I live at the south end of Miami-Dade county and I listen to an AM radio station based in Fort Lauderdale, Broward County – a fur piece away. During the day it comes in fine, but at night I can hardly get it at all. For FM stations, reception seems to be the same, day or night. Why is that?
For a reason that I’m too tired to investigate right now, many AM stations are required to cut their wattage at sundown.
This is why you can hear large stations (KOA Denver, WCCO Minneapolis, KMOX St. Louis, etc.) from a thousand miles away at night. No bandwith competition.
My Army radio training finally comes in handy.
AM waves during the day pass through the ionosphere, causing them to not travel as far, and so AM signals during the day that you can hear are ground waves - essentially, line-of-sight radio waves.
At night, the ionosphere doesn’t absorb AM signals, it reflects them, so they can travel an amazingly long way - they literally bounces off the sky, off the ground, off the sky again, off the ground, etc. Radio stations thus must reduce their signals or else they’ll interfere with one another.
AM propogation is much better at night than in the daytime because of the strengthening of the ionized layers high in the atmosphere. These reflect AM waves and allow long distance reception via the sky wave. Daytime reception is almost entirely, if not entirely, by ground wave.
So some stations are allowed run increased power in the daytime but required to reduce power at night to avoid interference between stations on the same frequency but a long distance apart.
Because the ionosphere recedes upward at night, signals can reflect much farther along the earth. So by design, AM waves will carry much farther at night than in daytime. For this reason, most stations are required to reduce their wattage and use directional antennas from sunset to sunrise, so that all the local stations in the country don’t start interfering with each other.
Basically, it has to do with the difference in the way the atmosphere reflects long-wave radio signals between day and night; radio signals can bounce off the ionosphere and travel many thousands of miles, whereas during the day this skybounce is almost nonexistent, thanks to the Sun.
This page has a good explanation of this phenomenon.
FM reception is restricted to line-of-sight between receiver and transmitter antennas. The received signal strength isn’t greatly affected by day or night propagation differences.
What does this mean, exactly?
Wikipedia now has a nice little article on the Heaviside layer.
That’s why there are “clear channel” AM stations. There are powerful AM stations that are the only ones to transmit at a designated frequency. No interference. They can be picked up from a thousand miles away at night. Stations that are not “clear channel” transmit at a frequency that is used by other AM stations. They have to cut their power at night to avoid interference.
There’s something called the “e-layer” that makes AM transmissions go farther at night. For that reason, I guess they make some stations reduce the wattage of their AM transmission. But I don’t know the policy. I do know that when I lived on the Caribbean coast of Colombia I could receive some AM stations from as far away as New Jersey and Cincinatti, but only at night.
Do you have a link to a list of these stations?
I don’t, but the FCC does.
The information above is accurate, however there may be another effect in play:
Medium wave signals do not reflect off the ionisphere at low angles during daylight, but they can have significant scatter at high angles. This can fill in coverage at over the horizon distances during daylight. Picture the way a torche lamp illuminates an entire room by reflecting light off the cieling. If you are recieving an AM station at 100-200 miles in daylight, this is likely the propigation mode you are exploiting.
At night, the ionsphere becomes more mirror-like, which can leave holes in the coverage at just-over-the-horizon distances.
While the transmitting antenna is optomized for groundwave transmission, it still has significant power at high angles…about half power at 45 deg. for example. The typical recieving antenna is located only a tiny fraction of a wavelength above ground (even in a third story apartment) so is well suited to high angle reception.
This is entirely possible. If this effect is what is causing the change in quality from day to night, the change should be gradual. It the change results from switching from high to low transmitter power the change will happen just as if someone flipped a switch. Which someone did.
I heard somewhere that radio waves travel further at night for some reason, so the stations have to use lower power.
Maybe the atmosphere is involved?
It was once, for about twenty minutes.
Here you go:
The “old-timers” from rural areas can tell you that their exposure to popular music was influenced by listening to clear channel AM stations late at night. It was the only way to get the good stuff that their local stations wouldn’t play.
At a time when I was driving a car with AM radio only, whenever I was on a long nighttime drive I found it entertaining to tune in stations from across the country. Can you still do that?
Immediately after the L.A. earthquake in 1994, there was no radio, TV, or electricity, but I was able to get the San Francisco affiliate of KNX on my car radio and find out what was going on, besides the obvious, of course.
Oh sure. There are still clear channel 50 KW AM stations all over. I’ve heard WHO Des Moines, IA from here on the desert 1359 great circle miles away (according to How Far Is It?).