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.

The ionosphere is stratified, or in other words, has layers. It's the E-layer which,
in the course of the day, charged by sunshine, actually absorbs medium wave
frequencies...it's only when the wavelengths are shorter (shortwave) where the
E-layer reflects for daytime skywave propagation. The E-layer, when particularly
dense like during solar maxima, will even reflect VHF frequencies (normally it passes
them day and night). When that happens you'll hear old radio people call it
"sporadic-E", since it's so, well, sporadic in nature - there's really no predicting
when it will happen. But when it does, you'll sometimes find FM stations from 1000
and more miles away coming in on a car radio, so strong sometimes that it'll blow
over a local station which has only direct line-of-sight propagation to get to the
same receiver.

At night, without the sun to ionize the gases there, the E-layer "evaporates", and
then the F-layer comes into play. The F-layer reflects medium wave frequencies
pretty well, except when they're disturbed by a solar flare or other phenomena. The
F-layer actually has two parts (F-1 and F-2, naturally enough). A shortwave station
which will skip 1000 miles during the day will skip twice that or more at night,
since the F-2-layer is higher and is more in play during the night.
--
For direct replies, take out the contents between the hyphens. -Really!-


"Steve Sundberg" wrote in message ...
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.

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


An ideal non-directional radiator which is 180 degrees or shorter has no
radiation at 90 degrees to the horizontal plane.

The taller a radiator is, over 180 degrees and up to 225 degrees, the higher
the radiation is in the horizontal plane, and the less the radiation is in the
vertical plane except for 90 degrees, where the radiation increases above 180
degrees to reach a maximum at 360 degrees.

A 360 degree radiator which is center fed and has the base grounded through a
capacitance is called a Franklin, and is the most efficient radiator known.
This is essentially two colocated 180 degree radiators.

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.

On 15 Jan 2004 17:48:19 GMT, inch (Peter H.) wrote:

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.

Then I think most of us are using the popular term which meant there
was only one station on the frequency. Not even a daytimer shared it.

Rich

Then I think most of us are using the popular term which meant there was only
one station on the frequency. Not even a daytimer shared it.


U.S. Class I-A clears a 640-670, 700, 720, 750-780 820-840, 870-890,
1020-1040, 1100, 1120, 1160, 1180, 1200-1210

Canadian Class I-A clears: 540 (first), 690, 740, 860, 990, 1010, 1580

Mexican Class I-A clears: 540 (second), 730, 800, 900, 1050, 1220, 1570

I'm having trouble thinking of a single U.S., Canadian or Mexican Class I-A
clear channel which did not have at least one co-channel daytimer somewhere in
the U.S.

And, remember that stations in the U.S. territories (and former territories,
now states) were allowed to operate on U.S. Class I-A clears many decades
before "Rio" caused all clears to be completely broken down.