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  #1  
Old 10-10-2006, 08:44 PM
BrainGlutton BrainGlutton is online now
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Why is AM radio reception worse at night?

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?
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  #2  
Old 10-10-2006, 08:48 PM
Lamar Mundane Lamar Mundane is offline
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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.
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  #3  
Old 10-10-2006, 08:54 PM
RickJay RickJay is offline
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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.
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  #4  
Old 10-10-2006, 08:55 PM
David Simmons David Simmons is offline
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Quote:
Originally Posted by Lamar Mundane
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.
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.
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  #5  
Old 10-10-2006, 08:57 PM
HMS Irruncible HMS Irruncible is offline
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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.
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  #6  
Old 10-10-2006, 08:58 PM
Q.E.D. Q.E.D. is offline
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Quote:
Originally Posted by Lamar Mundane
For a reason that I'm too tired to investigate right now, many AM stations are required to cut their wattage at sundown.
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.
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  #7  
Old 10-10-2006, 08:59 PM
David Simmons David Simmons is offline
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Quote:
Originally Posted by BrainGlutton
For FM stations, reception seems to be the same, day or night. Why is that?
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.
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  #8  
Old 10-10-2006, 09:08 PM
Q.E.D. Q.E.D. is offline
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Quote:
Originally Posted by Brain Wreck
Because the ionosphere recedes upward at night...
What does this mean, exactly?
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  #9  
Old 10-10-2006, 09:14 PM
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Quote:
Originally Posted by David Simmons
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.
Wikipedia now has a nice little article on the Heaviside layer.
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  #10  
Old 10-10-2006, 09:17 PM
R. P. McMurphy R. P. McMurphy is offline
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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.
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  #11  
Old 10-10-2006, 09:27 PM
guizot guizot is online now
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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.
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  #12  
Old 10-11-2006, 07:35 AM
Bosda Di'Chi of Tricor Bosda Di'Chi of Tricor is offline
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Quote:
Originally Posted by Spartydog
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.

Do you have a link to a list of these stations?
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  #13  
Old 10-11-2006, 10:34 AM
Q.E.D. Q.E.D. is offline
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Quote:
Originally Posted by Bosda Di'Chi of Tricor
Do you have a link to a list of these stations?
I don't, but the FCC does.
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  #14  
Old 10-11-2006, 11:04 AM
Kevbo Kevbo is offline
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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.

More Info

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.
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  #15  
Old 10-11-2006, 03:13 PM
David Simmons David Simmons is offline
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Quote:
Originally Posted by Kevbo
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.

More Info

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.
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  #16  
Old 10-11-2006, 06:05 PM
iamthewalrus(:3= iamthewalrus(:3= is offline
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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?
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  #17  
Old 10-11-2006, 06:31 PM
Squink Squink is offline
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Quote:
Originally Posted by iamthewalrus(:3=
Maybe the atmosphere is involved?
It was once, for about twenty minutes.
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  #18  
Old 10-11-2006, 06:36 PM
R. P. McMurphy R. P. McMurphy is offline
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Quote:
Originally Posted by Bosda Di'Chi of Tricor
Do you have a link to a list of these stations?
Here you go:

AM clear channel stations

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.
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  #19  
Old 10-11-2006, 06:48 PM
Spectre of Pithecanthropus Spectre of Pithecanthropus is offline
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Quote:
Originally Posted by David Simmons
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.

.
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.
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  #20  
Old 10-11-2006, 08:01 PM
David Simmons David Simmons is offline
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Quote:
Originally Posted by Spectre of Pithecanthropus
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?).
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  #21  
Old 10-11-2006, 08:10 PM
Bearflag70 Bearflag70 is offline
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Night air bouncy-bouncy, so they turn it down.
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  #22  
Old 10-11-2006, 08:21 PM
kunilou kunilou is offline
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Directional patterns play a part, as well.

By using multiple antennas, it's possible for AM stations to direct their signals in different patterns. Because of the potential for interference, stations may have to change their directional patterns at night.

Switching from daytime to nighttime pattern would also cause the signal to change, as David Simmons described it, at the flip of a switch.
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  #23  
Old 10-11-2006, 10:02 PM
Chronos Chronos is offline
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No, you're all wrong. You can't pick up AM stations at night, because that's PM.

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  #24  
Old 10-13-2006, 06:33 AM
Common Tater Common Tater is offline
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AM band is still a useful medium, or would be, if it weren't for the thousands of electronic devices that interfere with good reception. Light dimmers, computer power supplies, flourescent lights, monitors, etc. ruin decent reception. Lack of decent programming, i.e. music has relegated AM to a sort of backwater, and the rise of pay-services and currently advertisement-free XM and Sirius seals the deal. But it's still cool to listen to WSM on a big 1930's console..
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  #25  
Old 10-13-2006, 07:49 AM
Bosda Di'Chi of Tricor Bosda Di'Chi of Tricor is offline
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Quote:
Originally Posted by Common Tater
AM band is still a useful medium, or would be, if it weren't for the thousands of electronic devices that interfere with good reception. Light dimmers, computer power supplies, flourescent lights, monitors, etc. ruin decent reception. Lack of decent programming, i.e. music has relegated AM to a sort of backwater, and the rise of pay-services and currently advertisement-free XM and Sirius seals the deal. But it's still cool to listen to WSM on a big 1930's console..
Or a moderm replica console, like the one I've got.

LIKE THIS ONE! THAT'S MY REVIEW, BTW!

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  #26  
Old 10-13-2006, 10:01 AM
Common Tater Common Tater is offline
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egads, I would not want a replica! Pre-war consoles are plentiful and cheap, though they need a fair amount of work to overhaul. Definitely better performance and sound quality though.
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  #27  
Old 10-13-2006, 10:12 AM
David Simmons David Simmons is offline
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Quote:
Originally Posted by Common Tater
Pre-war consoles are plentiful and cheap, though they need a fair amount of work to overhaul. Definitely better performance and sound quality though.
But where do you get a 6L6 vacuum tube when you need one?
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  #28  
Old 10-13-2006, 11:28 AM
CJJ* CJJ* is offline
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Quote:
Originally Posted by kunilou
By using multiple antennas, it's possible for AM stations to direct their signals in different patterns. Because of the potential for interference, stations may have to change their directional patterns at night.
Most non-clear channel AM stations do have separate antenna patterns for day and night operation. One typically sets a pattern that "pulls a null" in the direction of another established station on the same frequency. I knew of a station in Toledo, OH that had a nighttime pattern that pulled a null in the direction of Pensacola, FL without altering power; one problem with this was that there was a fairly well-defined poor reception line going SSW of the city; you could be driving on a road just south of the city that crossed this line and here a significant drop in signal at night over a stretch of, say, less than a mile.

The FCC takes antenna patterns seriously. You are required once a month to go to designated points in your broadcast area and make daytime field-strength measurements for each pattern (you are allowed to switch to the nighttime pattern during the day to make these measurements). If another station lodges a complaint, you must produce these records.

Skywave is the reason for the change in signal, and as many posters note this is because of ionosphere changes between day and night. Essentially, the ionized E-layer high in the atmosphere is always ready to act as a mirror for signals in the AM frequencey range, but in the daytime sunlight ionizes an additional D-layer much lower in the atmosphere, effectively blocking skywave.
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  #29  
Old 10-13-2006, 11:48 AM
Barbarian Barbarian is offline
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Originally Posted by David Simmons
But where do you get a 6L6 vacuum tube when you need one?
Russia. Seriously
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  #30  
Old 10-15-2006, 12:17 PM
Common Tater Common Tater is offline
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Strangely enough, vacuum tubes are the easiest part of the equation. The hobby (demand) is small, and the number of tubes manufactured was staggering. Russia (and others) do make some of the more expensive audio output tubes that are becoming scarce or expensive but they aren't generally a factor for radios.
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  #31  
Old 10-15-2006, 03:38 PM
danceswithcats danceswithcats is offline
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Quote:
Originally Posted by David Simmons
But where do you get a 6L6 vacuum tube when you need one?
My Dad likely has some in the basement. He has a few hundred tubes from the minis back through to loctal base.
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  #32  
Old 10-15-2006, 03:45 PM
David Simmons David Simmons is offline
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Quote:
Originally Posted by Common Tater
Strangely enough, vacuum tubes are the easiest part of the equation. The hobby (demand) is small, and the number of tubes manufactured was staggering. Russia (and others) do make some of the more expensive audio output tubes that are becoming scarce or expensive but they aren't generally a factor for radios.
The 6L6 is a beam power audio amplifier and might be used in a consol radio. Although I suppose the same thing in a lower power version like the 6V6 might be more likely.
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  #33  
Old 10-15-2006, 04:36 PM
Common Tater Common Tater is offline
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6L6 is used in few radios, no doubt. The problem is that certain audio gurus have latched onto certain ancient tubes as having superior sound for their homebrew stereos, particularly in Asia. Tube types 45 and 50 in particular. They were used as final audio outputs in some sets in push-pull, so restoring these sets can get kind of pricey. This is where Russia and other tube manufacturers come in, the EL34, 6L6, 6550 types are made today. But it is very unlikely that the other radio signal tubes will ever be manufactured again as there is little demand, so what is left is what we got.
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  #34  
Old 10-15-2006, 07:04 PM
David Simmons David Simmons is offline
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Quote:
Originally Posted by Common Tater
The problem is that certain audio gurus have latched onto certain ancient tubes as having superior sound for their homebrew stereos, particularly in Asia.
Aficianados can talk themselves into almost anything.
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