I am ignorant when it comes to the physics of radio waves. I listen to AM radio in the car on the way to work. I leave the house when it’s still dark (about 5:45 AM EST) and in the summer months the sun usually comes up during my 50 mile commute. As the days get shorter it’s usually just getting light out as I get to work. The signal strength of the station I listen to decreases as I get closer to work (and farther from the radio antenna, I presume.) That I understand.
Here’s what I don’t understand: When the sun is up the signal stays strong miles farther than when it’s still dark. This time of year I lose the signal to static about 10 miles from my office on my drive in. However, when I go out to lunch around noon the same station comes in crystal clear. Is there a physical reason why this happens? There must be more to it than available sunlight, no? I’m not imagining it, it starts to happen every year about this time.
The short answer is that the ionosphere consists of three layers, the D, E, and F layers. At night the D layer, which absorbs radio waves, particularly in the range below 10 MHz largely disappears, with only small residual activity due to cosmic rays. This lets radio waves propagate further by bouncing off the E and F layers. It’s going to skip, hitting the ionosphere, bouncing back towards the earth, and bouncing back up again.
You’re probably located in the area between bounces. Although during the daytime the signal travels further, it will leave dead spots in between bounces. The signal is missing you like a chopped single going over the third baseman’s head.
I believe that because signals at travel further at night, some radio stations operate on reduced power to avoid interference with other stations on similar frequencies. The net effect is a reduction in strength at night.
Correct. Many AM stations reduce power at sunset (or go off the air entirely) in order to reduce interference. Also, since signals travel further at night, you may get interference from other stations at night that would be out of range during the day.
When I was a kid, we lived in Eastern South Dakota and at night we would listen to KGO, a radio station in San Francisco, CA. It would come through quite clearly.
In addition, most AM stations operate with a directional antenna at night, to reduce interference to other stations in certain directions. So you might find that the signal stays relatively strong in one direction, while rapidly dropping off in another. One example is the East Coast, where most US stations send a full signal out toward the empty Atlantic, and weaker signals to the west where the other stations are.
KGO and WWL are both examples of what are (or were) known as clear channel stations (which has nothing to do with the company of the same name). Clear channel stations were allowed to operate at full power at night and had essentially no other stations on their frequency, allowing them to be heard over pretty much the entire country at night.
As for AM vs FM, FM signals travel a much shorter distance than AM signals so FM stations can be crowded more closely together geographically.
Also, the way AM and FM stations interfere with each other is different. When you are in reception range of multiple AM stations on the same frequency you hear all of the stations at the same time. The stronger signals are more distinct but you hear all of them.
With FM the stronger signal tends to dominate so if you have both a strong signal and a weak signal then you pretty much just hear the stronger one and don’t hear anything at all from the weaker.
They really are different systems all together, with the FM & AM frequencies being very far apart in the firstplace.
AM operates in a band of frequencies around 1Mhz. The band runs from 520–1,610 kHz. Channels are 10kHz apart. It’s really only good for voice, as the bandwith is thin, and the frequency, and modulation technique are prone to noise. It operates via ground wave propagation, AND ionospheric skip.
Boston’s WBC 1030 AM is 1030 kHz, or 1.03 Mhz. (It also happens to be a clear channel station, in that it’s the only one on that frequency, and they utilize it to the fullest. Folks often call in at night from as far away as Alaska… not often, but it can happen) I once got a much smaller station in Boston, WRKO 680 when I was working in Augusta GA. I tuned the car radio at night, and there was the show I wanted!
FM Operates in a band about 95 times higher. 87.8 to 108.0 MHz is the band, with channels being offset by 200kHz (.2 Mhz) in the US. Channels on the odd decimal.
FM carries a lot more information, and is less prone to noise due to both frequency, bandwith, and modulation technique. It’s higher frequency does not tend to skip on the ionosphere. It propagates via ground wave only.
I’m not one for wiki posting to answer a question, but this one is fairly complete, though basic, and does a better job than I would.
AM DXing is fun if you’re driving around at night. I often listen to NY stations in the car, and I’ve certainly heard some funny accents (Quebec & Other parts of Canada, and the south) while trying to find something to listen to out in ‘the sticks’. Sometimes the signals don’t last long, but it’s fun to try and figure out just where the station is.
Southern Yankee, which station, and where is it? We might be able to dig more out with regard to the station. I personally have a station that I investigated the signal from. It’s in Manchester NH, and in my home, 20 miles East from the transmitter, I get horrible signal. If I go to Boston (35 South), I get clear coverage.
The station in question is WFNZ which is a CBS station and operates at 610 and 1660 in the Charlotte metro area. 610 is the stronger of the two in my area and the one I’m discussing.
For the FM broadcast band and other VHF communications, there’s other ways to extend the propagation, since (as previously noted) VHF doesn’t refract enough to use the ionosphere. One technique is tropospheric ducting; weather conditions create an internal refraction channel that can carry a radio wave a long distance, somewhat similar to a light beam in a fiber optic cable. People in California have been known to talk with people in Hawaii on VHF via tropo ducting.
Though this isn’t reliable enough for broadcast radio.
AM is not limited to voice. While it is more susceptible to interference, most people would be amazed at how good AM can sound with proper engineering at the broadcast station and a decent quality home or automobile receiver. Both are, unfortunately, in short supply in many areas. Most station managers are more interested in a “loud” signal than in a clean, low-distortion signal. A 10 kHz channel spacing does not mean that the audio bandwidth is limited to 5 kHz or less (AM with 5 kHz sidebands). FCC regulations allow the audio bandwidth to start rolling off at 10 kHz. See FCC regulation §73.44 for the gory details.
The problems with AM broadcasting audio quality are often economic, not technical. I’ve heard jazz and classical music broadcasts on AM that were definitely high-fidelity. The sad thing is that FM, which is technically better in many ways, often sounds like crap due to insanely aggressive audio processing.
WFNZ operates with 5,000 watts during the day, and reduces power to 1,000 at night. To avoid interfering with other stations, the station has a directional antenna both day and night, with the strongest signal sent to the southeast. The following links show the the stations’ local (“groundwave”) signal coverage: daytime, nighttime. (The red cross on the map is the location of the station transmitter. Note that the two maps don’t use the same scale, so the daytime coverage is much greater than nighttime).
