The clarity of local AM radio stations is less at night. Is there
some sort of law that radio stations have to cut their output when it
gets dark? I've noticed that it's the amount of light outside, not
the time of day. That is, the lack of clarity occurs whenever it gets
dark, not at a certain time each day. I always presumed the stations
were cutting power to save money, but then why would it be based on
light vs dark, rather than on time of day?

"Matt Beckwith" wrote in message
...
The clarity of local AM radio stations is less at night.

At night, nearly every AM in the US gets skywave intereference, due to the
nature of AM night propagation. Distant signals interfere with what are
unobstructed day signals, even if the station has the same power.

The only exceptions are clear channel stations, which get a coverage
increase due to skywave and being on realtively protected channels.

Is there
some sort of law that radio stations have to cut their output when it
gets dark?

Stations, to operate at night (nearly 35% don't) have to protect other
stations on the same frequency, sometimes as much as 1000 miles or more
away. Because of this, at night they may cut power or directionalize where
htey send it to avoid interference, or both.

I've noticed that it's the amount of light outside, not
the time of day. That is, the lack of clarity occurs whenever it gets
dark, not at a certain time each day.

The ionesphere makes AM signals skip form just before sunset to just after
sunrise. This is why many US stations are daytime only, since they could not
run at night without causing interference to others.

I always presumed the stations
were cutting power to save money, but then why would it be based on
light vs dark, rather than on time of day?

It's due to the laws of physics. Newer stations protect older ones. And many
can not even run at night.

David, it seems that you understand the answer to my question
beautifully. Unfortunately, I can't understand it. Would you mind
dropping the complexity of your answer a couple of notches and
re-iterating? Thanks. --Matt

"David Eduardo" wrote

At night, nearly every AM in the US gets skywave intereference, due to the
nature of AM night propagation. Distant signals interfere with what are
unobstructed day signals, even if the station has the same power.

Huh? Why only at night? Why unobstructed in the daytime only? What
is a skywave?

The only exceptions are clear channel stations, which get a coverage
increase due to skywave and being on realtively protected channels.

What is a clear channel station?

Stations, to operate at night (nearly 35% don't) have to protect other
stations on the same frequency, sometimes as much as 1000 miles or more
away. Because of this, at night they may cut power or directionalize where
htey send it to avoid interference, or both.

Why do they have to protect other stations? From what? And why only
at night?

The ionesphere makes AM signals skip form just before sunset to just after
sunrise. This is why many US stations are daytime only, since they could not
run at night without causing interference to others.

How does the ionosphere make AM signals skip, and why only at night?

It's due to the laws of physics. Newer stations protect older ones. And many
can not even run at night.

Why do newer stations protect older ones? Why can't they run at
night?

Why only at night? Why unobstructed in the daytime only?

Radio waves radiate more or less in all directions from an AM tower, meaning
that some of them radiate skyward.

The ionosphere, in the presence of solar radiation (i.e., during the day),
absorbs the radio waves. In the absence of solar radiation (i.e., during the
night), it becomes a reflector, bouncing the signals back to earth hundreds or
thousands of miles beyond the transmitting tower. This can create interference
with stations on the same or adjacent frequencies at night, where no such
interference would occur during the day. To avoid such interference, many
stations must either cut transmitter power at night, or employ directional
antennas...or sometimes both.

What is a skywave?

Radio waves reflected by the ionosphere under nighttime conditions.

What is a clear channel station?

It's now an obsolete term, but "clear channels" were AM frequencies with only
one station on them. What are now referred to as "clear channel" stations (not
to be confused with Clear Channel Communications, the group station owner) are
for the most part the 50-kW AM stations using non-directional antennas, many of
which have been around since AM radio's early days of the 1920's.

Why do they have to protect other stations?

The AM band is organized as "first come, first served." Stations that were
broadcasting first get priority, and are protected from interference by
stations that came later, through the use of lower power or directional
antennas.

why only at night?

See above, re the changes in the ionosphere depending on whether or not the sun
is up.

Why do newer stations protect older ones?

FCC rules, plus international treaties to which the United States is a
signatory.

ospam (Sid Schweiger) wrote

Radio waves radiate more or less in all directions from an AM tower, meaning
that some of them radiate skyward.

The ionosphere, in the presence of solar radiation (i.e., during the day),
absorbs the radio waves. In the absence of solar radiation (i.e., during the
night), it becomes a reflector, bouncing the signals back to earth hundreds or
thousands of miles beyond the transmitting tower. This can create interference
with stations on the same or adjacent frequencies at night, where no such
interference would occur during the day. To avoid such interference, many
stations must either cut transmitter power at night, or employ directional
antennas...or sometimes both.

Thanks, that's very interesting. Now, why is it that the ionosphere
absorbs radio waves in the presence of solar radiation, but not in its
absence?

On 14 Jan 2004 22:18:49 GMT, (Matt Beckwith)
wrote:

(Sid Schweiger) wrote

Radio waves radiate more or less in all directions from an AM tower, meaning
that some of them radiate skyward.

The ionosphere, in the presence of solar radiation (i.e., during the day),
absorbs the radio waves. In the absence of solar radiation (i.e., during the
night), it becomes a reflector, bouncing the signals back to earth hundreds or
thousands of miles beyond the transmitting tower. This can create interference
with stations on the same or adjacent frequencies at night, where no such
interference would occur during the day. To avoid such interference, many
stations must either cut transmitter power at night, or employ directional
antennas...or sometimes both.

Thanks, that's very interesting. Now, why is it that the ionosphere
absorbs radio waves in the presence of solar radiation, but not in its
absence?

That's just the nature of, uh, nature.

Sid Schweiger ) writes:

What is a clear channel station?

It's now an obsolete term, but "clear channels" were AM frequencies with only
one station on them. What are now referred to as "clear channel" stations (not
to be confused with Clear Channel Communications, the group station owner) are
for the most part the 50-kW AM stations using non-directional antennas, many of
which have been around since AM radio's early days of the 1920's.


Also known as single stick stations, as their antennas use only a single
tower and radiate omnidirectionally.

--
Craig Jackman - Audio Production and Sound Design
Multi-award winning Creative Production, Station Imaging, Comedy, Voices
"Pride is the attitude that separates excellence from mediocrity!"
Ottawa, Ontario, Canada

What is a clear channel station?

It's now an obsolete term ...


Hardly.

Clear channels a 540, 640-780, 800-900, 940, 990-1140, 1160-1220, 1500-1580.

Local channels a 1230-1240, 1340, 1400, 1450, 1490

All other channels are Regional channels.

It's clear that the most channels are clear channels.

The popular meaning of the term may have changed, but the legal definition
hasn't.

There is at least one Class I station (either Class I-A or Class I-B) on every
clear channel, and all such clears are clearly identified in the NARBA and Rio
treaties as to power and antenna efficiency (although for some Mexicans, the
antenna efficiency may be bogus).



Also known as single stick stations, as their antennas use only a single tower
and radiate omnidirectionally.


In general, Class I-As are single-stick.

But, there are as many Class I-Bs which are single stick as there are Class
I-As which are DA-1.

Class I-As which are or have operated DA-1: 660, 870, 1030, 1100, 1580.

Class I-Bs which operate ND: 680, 810, 850, 940, 1070, 1550


Also, there is a single stick Class II-B on a clear channel where the Class I-A
is DA-1: 1580

And, there is a Class I-A which protects a Class I-B: 1540, protecting KXEL,
which is a Class I-B.

Finally, there are Class I-Bs which protect Class II-Bs: 710 and 1510.

Why do newer stations protect older ones?


Because AM is a "demand allocation" system, not a "table allocation" system.

IOW, for AM, the first station on a frequency has absolute priority over every
subsequent station on the same and adjacent frequencies.

inch (Peter H.) wrote

Because AM is a "demand allocation" system, not a "table allocation" system.

IOW, for AM, the first station on a frequency has absolute priority over every
subsequent station on the same and adjacent frequencies.

What is a table allocation system?