My car stereo has terrible reception on a single AM station. Other cars get the signal fine and other channels on radio are received fine. I have contemplated some sort of booster for the antenna or replacing the antenna entirely but not pursued either option as other stations are coming in fine.

The reception on this single station gets significantly worse when I use the USB ports to charge a device. I can not detect, however, any change as the engine revs/speeds change. None of the other stations degrade when the USB is in use.

I am obviously not a radio enthusiast/expert and apologize if this is not the proper forum for this type of question. Thanks in advance, though, for any guidance/feedback anyone might be able to provide.

I'm going to answer this in general terms first, then address your specific case.

Two things make your reception bad:

  • too little power that makes it from the sender to your radio receiver
  • too much power of noise that reaches your radio receiver, compared to the power from the sender.

Your problem seems to be of the second category. Specifically, the noise we're talking about seems to be electromagnetic interference from a bad USB charger. (and not, for example, of thermal noise that happens to different degrees in any non-superconductive amplifier in this universe)

That interference seems to contain a lot of energy at a specific frequency – the channel you'd like to receive. That's nothing unusual. Chargers typically contain a so-called switch-mode power supply, which basically switches of an energy-storing and -releasing element to step down the car's 12 V to something that is USB-compatible (5 V). That comes inevitably with some noise. And if the amount of times said switch is toggled per second happens to be the frequency of your station, or an (odd) integer factor of that, then you'll have something that makes receiving AM impossible on that particular (set of) frequency (ies). And: that switching has, albeit only with little power, to happen even if nothing is plugged in, so that the USB voltage is there the moment you plug something in.

The noise is inevitable, but it can be well contained. And in fact, devices must be designed that they don't interfere with radio. But that's a question of design quality and cost, and I'd guess that car USB chargers aren't exactly stellar in that category.

Get rid of that USB charger; that might imply you need to unplug it internally (somewhere under the plastics in your dashboard, for example), maybe it even has its own fuse. Adding an antenna booster is not really likely to help – it would possibly amplify the noise just as well as your current radio does. (The fact that it might be mounted far away from said USB charger might help, though – but the fact that you're adding more connectors and cables to your receiver might at the same time hurt.)

  • Thanks for the excellent, in-depth feedback, Marcus. Awesome stuff! – J Greene Jun 13 at 18:49

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