A controversial bill that would require all new cars to be fitted with AM radios looks set to become a law in the near future. Yesterday, Senator Edward Markey (D-Mass) revealed that the “AM Radio for Every Vehicle Act” now has the support of 60 US Senators, as well as 246 co-sponsors in the House of Representatives, making its passage an almost sure thing. Should that happen, the National Highway Traffic Safety Administration would be required to ensure that all new cars sold in the US had AM radios at no extra cost.
Amateur radio operator here, I see some things in this thread that are kinda correct in this case but not technically accurate, and I’d like to provide the actual details:
I see folks asserting that “AM goes farther than FM.” This is only accidentally true in this case. The stations that your car radio’s AM setting can pick up are transmitting in the Medium Frequency (MF) spectrum, around 500 to 1600 kHz. The wavelengths here are around 200 meters. Radio waves with these wavelengths can refract around the Earth’s surface, or at night bounce off the higher layers of the ionosphere, to be heard hundreds or thousands of miles away (respectively).
By contrast, commercial FM broadcasts are done right in the middle of the VHF band, from 88 to 107 MHz in the US. These waves don’t refract off the ground or sky, and only travel in straight lines, so you have to be fairly close (within 75 miles depending on the height of the antenna) to hear them.
The terms “AM” and “FM” refer to how the audio signal is encoded on the radio wave; the short answer is Amplitude Modulation gets brighter/dimmer, FM gets redder/bluer. This is very little to do with how far the signal will travel, though it can have ramifications for how easy it is to hear near the edge of range and rising above the noise floor, etc. Both modes are relatively inefficient with power and bandwidth for different reasons; AM is primitive and FM is fancy.
AM radio is kept deliberately simple. Like, pre WWII simple. You can improvise an AM radio receiver with stuff you have lying around your house. I’ve seen a diode made out of a rusty razor blade and a pencil. It doesn’t have the best audio quality, it’s a bit of a waste of bandwidth compared to something like Single Sideband (which is cool but beyond the scope of this comment. Ask me about SSB if you want to hear about some cool radio shit) but it’s easy to find or make a working AM radio. There’s nothing that says you have to use AM on lower frequencies; aviation communication radios are AM (because we started using radios in planes before FM was invented) and they’re in the VHF spectrum just above the FM broadcast band and just below the amateur 2 meter band.
FM is fancier, it chews up even more bandwidth depending on the exact implementation and is less power efficient, but it offers the potential for less natural interference, better audio quality, and they even broadcast it in stereo. There’s just so narrow in terms of bandwidth you can make FM, which is why you don’t find it in use below 10 meters (about 28 MHz); there’s just no room for it. FM radios are more complicated, compared to a smart phone or PC they’re pretty basic but you’re not as likely to successfully improvise one out of shit you find in a tool shed.
In terms of sources for emergency information? I want both and more. AM broadcasting is the very definition of “good old” and relatively few stations can reach a national audience fairly easily, though FM stations typically take a LOT less power to run so I think they’d plausibly be up and running in a partial grid failure, and they’re more localized. So for a national emergency I might tune into AM, for a local emergency I’m going to tune into my town’s FM station that is 6 miles from my house.
I would also want a WX band radio. For those who aren’t familiar, the National Weather Service operates a network of transmitters that broadcast continuous weather information on one of a dozen channels in the 160 MHz range, narrow band FM. If you’ve had a “weather radio” that’s what that is. I know of very few car stereos that are equipped to receive WX band broadcasts and now that I think about it I can’t imagine why it hasn’t been mandatory for decades.
Man you seem like you’d be awesome to have a drink with and just learn… everything.
Not only that, it can be powered by the radio signal itself.
https://en.wikipedia.org/wiki/Crystal_radio
It should be noted though that you’ll need a fairly nearby radio station that is transmitting with a whackton of power.
Tell us about ssb. I also just bought an SDR to get my gas meter readings. Any other cool stuff I should check out?
Single Sideband is basically AM 2.0, so to talk about it in detail, we have to take a closer look at good old fashioned 19th century AM.
The graphic above is from Wikipedia. The top graph, the “baseband” signal, is the audio, aka the signal coming out of the microphone. It’s vaguely what a human voice would look like on a specrograph.
The second graph is what AM looks like. The spike in the middle is called the “carrier.” Let’s say you’re transmitting on 5 MHz with a 10 watt radio. When you push the talk button and then say nothing into the mic, you start broadcasting a 5 MHz wave with a power of 10 watts. A receiver tuned to 5 MHz will hear the background static go away, because you are transmitting a carrier wave that does not modulate much louder than the background noise, you are effectively holding the receiver’s speaker still. A signal coming in strong enough to do that we call “full quieting.”
The carrier carries no information. Another way to look at these graphs is, picture the sine wave. You may have seen something like this before:
That middle waveform represents AM, notice how there’s always some squiggles going through the middle, and it varies toward the top and bottom edges? That’s what the upper graphic is representing, that big center spike is always there for that reason.
You may notice that the two lobes to either side are the same shape as the base band signal; or one of them is, the other is a mirror image. We call these sidebands. That’s actually where the audio is. In the second graphic, you can see how the top and bottom edges of the AM waveform resemble the baseband signal. Turns out, AM radio uses twice as much bandwidth and more than twice as much power to transmit the usable signal.
So what if we built the transmitter to only transmit one of the two sidebands and suppress the carrier and the other sideband? The same audio information goes over the air, and we take up less room on the radio spectrum to do it. What’s more, since we’re transmitting less overall “stuff,” the radio’s power is more focused on the part we do transmit, so a single sideband transmission comes across as “louder” than an equivalent AM transmission.
There are some cool upshots to how SSB works: the first is that the radio uses less power and overall stays cooler. AM (and FM) transmit with their full power all of the time, doesn’t matter if you whisper or scream into the microphone you’re putting whatever power your amplifier is set to out to the feed line. You might be transmitting silence, but it is very loud silence. SSB doesn’t do that; the louder you talk into the mic, the more power goes out the antenna. You aren’t constantly transmitting that carrier, so if your mic goes quiet, so does your antenna. Thus, your transmitter gear takes less power, runs cooler, and if you are on some consumable power source like batteries, you can transmit more effective power for longer.
Even cooler than that is the lack of collisions. If you’ve played with radios much, even listening to music radio stations near the edge of their ranges where you can kind of hear both, you know they interfere with each other. Happens all the time with aviation radios, pilots will transmit on the same frequency at the same time and anyone else listening gets to listen to the psychedelic sounds of two carrier waves interfering with each other. On FM radios usually the louder signal “wins” and the other one just sounds like static or interference under it. SSB doesn’t do this.
If two people transmit at the same time on the same frequency on SSB, a third person listening just hears two voices, just like if two people talked at the same time in a room. Hams hold contests to see who can make contact with the most people from the most places, and folks from somewhere rare will end up asking for contacts, everyone else says their callsigns at once like it’s the floor of the stock exchange, the one station will pick someone he heard to exchange details with, rinse and repeat. It’s 1950’s Discord.
Hams also use this technique to send text back and forth extremely efficiently. If you tune your software defined radio to 14.070 MHz (or use one of several people make available online) you might hear what sounds like several strange warbling whistles that come and go. That is PSK31, a digital text mode designed to use an ordinary PC sound card as a modem, and an ordinary SSB transceiver to send the signals across the air. Using software like FLDigi, you can receive and transmit text over the air, and each text transmission is very narrow in bandwidth. Over a dozen can take place in the same space as a normal voice channel, you leave the radio tuned to 14.070 and choose which transmission to listen to by clicking on them in the waterfall, ie choosing what audio frequency to listen to, which only works using SSB because no carrier collisions.
For all its advantages, there are some disadvantages. You cannot transmit “quiet” with SSB the way you can with AM or FM; so the background static, the “noise floor” is always there. Makes it not so nice for listening to music, which is why you basically only ever see it on communication radios. And it also requires a much more complicated transmitter and receiver while achieving the same or slightly worse audio quality than AM. You don’t see SSB used much in VHF and above because there’s so much room for activities/line of sight limits how far your signal goes so the efficiency advantages of SSB are less important, which is why we tend to use FM (or AM for old shit like airplanes) at wavelengths shorter than 6 meters or so.
100% accurate, thanks for the clear write up. Please stick it up on Wikipedia if you can :)
And I’ll add a bit about Clear Channel AM (unrelated to the billboard advertising company) - there were originally a handful of said stations that broadcast on a few AM band frequencies that are reserved just for them, so their broadcast range is impressive.
One for example is WOR radio in Chicago.
Fun factoid - you can see on very old AM radios those clear channel frequencies marked by a diamond or similar symbol on the dial.
Holy crap, I have an old radio (a few feet from me) for a decade and I had no idea what the diamond symbols on it were for. Thank you!
See this right here is what I’m on here for, these kinds of connections.
This is just making me think that there should be a fundamentals of modern technology class in high school somewhere between a shop and physics class. It’d be a nerd elective but by fuck am I that nerd
You had shop?
lol no I’m a millennial
Something between shop class and engineering. Like Technician class. I like it.
I dunno, maybe we need something more General.
Aren’t you being a little Extra?
He’s just advanced, but you’ll never reach that state these days in afraid.
In today’s world, it’s basically impossible to even start out as a Novice.
Yeah as an engineer I think it would’ve been far more useful than the engineering class I took that was basically how to do autocad and measure things. And it would’ve been useful for everyone.
There are two things at play here. 1) one of the primary purposes of the United States’s education policy is to produce engineers. This is an economic and military strategy. And 2) we live in a world where technology abounds and yet so few people understand it. A robot isn’t a magic person made of metal, it’s the manifestation of the laws of physics as applied for our own desires.
Making a radio receiver and a telegraph and a record and telephone and a basic battery etc makes for more grounded adults. Show the teenagers the way the real world works, clever applications of natural phenomena
I suppose if we discovered magic and understood its principles, it wouldn’t really be magic any more.
— Arthur C. Clarke
Well now you’re just where I fell when I learned about the imaginary plane in electronics.
Very nice write up. I am curious, is the long wavelength Responsible for the rise and fall in audio quality depending on where you are. I have had this happen as I was driving, the sound quality seemed to pulse.
Also I used to live right at the base of a tall mountain range there was a AM transmitter on the other side less than 100 miles away. During the daytime I could never receive it, at night it would bounce over the mountain and it was pretty clear.
I doubt the wavelength is a factor there; depending on the circumstances it could be anything from atmospheric waves to something in your car causing intermittent interference.
The Earth’s ionosphere exists in several layers. During the day, solar radiation ionizes gas deeper into the ionosphere causing a layer that doesn’t usefully refract most radio waves; you can reach beyond the horizon on some of the higher HF bands, but down in the MF, you’ve basically got ground wave. At night, without the sun around to cook the atmosphere, that lower level dissipates, revealing a higher ever-present layer, and the geometry is right to refract signals for hundreds or even thousands of miles.
Skywave propagation can be really fun to play with.
That truck was a old fucker. It had AM and a 8 track. So I wouldn’t be surprised if the radio shielding was limited and degraded…
There likely was no “shielding” in a truck of that era, just simply the truck was made of metal as was the chassis of the radio, bolt 'em together and you’ve got a reasonable ground.
But, I do know from experience that there are items on a pickup truck that can produce radio interference especially when worn. A worn distributor is a spark gap transmitter, as I learned when I installed a mobile radio in my S10. The audio on my radio got a lot better after a good service of the ignition system.
Good info here.
Reminds me of that kitchen sink that started picking up AM radio.