Why can I pick up a faraway AM radio signal late at night?

[GilaB]

On vacation recently, I wondered (as I do every year) why we can pick up the news on AM radio from home (400 miles/640 km away) late at night, but not during the day. Is there less interference at night? Do the AM radio waves (for some unfathomable reason) just travel better in the dark?

Why doesn’t this happen with FM - does one frequency just travel better than the other?

[Bob_Gaughan]

AM broadcast radio stations generate two kinds of radio waves: Groundwave and Skywave.

Certain radio frequencies reflect off the ionosphere and the ground so that the radio wave keeps bouncing back and
forth.

While groundwave signals “hug” the surface of the earth, skywave signals work by “bouncing” off of the ionosphere. During the day this particular layer of the atmosphere absorbs radio signals.

But at night, it acts as a reflector bouncing medium wave radio signals back to earth. This bounce or “skip” allows these signals to travel great distances.

[Derleth]

FM doesn’t skip. Which is to say, it can’t bounce from the ionosphere (an ionized part of the upper atmosphere) to the ground and back a few jillion times, which is how AM travels over the horizon.

AM and FM don’t necessarily refer to the standard AM and FM bands, though. There are three main ways to encode a signal on a carrier of a given frequency:
[ul]
[li]CW: Continuous Wave (I might have that name wrong, it’s been a while). This is morse code. The modulation happens by turning the carrier on and off. This modulation can skip very well: As long as you can get the carrier, you can pick up the message.[/li][li]AM: Amplitude Modulation. This is used not only in the AM band, but in shortwave as well. It is done by changing the strength, or amplitude, of the carrier to encode the signal. It also skips pretty well, enabling me to pick up our propaganda broadcasts to Cuba (all the way from Miami) in Montana.[/li][li]FM: Frequency Modulation. This is done by changing the frequency of the carrier. This is used in places where you don’t care how far the signal goes, because the carrier skipping completely destroys the modulation. It’s pretty much line-of-sight, which means it also can be blocked by obstacles (like mountains).[/li][/ul]
As to why you can pick it up better at night, it’s because a huge RF source has gone away: Once the sun sets, the ionosphere becomes more able to bounce signals and your radio unit has less noise to put up with.

[Mr.Blue_Sky]

I’ve experienced the same thing. Way back in the mid-70’s, I picked up a station from Chicago (I’m in Savannah, GA).

It also happened with TV. After a major storm, I was able to pick up a station from St. Louis for about 15 minutes.

[whitetho]

During the day, AM (mediumwave) signals travel across the ground for reception. But at night, changes in the ionosphere allow it to reflect the signals back to earth, instead of absorbing them. Below is the explanation I have on my webpage, covering events as they stood in the 1920s.

Forgotten in the “cult of the groundwave” was the fact that not all of a station’s signal is groundwave – some of it does indeed travel “through the air”. Originally it was thought that these “skywave” signals merely fled into the cosmos, never to be heard again. However, soon there was evidence that something strange was happening, especially at night. Somehow, some of the signals were coming back to Earth at distant points. English physicist Oliver Heaviside did pioneering work on the subject, and found evidence that high above the Earth there is an encircling layer of charged particles. This was originally called the Heaviside Layer, but is now known as the ionosphere, and is the cause of the reflected signals. At first it was mainly viewed as a curiosity, responsible for “freak” reception. Unlike the groundwave signal, which is unaffected by the sun, and has the same strength day and night, the strength of the reflected skywave signal is variable, and usually was too weak to be readily detected by the primitive receivers then in use. Also, on the wavelengths then in use there normally wasn’t any skywave signal during daylight hours, so daytime reception was completely dependent on the groundwave signal. In fact the skywave signal was seen mainly as a nuisance, since it interacted with the groundwave signal, causing fading.

With the introduction of broadcasting, information about skywave signals suddenly became important. However, a full understanding of what was taking place did not exist in the early twenties. It was obvious the sun was involved, since in most cases skywave signals appeared only at night. Eventually it was determined that the ionosphere is composed of layers, each with distinctive characteristics. What became known as the “E” and “F” layers are responsible for reflecting radio signals back to Earth. (Unlike groundwave signals, the strength of reflected skywave signals are essentially the same across the entire 200 meter to 600 meter band.)

Due to the ionizing effect of the sun, these reflecting layers actually are more concentrated, thus more effective at reflecting radio signals, in daylight hours than at night. Therefore, in theory skywave signals should be even stronger during the daytime than at night. However, it turned out that a inner “D Layer” also existed. And the D Layer absorbs signals in the wavelengths that happened to be assigned for broadcasting, blocking them before they have a chance to reach the reflective outer layers. But unlike the E and F Layers, the D Layer only exists during daylight hours, which is why skywave signals disappear during the day but return at night. An analogy is that, when talking about the wavelengths assigned to broadcast stations, the E and F Layers act as a mirror reflecting signals back to Earth, while the D Layer is a curtain drawn in front of the mirroring layers during daylight hours.

In addition, KFI is a 50 kilowatt “clear channel” station, i.e. it is allowed to use high power with a limited number of other stations on the same frequency, in order to to protect its long-range nighttime coverage.

The higher frequencies used by FM and TV stations normally pass through the ionosphere into space day and night. However, occasionally there is enough solar radiation to strengthen the E layer of the ionosphere so that it reflects FM and the lower TV channel signals back to earth – this most commonly occurs around noon in the summer. Thus, I once heard a Maine FM station in North Carolina in the middle of the day. Also, temperature inversions in the troposphere can sometimes reflect the FM and TV frequencies.

[pldennison]

In addition to the already great explanations, there are also the facts that some AM stations are permitted to broadcast at higher wattages at night than they do during the day; and that other, small stations on the same or near frequencies may go dark, or stop broadcasting altogether, at night, eliminating another source of interference.

[Berkut]

*Originally posted by Derleth *
FM doesn’t skip. Which is to say, it can’t bounce from the ionosphere (an ionized part of the upper atmosphere) to the ground and back a few jillion times, which is how AM travels over the horizon.

FM does skip, the same as AM. The reason it isn’t usually associated with skip is the fact that it is used primarily for VHF and above.

[GaryM]

Under normal conditions lower frequencies skip and higher frequencies don’t. As Joey G pointed out, this happens regardless of the modulation method used. The point (frequency) where this occurs depends on atmospheric conditions. Generally skip is better at night when the reflective layers of the atmosphere are stable because the sun has set and they are not “stirred up” by solar radiation.

The 2 meter ham band is usually line of sight. But sometimes when conditions are right, the signals may be received hundreds of miles away.

[ftg]

[Back in my day …]

I am amazed that people don’t know that AM band radio used to be intended to be heard hundreds of miles away at night. Listen to the Blackhawk games in Florida. Pick up those giant Mexican stations in Canada. Etc.

I used to listen to stations 1500 miles away on a crystal radio. No tubes, no transistors, no power. NYC stations at 4am on a tube radio on the west coast.

But the consumer grade AM band radios made today (especially the digital ones) are worthless for such “DXing”. TVs that could pull in far off stations more or less went out with tubes.

There are times when TV/FM band stations “skip”. Meteor storms for example (ionization trails). Once got a South Dakota TV station on the west coast. (Saw “Sea Hunt” an hour or two early.) But with so many people on cable (or TVs programmed to skip “unused” channels) very few notice these anymore.

[/Back in my day …]

[Tedster]

With modern digital and satellite technology, the internet, and all that not much thought is put into AM receivers anymore. One of the best DX radios are old GM car radios from the 50’s, for some reason. Here in the Midwest I get a good selection of DX from New York, Boston, San Antonio, Salt Lake, Denver, etc. My el cheapo radio shack am/fm/sw won’t pick up any of them, and even the far away stations it will are unlistenable due to the noise floor. Vintage radios have superb audio for casual listening to ball games and stuff like that.

[manhattan]

OK, is anyone else sure that Cecil answered this at some point but can’t find it in the archives?

[spriteman11]

One time I was camping, and my Dad had packed along a pocket radio, camping in Washington, we were able to pick up AM radio from New York. It was pretty amazing, to say the least.

[nearwildheaven]

Do zombies still listen to AM radio?