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?

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?

What is a table allocation system?


Tables are published for FM and TV, which allocate FM channels and TV channels
to specific cities.

Yes, FM has channels, starting with 200, although 201 is the lowest channel
actually allocated, while 200 is reserved for situations where 201 or higher
won't work.

The tables can be read on-line.

Modifications to the tables have been done from time to time.

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

All other channels are Regional channels.

If the expanded band is regional (these are local, I thought...) then there is
52.


Expanded band stations are Class B, for now, although I wouldn't put it past
some country to try to notify one of theirs as a Class A.


Again, the 60 clears are total for North & South America, or am I mistaken?


Total for this ITU region.


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).

Where would this listing be located on the web?


The FCC AM site.

List all stations on any clear channel (by specifying no call letters and
restricting the frequency range to any one specific clear channel) and observe
the class letter.

"A" is Class A, "B" is Class B, etcetera.

On a clear channel, you should find only As, Bs and Ds.

Central American stations will show up, too, and some of these are notified as
described, but many are not.

My assumption is if the FCC database has no class letter, then the United
States has not "accepted" the station's notification, or the class is
indeterminate.

It is common for any change to be submitted to Canada and Mexico for
acceptance, particularly in border areas, and vice versa.

This is one reason why almost all of L.A.'s FM's are operating way above Part
73 power ... because they started out that way, and when it came time to
formalize those existing operations, Mexico was requested to accept those
operations, which Mexico did, thereby "grandfathering" those stations with very
high (most) or even super power (KPFK).