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Different Power at Different Times of Day
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. |
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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. |
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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. |
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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). |
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. |
"Peter H." wrote in message ... 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. Very old examples would be WOI in Ames on 640, KVFD/KPOP/KGBS on 1020, KOB and WEW on 770, Kannapolis on 760, Lansing on 870, Oklahoma City on 890. 1100 in San Francisco. 1160 in Chicago. 660 in Dallas. 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. 650, 760, 830, 870, 1040, 1210 in Hawaii. 660, 700, 750 in Alaska. |
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