actually helps.
