Wednesday, July 22, 2009

What happened to AM-music radio?

Back in the 50's and 60's, AM was King --- it had everything, whether it was the Top 40, the news, or the weekly serials. Stations like KAAY took advantage of AM's reach to broadcast to a huge portion of the U.S. and abroad. But it is difficult these days to find an AM station that plays music, and the finances of AM radio are tight.

To get some insight on the situation, I consulted Ron Henselman, our resident DXer and RF expert, for some technical info about music broadcasting on AM. Here's what Ron had to say:
AM stations have certain limitations which make them inferior to FM for broadcasting music. They have a limited audio frequency response of up to about 5000 Hz although the current FCC legal limit is 10,200 Hz.  Most of the audio program is below 5000 Hz.

(To give you some perspective, all musical tones have a frequency. For instance, middle "A" is 440 Hertz (vibrations per second), and Middle "C" is 262 Hertz. A modern piano might have frequencies as high as 4186 Hertz. In what follows, remember that "KHz" means "times 1000 Hz", for example, 5000 Hz means the same as 5 KHz. So, a piano plays notes in the range from about 100 Hz to 5 KHz.)

There is a technical reason for limiting AM frequency transmission to a max. of 5000 Hz: AM modulation is a non-linear frequency mixing process. If we transmit a 5000 Hz tone on 1090 KHz, the two frequencies mix together. We end up with the original 1090 KHz, but we also end up with a signal at 1085 KHz and another at 1095 KHz. If we transmit any tones higher than 5000 Hz, we are occupying bandwidth of the stations right above and below our station.

For instance, KMOX in St Louis has a center frequency of 1100 KHz. KMOX's channel is actually from 1095 to 1105 KHz --- it's exactly adjacent to KAAY's. Now, say that KAAY transmits a bass note or tone at 200 Hz. At the instant that tone is transmitted, the occupied bandwidth of KAAY would only be 1089.800 KHz to 1090.200 KHz. You just subtract and add the audio frequency from the RF carrier frequency. AM channels in the USA are spaced 10 KHz apart for that reason. The AM stations in many other parts of the world are spaced 9 KHz apart.

On the other hand, one of the good things about AM is the radio frequency range can travel long distances at night. The signals which normally are lost into outer space during the day are reflected back down by the ionosphere during hours of darkness.

There is another reason AM is no longer popular for transmission of music. The frequencies used for AM broadcast suffer from static crashes or noises when storms are present. This is worst during the summer months. Any type of electrical pulse noise is also picked up by an AM receiver. I hated it when somebody in my household ran an electric mixer or electric razor when I was trying to listen to KAAY. Light dimmers also are notorious for interfering with AM. It is even a bigger problem with spark plug noise (ignition noise) in automobiles. It is sort of obvious why most of the music broadcasters moved over to FM.

FM is somewhat immune to pulse noises due to something called a limiter stage in the receiver. Storm static is much reduced on the higher RF frequencies, and the limiter stage pretty much reduces what is there. The method of modulation for FM is not nearly as simple as the simple mixing method of AM. FM also occupies more bandwidth when the audio frequencies become higher, but this was engineered right into the band plan from the FCC. FM stations are spaced 200 KHz apart, so one signal can occupy quite a bit more bandwidth than an AM station. The highest tones transmitted on FM are quite a bit higher than the 5000 Hz limitation for a typical AM station.

FM antennas can be smaller and still work fairly well. AM antennas are always a compromise with short lengths.

There usually aren't any sky waves present on the FM frequencies, so interference from other stations is not as likely unless you are located some distance from the station. Even then, there is something called capture effect on FM which basically means you pretty much only hear the stronger of two or more stations.

So why would anybody try to broadcast music on the AM band these days? Well, most major markets don't have any spectrum available for new stations in the FM band. Many people consider the AM band to be a poor investment due to its waning popularity with listeners; consequently, it is easier to take over an existing AM station.

Thanks, Ron!

As Ron makes clear, there are technical reason why FM is better than AM for music broadcast. On the other hand, as Ron notes in his last paragraph, music on AM might make a comeback. The last few summers, my wife and I have been taking road trips into New Mexico and Arizona, and the best radio we find is on the AM side --- the Navajo and Hopi Indian Reservations have really good AM music stations, and there is a lot of great Mexican and World music elsewhere on the band. In contrast, the FM side sounds like computer-programmed "Jack FM.'' Maybe innovation is slowly happening in music radio...on the AM side....

---Dave S. ( )


  1. Automated stations have been around for many years, but they sure sounded better than Jack FM. The oldies station in Chicago (WJMK)became Jack FM. They brag how they play what they want; I thought the idea was to play what the audience wants to hear.

    Ron Henselman

  2. Someone has asked me why AM has a skywave at night, and why FM usually doesn't. It is because of the RF frequency range used to transmit the signal and not the type of modulation employed. FM is transmitted from 88 to 108 MHz. If we moved FM down to the AM band, .540 MHz to 1.7 MHz, there would be skywave present during hours of darkness. So why not do this? The problem is FM is a wide signal. It is possible that when the highest RF frequency is being received well, the lowest part of the signal might be experiencing a fading condition. This would result in severe distortion of the received signal. The AM band is not large enough to hold many stations spaced .2 MHz (200 KHz) apart. Skywave does exist in the FM band of frequencies, but it is a rare condition. Some radio enthusiasts make a hobby of receiving far away stations on the FM band.

    Ron Henselman

  3. During my almost 10 years at KAAY, I assisted Felix McDonald in the annual Proof of Performance measurements. I can tell you that we took extra pains in getting every ounce of performance out of the RCA transmitter. Our goal was to be the a) cleanest, and b) loudest on the dial. We spent a LOT of money and effort on getting our audio processing just right. We wanted a lister, who might be spinning the dial on his radio, to have KAAY literally "jump out" of the speaker and grab the listener by the ears, and say "you need to stop here and listen for a while". Dave M//

  4. Dave M., you're correct- KAAY DID jump out and get me!

    Bud S.

  5. KAAY certainly jumped out at me from the beginning.

    I want to make a correction to what I implied in the article above. I was looking at the audio frequency limitation from a DX'ers standpoint. The actual audio frequency limitation is legally 10,200 Hz. If I remember correctly, there are 12,500 Watts in each of KAAY's sidebands. I wondered why I rarely heard what I considered splatter into the adjacent channel. It is because so little of the program audio was between 5000 Hz and 10,20 Hz, so rarely did a sideband extend out that far into the adjacent channel. In fact, when KAAY began, the legal audio frequency limitations were actually higher than they are now. Since so many of the AM stations are now talk radio, we find the bandwidth is usually much less than the legal amount. Nobody challenged me on this, but it bothers me when I disseminate misinformation.


  6. KMOX is on 1120, not 1100.

  7. My problem is when I try to tune to an AM broadcast in Arkansas there is not a single station on there anytime of the day or night and if I do pick one up on there it is barely audible I don't know what it is but I can't pick up anything on the AM band so I'm lost as to what happened to AM Radio in Arkansas.

  8. Although AM may have a frequency limitation, a well designed receiver CAN make good use of what is there. In fact, in the Philadelphia market, where I live, I can still find a couple AM music stations, 640 AM (Radio Disney), 1480 AM (WDAS) and 1240 AM (WSNJ). When I listen to them on a 1941 Philco, it can make that AM sound GOOD. So good that my wife, who teases me about listening to Radio Disney found herself unconsciously "bopping" to the rock coming out of that old goat. When I pointed out what she was doing, she jokingly called me a "turkey" for getting her to like Radio Disney and AM. :-)

    Also, I make extensive use of an external antenna called the Terk Advantage (I get mine on Amazon). It can be oriented in a way that it will NULL out a static source and often amplifies the desired signal at the same time. I can often make AM as quiet as any FM station with the antenna properly oriented...