Analog Angle Blog

The Final Knock-Down for Auto AM Radio?

It’s not news that the car is an electrically hostile environment. Given all the switching power converters ranging from a few hundred mA to many tens of amps, and the various motors and their drivers in the vehicle, plus other noise sources, getting acceptable broadcast AM radio reception (approximately 550 to 1600 kHz) is a serious challenge, see Figure 1 .

Figure 1

The broadcast AM band is in a fertile region for EMI/RFI, and the amplitude modulation is very sensitive to noise. (Image source: HyperPhysics, Georgia State University)

The broadcast AM band is in a fertile region for EMI/RFI, and the amplitude modulation is very sensitive to noise. (Image source: HyperPhysics, Georgia State University)

Now there’s another reason to forget about AM radio in the car: the increasing number of all-electric and hybrid) vehicles (EV/HEVs) on the road, with high-power motors and associated circuitry which create a lot of RF noise. A recent article in The Wall Street Journal “Your Tesla Can Go Zero to 60 in 2.5 Seconds But Can’t Get AM Radio” (sorry, it may be behind a paywall) explored the subject and current state of the car radio. It’s yet another example of the law of unintended or unexpected consequences of technology advances.

In brief, this is what is happening: some of the newer EV/HEVs no longer even come with the AM radio, either as a standard feature or even as an option. Some owners are very upset, while others don’t even notice it is missing.

There’s no denying the reality that legacy AM/FM broadcast radio – especially noise-sensitive AM, much more than noise-resistant FM – is no longer important to many people. Sure, you can get traffic, weather reports, and news headlines, but you can also get those easily by other means when in the car. The same is true for music, talk shows, and other content.

Of course, the auto industry is aware of the issues that RFI/EMI can cause. Vendors use low-emission components with tailored rise/fall edges, filtering, shield cables and other design techniques, as they need to meet fairly strict regulatory standards for these emissions. This effort starts with CISPR25, which defines several test methods and with suggested limits, in order to evaluate the level of radiated emissions, Reference 1 . However, even if each subsystem meets the standard, noise is additive, and all those sources add up, both in terms of amplitude and spectrum. Further, noise-reduction techniques often degrade as the car ages, corrosion sets in, and grounding becomes less effective.

There are really two questions to consider as AM radio faces a greatly diminished role or even exclusion from newer vehicles: 1) does it matter? and 2) should we even care? There are certainly alternatives to AM radio, and no one will be in a communications “desert” so long as there are cell towers nearby or they are not in a satellite-blocking tunnel. On the other hand, one of the benefits of broadcast radio is its reach and coverage. Further, there is no infrastructure cost to support more uses (only additional receivers). Its required infrastructure is modest – just a transmitter and receiver, no intervening links, servers, local base stations, cables, fiber links, and similar – just the free-space transmission path. That means it can be both reliable and scalable.

There’s an ironic “back the future” aspect as vehicle noise makes in-car AM radio reception a challenge and perhaps a non-entity. When Paul Galvin of Galvin Manufacturing Corporation introduced the first radios in cars in 1930 – he called this the “Motorola” radio, as a mash-up of “motor” for car and “ola” for sound – their vacuum tubes needed a voltage higher than the 6V battery could supply (the upgrade to 12V occurred in the 1950s), Figure 2 .

Figure 2

Galvin Manufacturing Corporation -- later known as Motorola -- introduced the first widely adopted, mass-market, tube-based radio for cars. (Image source: Motorola)

Galvin Manufacturing Corporation — later known as Motorola — introduced the first widely adopted, mass-market, tube-based radio for cars. (Image source: Motorola)

To convert the 6V DC into the 50V to 125 V which the tubes needed, a device called a vibrator was used to chop the 6V rail, then transform it into a rough square wave which could then go through a transformer, be stepped up, and then rectified back to DC. In effect, they created a crude but effective switching power supply for cars.

This vibrator was a simple four-terminal component consisting of a relay configured so the applied power pulled in the armature, which opened the circuit, which released the armature, thus closing the circuit, which energized the relay…you get the idea. (if you don’t know how to wire a relay to do this, review your “Electricity 101” notes.) The waveform that resulted from this action was a strong RFI source which wiped out much of the radio signal (it also made a fairly loud audible buzz), so Galvin’s company also had to develop low-cost, effective RF filters. Note that for antique auto enthusiasts, there are vendors who sell solid-state replacements for the vibrator, so the original tube radio can still be used, see here.

Will we miss AM radio in cars? Probably not. But I do feel some nostalgic pull for AM radio in general. I suspect that for many readers of this column, one of their first projects in electronics and electricity was to build a simple crystal-diode AM radio which was self-powered from the received AM signal, and used a long-wire antenna and high-impedance headphones. You can even buy kits for budding STEM students which have all the parts and set-up for $10-$20, Figure 3 .

Figure 3

You can buy basic crystal-radio kits complete with all parts, or make some parts and buy the others on your own -- it's that simple. (Image source: Carl's Electronics/Electronic

You can buy basic crystal-radio kits complete with all parts, or make some parts and buy the others on your own — it’s that simple. (Image source: Carl’s Electronics/Electronic

It’s hard to describe the feeling of elation which results from building one of these from parts you could find or buy cheap (wire, a tube to roll it on, a small general-purpose diode such as 1N34, a commercial or self-made capacitor, headphones, and a real breadboard) and then receive an AM station come in over the air. (You didn’t even need to understand the principle of energy harvesting or envelope detection to build and use it!)

What are your thoughts on the apparent demise of AM radio as cars get more electronics, and EV/HEVs become more common? Do we need that radio in the car for backup, or is it an artifact of the past that is nice but not really needed?


  1. In Compliance, “Meeting Automotive EMC/EMI Requirements
  2. Motorola Timeline
  3. Vintage Car Radio, Radio Vibrators
  4. Wikipedia, Vibrators

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6 comments on “The Final Knock-Down for Auto AM Radio?

  1. SpatialKing
    November 30, 2018

    Hi Bill,

    Good article – I have to say I am surprised that the auto manufacturers didn't bother squish their emissions enough to put AM in their cars.   Where are the analog guys when you need them?

    One issue I see is the Emergency Broadcast Alert, which is sent out on AM radio, not FM, and not on cell phones either.

    I see some legislation in our future here….

    Robert D

  2. Steve Taranovich
    December 5, 2018

    Check out this article that fixes an AM noise problem in my friend's car 

  3. Measurement.Blues
    December 5, 2018

    I've reached a point where I can't listen to any form of commercial radio. The ads are too annoying. As for noncommerical or public radio, why bother? I stream using my phone.

  4. Andy_I
    December 6, 2018

    I'm pretty sure Emergency Alert Notifications go out on all broadcast stations, not just AM.  I don't think any are exempt.

    The cell phone situation is changing.  But some cell phones do not receive alerts.  No attempt has been made to reach all cellphones.

  5. Andy_I
    December 6, 2018

    In some markets (areas), some radio programming might be available only on AM.

    Boston, MA is SOMEWHAT an example.  WBZ (AM) is known for its comprehensive traffic reports updated every 10 minutes.  I think WBZ might simulcast on an “HD” channel on FM, but I have not driven a car that can receive that.  So *my* only option in a car is to hear them on an AM radio.  Would FM stations pick up the void if AM radios disappeared?  Doubtful.

  6. SpatialKing
    December 6, 2018

    Actually, now that I think about it, the only time we had a real Emergency Broadcast Alert here in the Bay Area was during the big quake of 1989.  All radio stations went off the air except the two authorized to broadcast during an EBA.  We have had a few alerts for storms, fires, etc, but those were warnings, not emergencies.   I wonder if the rules have changed for EBA?

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