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General engineering question: Why do ERP levels vary and what are the?

F

fordranger797

I'm curious as to why the limitations for power output for radio stations around North America varies so greatly depending on the market. I find it interesting that many of the Chicago FM stations broadcasting from the Willis tower only put out a few thousand watts (similar situation it seems for stations in New York on the Empire State Building), yet there are stations in Toronto broadcasting at 40kw from the CN tower (which is even taller). Is this due to the fact that other markets need to be protected, and thus lower powers need to be used?

To provide a better example, the difference between the Seattle, WA and Los Angeles market. In Seattle, there are stations broadcasting from a HAAT of 2,300 feet in the mountains and an ERP of 55kw, and yet a station in Los Angeles may have a HAAT of 3,000 (on Mount Wilson) but may have an ERP of only 8kw. Why such a steep decline in power output over x height?

In that same vein, would a broadcaster ever see it as more advantageous NOT to transmit from the highest point in a metropolitan area to allow for a higher ERP? I believe that I was told once that many radio stations in New York City did not broadcast from the original World Trade Center, as there was such a low power output that reception suffered.

Thanks for explaining
 
fordranger797 said:
I'm curious as to why the limitations for power output for radio stations around North America varies so greatly depending on the market. I find it interesting that many of the Chicago FM stations broadcasting from the Willis tower only put out a few thousand watts (similar situation it seems for stations in New York on the Empire State Building), yet there are stations in Toronto broadcasting at 40kw from the CN tower (which is even taller). Is this due to the fact that other markets need to be protected, and thus lower powers need to be used?
Click to expand...

First, let's compare US stations only. Canada, which populates its band less densely than the US tends to have fewer stations and fewer lower power stations.

For many years, the US had only two classes of station in any geographic zones. The areas that were most densely populated when the original Table of Assignments was made got Class A and Class B stations. 3 kw at 300 ft or 50 kw at 500 feet. The other zones got A and C classes, with C being 100 kw at 2000 feet.

(Later, the FCC would establish sub-classes like C0, C1, etc., and increase A's to 6 kw and some stations upgraded from A's to a higher class under one of the new sub-classes to protect other stations. Also, the Commonwealth of Puerto Rico has modified B's and A's at considerably greater height. )

A B in Chicago on the Willis or in NY on the ESB would be at around 1500 feet, so the equivalent of 50 kw at 500 feet means reducing the power to about somewhere in the vicinity of 5 kw. That gets the same theoretical coverage as a conforming Class B, but with the height allowing obstructions from other buildings and hills to be minimized.

To provide a better example, the difference between the Seattle, WA and Los Angeles market. In Seattle, there are stations broadcasting from a HAAT of 2,300 feet in the mountains and an ERP of 55kw, and yet a station in Los Angeles may have a HAAT of 3,000 (on Mount Wilson) but may have an ERP of only 8kw. Why such a steep decline in power output over x height?
Click to expand...

LA has some very high power non-conforming B's because they were licensed before the classes were established. This is called being "grandfathered" as the existing licenses prior to the class specifications were allowed to continue to operate. The power of 8 kw for KIIS is closer to a conforming B than most of the stations on Wilson, but still above the level.

In that same vein, would a broadcaster ever see it as more advantageous NOT to transmit from the highest point in a metropolitan area to allow for a higher ERP? I believe that I was told once that many radio stations in New York City did not broadcast from the original World Trade Center, as there was such a low power output that reception suffered.
Click to expand...

The reasons many NYC stations did not go to the WTC are two. First, existing protections like WCBS-FM with WBEB would not allow CBS-FM closer to Philly. Second, moving farther south did not pick up additional territory in the NYC metro, but put a station farther away from the areas in NY state to the north of the city and farther, too, form some NE New Jersey counties. The site is just not as good for market coverage.

There is a point at which height forces such a power reduction that the signal covers a lot, but does not penetrate buildings. Each engineer and consultant will have preferences, but in general there is point where too much height is not a good thing.

This is a very simplified explanation, but should give you an idea of the situation.
 
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fordranger797 said:
I'm curious as to why the limitations for power output for radio stations around North America varies so greatly depending on the market. I find it interesting that many of the Chicago FM stations broadcasting from the Willis tower only put out a few thousand watts (similar situation it seems for stations in New York on the Empire State Building), yet there are stations in Toronto broadcasting at 40kw from the CN tower (which is even taller). Is this due to the fact that other markets need to be protected, and thus lower powers need to be used?

To provide a better example, the difference between the Seattle, WA and Los Angeles market. In Seattle, there are stations broadcasting from a HAAT of 2,300 feet in the mountains and an ERP of 55kw, and yet a station in Los Angeles may have a HAAT of 3,000 (on Mount Wilson) but may have an ERP of only 8kw. Why such a steep decline in power output over x height?

In that same vein, would a broadcaster ever see it as more advantageous NOT to transmit from the highest point in a metropolitan area to allow for a higher ERP? I believe that I was told once that many radio stations in New York City did not broadcast from the original World Trade Center, as there was such a low power output that reception suffered.

Thanks for explaining
Click to expand...

Virtually every FM station in Los Angeles has its facilities "grandfathered" from before 1964. If they weren't, all primary LA stations would be Class B, 50KW @ 150 meters. The United States is divided into zones of B or C Class stations. Class B stations tend to be in heavily populated areas with other heavily populated areas nearby. Class C(100KW @ 600 meters)covers the rest of the country. As of the last 20 some years, these categories have been broken down further but everything is still based on being in one zone or the other. There are smaller stations, Class A that exist in both zones. If a station wants more antenna height, they must reduce power accordingly and there's a formula for that. (I just noticed that while I was typing, David answered the question. I'd like to know if it would be possible to move my response before his so I don't appear so stupid by comparison!) :)
 
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semoochie said:
Virtually every FM station in Los Angeles has its facilities "grandfathered" from before 1964. If they weren't, all primary LA stations would be Class B, 50KW @ 150 meters. The United States is divided into zones of B or C Class stations. Class B stations tend to be in heavily populated areas with other heavily populated areas nearby. Class C(100KW @ 600 meters)covers the rest of the country. As of the last 20 some years, these categories have been broken down further but everything is still based on being in one zone or the other. There are smaller stations, Class A that exist in both zones. If a station wants more antenna height, they must reduce power accordingly and there's a formula for that. (I just noticed that while I was typing, David answered the question. I'd like to know if it would be possible to move my response before his so I don't appear so stupid by comparison!) :)
Click to expand...

LOL.

You added a good point about the LA FMs. The FCC has a calculator at https://www.fcc.gov/media/radio/fmpower where you select the class of station and the antenna height (HAAT) and it gives the power. A conforming B at an average Mt Wilson location would have 690 watts!
 
DavidEduardo said:


First, let's compare US stations only. Canada, which populates its band less densely than the US tends to have fewer stations and fewer lower power stations.

For many years, the US had only two classes of station in any geographic zones. The areas that were most densely populated when the original Table of Assignments was made got Class A and Class B stations. 3 kw at 300 ft or 50 kw at 500 feet. The other zones got A and C classes, with C being 100 kw at 2000 feet.

(Later, the FCC would establish sub-classes like C0, C1, etc., and increase A's to 6 kw and some stations upgraded from A's to a higher class under one of the new sub-classes to protect other stations. Also, the Commonwealth of Puerto Rico has modified B's and A's at considerably greater height. )

A B in Chicago on the Willis or in NY on the ESB would be at around 1500 feet, so the equivalent of 50 kw at 500 feet means reducing the power to about somewhere in the vicinity of 5 kw. That gets the same theoretical coverage as a conforming Class B, but with the height allowing obstructions from other buildings and hills to be minimized.



LA has some very high power non-conforming B's because they were licensed before the classes were established. This is called being "grandfathered" as the existing licenses prior to the class specifications were allowed to continue to operate. The power of 8 kw for KIIS is closer to a conforming B than most of the stations on Wilson, but still above the level.



The reasons many NYC stations did not go to the WTC are two. First, existing protections like WCBS-FM with WBEB would not allow CBS-FM closer to Philly. Second, moving farther south did not pick up additional territory in the NYC metro, but put a station farther away from the areas in NY state to the north of the city and farther, too, form some NE New Jersey counties. The site is just not as good for market coverage.

There is a point at which height forces such a power reduction that the signal covers a lot, but does not penetrate buildings. Each engineer and consultant will have preferences, but in general there is point where too much height is not a good thing.

This is a very simplified explanation, but should give you an idea of the situation.
Click to expand...

Thank you, David. Extremely helpful. Just to clarify, the zones basically depend on how closely various markets are spaced, and what the population density is like in that region?

I was told that a large HAAT is the best possible scenario for putting out the best possible signal. However, if I understand what you are saying, it sounds like a massive HAAT is really not that effective in zones that only allow class A and B if a decent amount of power is also not being used. I also wonder if there are problems with "dead zones" in areas that are extremely close to the transmission site in city centres (such as Chicago or New York). Is this where vertical power output becomes important?
 
fordranger797 said:
Thank you, David. Extremely helpful. Just to clarify, the zones basically depend on how closely various markets are spaced, and what the population density is like in that region?
Click to expand...

§73.205 Zones.

For the purpose of allotments and assignments, the United States is divided into three zones as follows:

(a) Zone I consists of that portion of the United States located within the confines of the following lines drawn on the United States Albers Equal Area Projection Map (based on standard parallels 291/2° and 451/2°; North American datum): Beginning at the most easterly point on the State boundary line between North Carolina and Virginia; thence in a straight line to a point on the Virginia-West Virginia boundary line located at north latitude 37°49′ and west longitude 80°12′30″; thence westerly along the southern boundary lines of the States of West Virginia, Ohio, Indiana, and Illinois to a point at the junction of the Illinois, Kentucky, and Missouri State boundary lines; thence northerly along the western boundary line of the State of Illinois to a point at the junction of the Illinois, Iowa, and Wisconsin State boundary lines; thence easterly along the northern State boundary line of Illinois to the 90th meridian; thence north along this meridian to the 43.5° parallel; thence east along this parallel to the United States-Canada border; thence southerly and following that border until it again intersects the 43.5° parallel; thence east along this parallel to the 71st meridian; thence in a straight line to the intersection of the 69th meridian and the 45th parallel; thence east along the 45th parallel to the Atlantic Ocean. When any of the above lines pass through a city, the city shall be considered to be located in Zone I. (See Figure 1 of §73.699.)

(b) Zone I-A consists of Puerto Rico, the Virgin Islands and that portion of the State of California which is located south of the 40th parallel.

(c) Zone II consists of Alaska, Hawaii and the rest of the United States which is not located in either Zone I or Zone I-A.

I was told that a large HAAT is the best possible scenario for putting out the best possible signal. However, if I understand what you are saying, it sounds like a massive HAAT is really not that effective in zones that only allow class A and B if a decent amount of power is also not being used. I also wonder if there are problems with "dead zones" in areas that are extremely close to the transmission site in city centres (such as Chicago or New York). Is this where vertical power output becomes important?
Click to expand...

The shadow areas can be handled by beam tilt on the antenna.

An example of too much height and not enough power was KRCD in West Covina, part of the San Gabriel Valley area of the LA market. 570 watts at nearly 1000 feet. Covered a big area on the maps, but not well enough to penetrate homes and buildings. Station went to 6 kw at 100 meters, and tripled ratings with no change in programming.
 
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fordranger797 said:
Thank you, David. Extremely helpful. Just to clarify, the zones basically depend on how closely various markets are spaced, and what the population density is like in that region?

I was told that a large HAAT is the best possible scenario for putting out the best possible signal. However, if I understand what you are saying, it sounds like a massive HAAT is really not that effective in zones that only allow class A and B if a decent amount of power is also not being used. I also wonder if there are problems with "dead zones" in areas that are extremely close to the transmission site in city centres (such as Chicago or New York). Is this where vertical power output becomes important?
Click to expand...

Horizontal transmission works best with horizontal reception, such as a roof antenna. Vertical transmission is intended for vertical reception i.e. cars, for instance. Horizontal transmission in cars is not pretty! Having both is usually preferable.
 
DavidEduardo said:


§73.205 Zones.

For the purpose of allotments and assignments, the United States is divided into three zones as follows:

(a) Zone I consists of that portion of the United States located within the confines of the following lines drawn on the United States Albers Equal Area Projection Map (based on standard parallels 291/2° and 451/2°; North American datum): Beginning at the most easterly point on the State boundary line between North Carolina and Virginia; thence in a straight line to a point on the Virginia-West Virginia boundary line located at north latitude 37°49′ and west longitude 80°12′30″; thence westerly along the southern boundary lines of the States of West Virginia, Ohio, Indiana, and Illinois to a point at the junction of the Illinois, Kentucky, and Missouri State boundary lines; thence northerly along the western boundary line of the State of Illinois to a point at the junction of the Illinois, Iowa, and Wisconsin State boundary lines; thence easterly along the northern State boundary line of Illinois to the 90th meridian; thence north along this meridian to the 43.5° parallel; thence east along this parallel to the United States-Canada border; thence southerly and following that border until it again intersects the 43.5° parallel; thence east along this parallel to the 71st meridian; thence in a straight line to the intersection of the 69th meridian and the 45th parallel; thence east along the 45th parallel to the Atlantic Ocean. When any of the above lines pass through a city, the city shall be considered to be located in Zone I. (See Figure 1 of §73.699.)

(b) Zone I-A consists of Puerto Rico, the Virgin Islands and that portion of the State of California which is located south of the 40th parallel.

(c) Zone II consists of Alaska, Hawaii and the rest of the United States which is not located in either Zone I or Zone I-A.



The shadow areas can be handled by beam tilt on the antenna.

An example of too much height and not enough power was KRCD in West Covina, part of the San Gabriel Valley area of the LA market. 570 watts at nearly 1000 feet. Covered a big area on the maps, but not well enough to penetrate homes and buildings. Station went to 6 kw at 100 meters, and tripled ratings with no change in programming.
Click to expand...

It seems to me that another solution for KRCD would be to find a format aimed directly at motorists, whatever that is.
 
semoochie said:
It seems to me that another solution for KRCD would be to find a format aimed directly at motorists, whatever that is.
Click to expand...

The simulcast of KRCD/KRCV averages 3rd or 4th among all LA Spanish language stations, and was 8th overall in 12+ in the last book and it bills about what KSWD, KTWV and KYSR did in 2015 despite being a limited signal facility (I do not think it needs a new format).
 
semoochie said:
Horizontal transmission works best with horizontal reception, such as a roof antenna. Vertical transmission is intended for vertical reception i.e. cars, for instance. Horizontal transmission in cars is not pretty! Having both is usually preferable.
Click to expand...

For nearly a decade I ran a number of FMs in a very mountainous market with vertical polarization only. Much less multipath and very good fringe area reception.
 
DavidEduardo said:


For nearly a decade I ran a number of FMs in a very mountainous market with vertical polarization only. Much less multipath and very good fringe area reception.
Click to expand...

There are numerous stations running vertical only or very little horizontal power, often to avoid interference to other stations.
 
Great answers, thank you very much! I do have just one more quick question (for now...)

What is the difference between an antenna that uses multiple "bays" as opposed to "rods" that are mounted on the side of a pole? Are the various bays and rods used for different functions (i.e, sending out the vertical signal versus the horizontal signal, or even for directionality)? The station I work for has a tower with 4 or 5 bays on the tower, but i've seen other radio stations (mostly LP's) that just use a little rod on the side of a cellular tower. I'm curious as to how they function differently.
 
Look at the physical antenna. With the exception of some TV transmit antennas, Vertically oriented radiating elements are vertically polarized. Horizontally oriented are horizontally polarized. Antennas with some of each, may be circularly or elliptically polarized.

Stacked radiating elements, or "bays", increase the antenna gain (focus) toward the horizon, taking energy that would be sent equally up and down from the antenna and focusing that power outward. As an example; a typical 100kW E.R.P. FM would use a non-directional, circularly polarized full-wave FM antenna with 6 stacked elements. 30,000 watts into the antenna with 3:1 gain, would produce around 100kW to the horizon.

The size and spacing of the antenna elements depend on the wavelength for the channel or band the antenna covers. For example, an FM broadcast station operating at 100Mhz, one full wavelength is about ten feet, whereas a full wavelength at 5.8Ghz is 0.166 inches long.
 
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fordranger797 said:
The station I work for has a tower with 4 or 5 bays on the tower, but i've seen other radio stations (mostly LP's) that just use a little rod on the side of a cellular tower. I'm curious as to how they function differently.
Click to expand...

The one bay antennas are very inexpensive compared to a 5 bay.

The outlay for a larger antenna (and heftier tower to hold it up) makes more economic sense as you go up in ERP. You replace an upfront investment on the tower/antenna with a monthly return of a lower power bill.

Quick back of envelope math: 100kW station could run 100kW (+ transmission line loss and other efficiency factors!) out of transmitter into one bay antenna. You're looking at $500 a day in electric, over $150k a year, to run the transmitter. Use a larger antenna with a gain of 4.5, run 23kW out of transmitter, save $125k a year in electric.

Based on $0.15 per kWH, 80% efficiency.
 
Can anyone confirm whether new CPs for full power stations in the US are able to go vertical pol. only? I thought I'd heard a couple of years ago that you had to be horizontal only or have equal ERP in the H or the V planes. No new Vertical pol. operations were being authorized, IIRC. Which is a shame, really.

Circular polarization is better than being just horizontal or just vertical. Where I'm at, we have several translators all coming from the same hill top. The circular pol. did better with inside portable reception than the translator that was vertical only.

However, if one is planning an off-the-grid hillside installation for a class D FM where you still have the option for vertical only, then absolutely vertical is the way to go, with as much gain as is practical. Solar and or wind power requires a mighty big ($$$!) battery bank to get you through the night.
 
PTBoardOp94 said:
Quick back of envelope math: 100kW station could run 100kW (+ transmission line loss and other efficiency factors!) out of transmitter into one bay antenna. You're looking at $500 a day in electric, over $150k a year, to run the transmitter. Use a larger antenna with a gain of 4.5, run 23kW out of transmitter, save $125k a year in electric.

Based on $0.15 per kWH, 80% efficiency.
Click to expand...

I think I see what you're saying. Adding more antennas may cost more upfront, but in the long run will cost less to operate because less power is put through each one (but combines into a collective ERP)?
And that would be why the ERP level is typically much greater than the actual transmitter output, right?
 
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fordranger797 said:
I think I see what you're saying. Adding more antennas may cost more upfront, but in the long run will cost less to operate because less power is put through each one (but combines into a collective ERP)?
And that would be why the ERP level is typically much greater than the actual transmitter output, right?
Click to expand...

Sort of.

One bay is essentially a radiator that "sends" the signal on a full 180° arc from ground to straight up. That means that much of the power goes at upwards angles into the sky... like a normal light bulb.

When you add more elements, the beam of the signal changes from the light bulb to more of a spotlight. The more bays, the tighter the beam. In other words, it is the same amount of power, but aimed at the market area towards the horizon, with less low angle "undershoot" or high angle "star shooting".

In these cases, the elements are "stacked" vertically. The spacing between bays is also a consideration for gain and beam angle.

When you get over 6 bays in an antenna ("antenna" being the system, "bays" being the individual radiating elements of the antenna system) there begin to be artifacts which cause the signal beam to have nulls and spikes; the beam can be so narrow that significant areas are under- or over-shot. This is particularly noxious with some 10 to 12 bay antennas, but most stations do not use that many bays anymore.

Many engineers prefer low bay count. When a station is lower powered, like an A, two bays would be considered optimal, and perhaps four to six bays for higher powers. In installations I have worked on, I have stayed at 6 or under.

These considerations change when panel antenna systems (such as in common antenna systems used by many stations) are employed. Panels are arranged in a circle around a central point on a structure, instead of being stacked vertically and are subject to different radiation "beam" calculations.

This is very, very simplified.

Beam me up, Scotty.
 
The master FM antennæ in New York and Chi-town are probably one or two bays because much of the market is down.
On the other hand, the towers between Dallas and Fort Worth and between Miami and Fort Lauderdale are fairly remote and not on mountains, so theirs are a lot bigger.
(definitely for Miami except for size restrictions or height requirements, and Probably for Dallas)
 
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