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.

"Matt Beckwith" wrote in message
...
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?


Here is something the FCC prepared about AM radio at night:

Why AM Radio Stations Must Reduce Power, Change Operations, or Cease
Operations at Night
Most AM radio stations are required by the FCC's rules to reduce their power
or cease operating at night in order to avoid interference to other AM
stations. Our rules governing the daytime and nighttime operation of AM
radio stations are a consequence of the laws of physics. Because of the way
in which the relatively long wavelengths (see Footnote 1) of AM radio
signals interact with the ionized layers of the ionosphere several miles
above the earth's surface, the propagation of AM radio waves changes
drastically from daytime to nighttime. This change in AM radio propagation
occurs at sunset due to radical shifts in the ionosphere layers, which
persist throughout the night. During daytime hours when ionospheric
reflection does not occur to any great degree, AM signals travel principally
by conduction over the surface of the earth. This is known as "groundwave"
propagation. Useful daytime AM service is generally limited to a radius of
no more than about 100 miles (162 km), even for the most powerful stations.
However, during nighttime hours the AM signals can travel over hundreds of
miles by reflection from the ionosphere, a phenomenon called "skywave"
propagation. (Shortwave stations, which operate using AM modulation on
several bands between between 2.3 MHz and 26.1 MHz, also use this phenomenon
to broadcast still greater distances, up to thousands of miles). Because of
this change in signal propagation from daytime to nighttime, if every AM
station kept its daytime operating power at night, massive interference
would result. (See also Hours of Daytime-Only AM Broadcast Stations, First
Report and Order, BC Docket 82-538, 95 FCC 2d 1032 (1983) for a similar
description.)

The FCC was established by the Communications Act of 1934, and was given the
responsibility of making a "fair, efficient and equitible distribution of
radio service" to the various communities of the United States. In order to
meet this responsibility, and in recognition of the physical laws which
govern AM radio propagation, the FCC established three basic classes of AM
stations (Clear Channel, Regional, and Local), each of which operates on
specified frequencies, or channels There have been some changes since then,
but the main features of the plan have remained in place. Here is a quick
summary of the three types of AM stations:


a.. Clear channels. On these channels, only one station, called the
"dominant" station, was permitted to operate during nighttime hours in order
to provide service to a wide area. Footnote 2. A typical clear channel
station, transmitting with power of 50 kilowatts into an omnidirectional
antenna, provides nighttime skywave service to an area approximately 750
miles in radius. Some examples of clear channel stations are WCBS in New
York, WGN in Chicago, WHO in Des Moines and KFI in Los Angeles.

b.. Regional channels. On these channels several stations are permitted to
operate during nighttime hours, protecting each others' nighttime service
areas by use of directional antennas. As the name implies, stations on
regional channels provide nighttime service to moderately-sized or
"regional" areas.

c.. Local channels. Essentially, all stations on local channels are
permitted to operate during nighttime hours with the same facilities as
employed during the daytime. The nighttime service of such stations is
limited to very small "local" areas by massive interference from the
numerous other stations on the channel.
For the most part, the basic structure of the FCC's original frequency plan
has remained unchanged over the past several decades. Additional stations
gradually have been permitted to operate on the clear channels. In order to
preserve the wide-area nighttime service provided by the dominant clear
channel stations, when nighttime operation is permitted, many of these these
stations are required to use directional antennas in order to protect the
dominant clear channel station from interference to its nighttime skywave
service area. Most of these stations are also required to reduce power at
night, to avoid causing interference to the dominant stations and to each
other. Other stations, which cannot afford the necessary protection at night
to other AM stations, are presently licensed to operate during the daytime
hours only. (New daytime-only stations are no longer being authorized, since
December 1, 1987.)

To find out what daytime and nighttime operations are authorized for your
favorite AM broadcast station, you may use our AM Query. The Hours of
Operation for this Record field for the station will indicate what hours the
station may operate with the listed station parameters, either as Unlimited
hours operation, Daytime, or Nighttime. If the record shows the "Hours of
Operation" as "Daytime" or "Nighttime", then you can find the approximate
monthly times that the station will change from day to night operation (or
cease operation for the night in the case of a daytime-only station) by
entering the coordinates (latitude and longitude) into the the Sunrise /
Sunset Times Calculations program. Note that a station may have multiple
records, for different daytime and nighttime operations.

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?


Power and/or pattern change is done to accommodate a station of lesser class
against a station of superior class; or a station of the same class, but one
which was authorized before the subject station.

AM is a "demand allocation" scheme, and the first station to be authorized is
usually the superior station, with respect to subsequently authorized stations.

Extreme example: 710 in L.A. and 1510 in Boston are protected by stations of
higher class (710 in Seattle and 1510 in Nashville) simply because these
stations were authorized before Seattle and Nashville were authorized. L.A. and
Boston remain Class B; Seattle and Nashville became Class A.

Power and/or pattern change is not always at local sunset or sunrise at the
affected station. It could be local sunset and/or sunrise at another station
(see 1110, 1530 and 1560 for examples). There are historical reasons for this.

So-called DA-3 operation is also possible, where a station has defined day and
night operating parameters, but during so-called critical hours it operates
differently, usually with parameters in between day and night, although not
always.

Matt Beckwith wrote:
The clarity of local AM radio stations is less at night.

This is because all of the other stations everywhere else in the world
are coming in through skip at night, and interfering with local stations.

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.

Many smaller stations are forced to reduce power at night, or go off the
air altogether, to protect stations on the same frequency in other parts
of the country.

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?

Because the ionosphere is denser and closer to the earth at night, which
is why you can get transatlantic AM stations at night and not in the daytime.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article , "David Eduardo"
writes:

Useful daytime AM service is generally limited to a radius of
no more than about 100 miles (162 km), even for the most powerful stations.

The FCC apparently didn't take areas with high soil conductivity into
consideration when they wrote this.

Here in South Dakota, strong AM daytime signals propagate for 200 or more
miles. KFYR, WNAX, KSDN, KFGO, KSJB, etc. are all examples of this. And these
stations are only putting out 5,000 watts. Over in the Black Hills, KBHP runs
25,000 watts on 810 and can easily be heard up and down I-29 on the opposite
side of the state. That's 300-350 miles from the transmitter.

But that's okay. We're used to being ignored by the bureaucrats in Washington,
DC :-)

"SouthDakotaRadio" wrote in message
...
In article , "David Eduardo"

writes:

Useful daytime AM service is generally limited to a radius of
no more than about 100 miles (162 km), even for the most powerful
stations.

The FCC apparently didn't take areas with high soil conductivity into
consideration when they wrote this.

It's David who is oversimplifying. That statement couldn't have come from
anyone in the broadcast allocation part of the FCC.

The FCC rules definitely require consideration of ground conductivity. The
old printed rules contain a conductivity map and propagation curves for each
AM frequency for each conductivity. THAT is the basis upon which AM
stations are allocated and power and directional antennas specified.

Here in South Dakota, strong AM daytime signals propagate for 200 or more
miles. KFYR, WNAX, KSDN, KFGO, KSJB, etc. are all examples of this. And
these
stations are only putting out 5,000 watts. Over in the Black Hills, KBHP
runs
25,000 watts on 810 and can easily be heard up and down I-29 on the
opposite
side of the state. That's 300-350 miles from the transmitter.

But that's okay. We're used to being ignored by the bureaucrats in
Washington,
DC :-)

Your complaint is entirely unjustified.

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.

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.