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30 November 2001

AM/FM Radio Blues

I generally prefer silence on my beautiful drive between school and home on San Marcos Pass, but sometimes the car radio is good company, usually 91.9 FM or 1250 AM. I've had good reception of both local stations since school started in September. But winter is approaching, and the days are shorter now, so I often drive home in the dark. I've noticed that I usually lose my AM station in the dark while my FM station is still clear. Why does the time of day make such a big difference in my radio reception? Other AM/FM radio stumpers will be appreciated as well as answers!

Here are a few more AM/FM radio stumpers. If I owned a cell phone or Ham rig, I'd probably have many more. Don't expect me to answer all these!

  • Why is the stretch of Highway 101 through Gaviota Pass such a black hole for radio?
  • Why do portable radios (usually) work inside metal cars and airplanes as well as solid buildings and tunnels?
  • Why does one car antenna work for both AM and FM? (Or does it?)
  • Why do I get better AM reception at night for distant stations, but loose my local stations?
  • My car antenna is adjustable, so is there a best antenna length for a particular station, say KCSB at 91.9 FM?
  • Why does it make a difference for FM reception where I stand in my classroom at school, but not at home? The signal at school disappears if I stand in the wrong place!
  • Why do portable cell phones and scanners and transceivers use short "rubber ducky" antennas, but my car needs a much longer whip antenna?

The photo (barely) shows the radio and TV antennas on top of 4000+ foot Broadcast Peak and Santa Ynez Peak, the western summits of the Santa Ynez Mountains.


A car radio is a sensitive instrument for exploring the universe. High frequency radio signals like FM 91.9 mega-hertz travel by line of sight. Lower frequency signals like AM 1250 kilo-hertz can reflect back from the electrically charged ionosphere 30+ miles up. High energy UV radiation from the sun charges this reflective zone to even lower levels, but the ceiling rises again at night like a solar tide, so that distant stations can bounce much farther to interfere with my local radio. It's inconvenient, but it's pretty exciting that it's really the effect of space weather!

Notes:

Quick, if you're old enough to remember the 60s, what was Wolfman Jack's radio station? (I know, "If you remember the 60s, you weren't there", but try anyway!)

Wolfman Jack (aka Robert Weston Smith) started on XREF (1570 AM) (at 250 kw??) in Ciudad Acuņa, Coahuila, Mexico, just across the river from Del Rio, Texas. I'm dating myself, but I remember him on XERB (1090 AM) in Southern California. As I understand it, the Wolfman made tapes in LA (or Chula Vista?) that were trucked daily to be broadcast from Rosarita Beach, Baja Mexico, from the 50 kw XERB. "50 thousand watts of soul power, you gonna love it to death!" XERB was a "clear channel," free of FCC regulations. That means that no other station shared that frequency so they could broadcast at maximum power day and night. AM radio waves don't stop at the border, especially at night!

You could listen to the Wolfman anywhere, including high canyons up the Eastern High Sierra where you couldn't even get the local stations. Rumors were that you could hear him across America and Canada all the way to Alaska! There were other high-power AM stations that mostly featured preachers and gospel, but the Wolfman played gritty R&B and R&R. George Lucas captured some of the urban legend in his classic film American Grafitti. The Wolfman Jack Online Museum has a few archival recordings (as MP3s).

About the same time in the 60s (?), we were lucky enough to get Peter Bergman's Radio Free Oz on KPFK-FM (and here) all the way from Los Angeles to our Isla Vista apartment near UCSB. Either there was a clear path over the ocean or there was a local repeater. That radio show was the birthplace of the Firesign Theatre. I loved those radio shows. I wish I made recordings, but that was before the time of cheap cassettes. (Peter Bergman now has a new Radio Free Oz on the Web.) I hope kids today have equally strong radio memories. It won't be that pop and "classic rock" stuff that's so common on the radio, but our local college station KCSB (91.9 FM) might qualify.

AM and FM are very different. I hear that other radio bands have their quirks too, but they're not so familiar. FM is usually limited by the curve of the earth. That's why FM (and TV) antennas are up high on mountain peaks. But lower frequency AM atations can have their antenna on the flats because their signals can curve around the earth by "groundwave" propagation, and they can bounce back from the ionosphere and travel much farther at night. To reduce interference, the FCC requires local stations to drop their broadcast power at night. Here's the (rather good) FCC official explanation:

Most AM radio stations are required by the FCC's rules to reduce their power or cease operating at night in order to avoid interference to other AM stations. Our rules governing the daytime and nighttime operation of AM radio stations are a consequence of the laws of physics. Because of the way in which the relatively long wavelengths of AM radio signals interact with the ionized layers of the ionosphere several miles above the earth's surface, the propagation of AM radio waves changes drastically from daytime to nighttime. This change in AM radio propagation occurs at sunset due to radical shifts in the ionospheric layers, which persist throughout the night. During daytime hours when ionospheric reflection does not occur to any great degree, AM signals travel principally by conduction over the surface of the earth. This is known as "groundwave" propagation. Useful daytime AM service is generally limited to a radius of no more than about 100 miles (162 km), even for the most powerful stations. However, during nighttime hours the AM signals can travel over hundreds of miles by reflection from the ionosphere, a phenomenon called "skywave" propagation. (Shortwave stations, which operate using AM modulation on several bands between between 2.3 MHz and 26.1 MHz, also use this phenomenon to broadcast still greater distances, up to thousands of miles). Because of this change in signal propagation from daytime to nighttime, if every AM station kept its daytime operating power at night, massive interference would result.

In my case, our Santa Barbara local KEYT (1250 AM) is required to reduce their signal from 2.5 kilowatts to 1 kilowatt at sundown. (See the FCC AM Query database for details.) I'm sure that makes a big difference, since my car radio reception fails with interference almost exactly at sundown.

An additional factor might be that AM (amplitude modulated) radio uses the amplitude or volume of the signal to carry sound information, but FM (frequency modulated) radio uses small changes in the frequency. That means that FM signals are always full power, but AM signals reduce power for quieter sounds. Think of sending a code with a flashlight. AM sends the code by dimming the light, but FM sends the code by blinking the bright light at different rates. New digital radio signals also blink full power signals on and off with a different code.

There's lots more to say about AM/FM radio stumpers, but I'm out of time this week. Here are some links for further research:

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Copyright © 2001 by Marc Kummel / mkummel@rain.org