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What is the maximum dbu reading/measure from an active FM or AM transmitter site?

F

fordranger797

I’ve been curious about this for a little while now. I know about “city grade” reception, and the expected dbu levels for FM at various distances from the tower. With that being said, I’m curious what the “maximum” dbu measure could possibly be. This would be the “peak” of where reception maxes out.

Obviously most FM sites send most of the power off into the distance, so I doubt it would be very effective to go to the base of a site and take a reading. The highest reading I’ve ever seen is about 60, which is accurate for city grade reception (but I am sure there is still “maximum”). Is there a dbu level that is the confirmed “maximum”?

AM is a bit different. I understand it’s typically measured in m/v, but there is a conversion for dbu (but my radios with meters spit out a reading in dbu). I live pretty close to a lot of the AM sites in my current market. I see some pretty high numbers; usually around 70dbu. Last night I decided to take a quick drive by a 50kw site and see if I could figure out for myself what a “maximum” level for AM would be. Standing on the west side of the transmitter site, with all 50kw being thrown in my direction, I measured a peak of around 95dbu. I’m not sure if this is truly the “maximum” possible measurement, but it must have been close.

So is there a magic number that represents the “peak” or “maximum” of what is possible?
 
Is there a dbu level that is the confirmed “maximum”?
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No. In a purely mathematical sense, the maximum dBu you can observe is infinite, because of how the logarithm is defined. It is asymptotically approaching infinity, at 0 distance.

The FCC FM rules contemplate a 115 dBu blanketing contour. As I recall, certain stations that would be in each other's interfering contour cannot be constructed, although I don't recall the specifics. I think that is the highest field strength mentioned in the FM rules. But you're talking about a scale of a mile or two to the blanketing contour, obviously there would be a 120 dBu contour closer to the antenna. And when you find the 120 dBu contour, you can be assured that there is a 125 dBu contour at a point still closer.
 
PTBoardOp93 said:
No. In a purely mathematical sense, the maximum dBu you can observe is infinite, because of how the logarithm is defined. It is asymptotically approaching infinity, at 0 distance.

The FCC FM rules contemplate a 115 dBu blanketing contour. As I recall, certain stations that would be in each other's interfering contour cannot be constructed, although I don't recall the specifics. I think that is the highest field strength mentioned in the FM rules. But you're talking about a scale of a mile or two to the blanketing contour, obviously there would be a 120 dBu contour closer to the antenna. And when you find the 120 dBu contour, you can be assured that there is a 125 dBu contour at a point still closer.
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Thank you for sharing that! That’s an interesting scenario you described, as I’m on the west coast (where transmitter sites tend to be perched on mountains near population centers, but not IN the population center per se). That moves the receiver considerably further away from the antenna. I could see the scenario your describing be more applicable if you’re looking at a transmitter site that’s lower to the ground with the maximum ERP.
 
Let me try to explain how some of this works in practice.

Whenever I file an FM application with the FCC, I have to make at least one RF radiation study. For ANY FM or TV station, the FCC will only grant an application with a demonstration that the new signal won't cause the total level of RF radiation to exceed certain limits at ground level and within any area where the general public has access.

That's not measured in dBu, but rather in a more complicated equation that tells you whether the total contribution of all the stations at a site does or doesn't exceed limits set by OSHA and the FCC OET.

It's usually not a hard standard to meet, but there are certain circumstances with high power on short towers on mountains or rooftops, or tower farms with lots of TV stations, where the limits do come into play. Using a multiple bay FM antenna can often reduce RF at ground level to fix some of those issues.

For translators and LPFMs, there's often another study needed to show that there's no real-world interference to stations on second and third adjacent channels.

This one DOES use dBu, and here's how it works: let's say I'm putting a translator on 97.1 here in Rochester, where we have a class B local on 96.5, WCMF. It's located 2.7 km from where the translator is going.

Using the FCC's usual formulas, it turns out WCMF's field strength at the translator site is 112 dBu (that's very strong, by the way, though where I live, even closer to WCMF, I get something like 120 dBu).

It's really, really easy math from there: I need to show the FCC that there's nobody living or working in an area where my new signal will be more than 40 dB stronger than the adjacent station I'm protecting.

112 dBu + 40 equals 152 dBu, which is a very VERY strong signal. How far away is the 152 dBu contour from my 250-watt translator antenna? It turns out when I calculate it that it extends about 3 meters from the antenna. And since the antenna is on a locked roof where the public can't access it, I can easily show my translator won't interfere with WCMF.

(In the real world, I have taken a radio to the roof and I still get no interference to WCMF even right next to the 97.1 translator.)

If you think this through, it becomes obvious that it's easiest to show no interference when you're very close to the adjacent station, and harder when you're at the edge of the adjacent station's protected contour.

I've done translators that are very close to the 54 dBu contour of class B stations, which means I have to show there's no population within the translator's 94 dBu (54 dBu + 40) contour. A 94 dBu contour from a 250-watt translator can go out the better part of a mile if it's up high enough, so it can be hard sometimes to show there's no population in that radius.

Anyway, to answer the original question: if you're right next to a 100 kW (or bigger) FM, you might have a field strength that's up in the range of 150-160 dBu. But it falls off quickly as you move away from the antenna, of course.

It's unlikely in the real world that you'll experience more than about 125 dBu from an FM station anywhere that's not a restricted area around a tower.

Does that all make some sense?
 
fybush said:
Let me try to explain how some of this works in practice.

Whenever I file an FM application with the FCC, I have to make at least one RF radiation study. For ANY FM or TV station, the FCC will only grant an application with a demonstration that the new signal won't cause the total level of RF radiation to exceed certain limits at ground level and within any area where the general public has access.

That's not measured in dBu, but rather in a more complicated equation that tells you whether the total contribution of all the stations at a site does or doesn't exceed limits set by OSHA and the FCC OET.

It's usually not a hard standard to meet, but there are certain circumstances with high power on short towers on mountains or rooftops, or tower farms with lots of TV stations, where the limits do come into play. Using a multiple bay FM antenna can often reduce RF at ground level to fix some of those issues.

For translators and LPFMs, there's often another study needed to show that there's no real-world interference to stations on second and third adjacent channels.

This one DOES use dBu, and here's how it works: let's say I'm putting a translator on 97.1 here in Rochester, where we have a class B local on 96.5, WCMF. It's located 2.7 km from where the translator is going.

Using the FCC's usual formulas, it turns out WCMF's field strength at the translator site is 112 dBu (that's very strong, by the way, though where I live, even closer to WCMF, I get something like 120 dBu).

It's really, really easy math from there: I need to show the FCC that there's nobody living or working in an area where my new signal will be more than 40 dB stronger than the adjacent station I'm protecting.

112 dBu + 40 equals 152 dBu, which is a very VERY strong signal. How far away is the 152 dBu contour from my 250-watt translator antenna? It turns out when I calculate it that it extends about 3 meters from the antenna. And since the antenna is on a locked roof where the public can't access it, I can easily show my translator won't interfere with WCMF.

(In the real world, I have taken a radio to the roof and I still get no interference to WCMF even right next to the 97.1 translator.)

If you think this through, it becomes obvious that it's easiest to show no interference when you're very close to the adjacent station, and harder when you're at the edge of the adjacent station's protected contour.

I've done translators that are very close to the 54 dBu contour of class B stations, which means I have to show there's no population within the translator's 94 dBu (54 dBu + 40) contour. A 94 dBu contour from a 250-watt translator can go out the better part of a mile if it's up high enough, so it can be hard sometimes to show there's no population in that radius.

Anyway, to answer the original question: if you're right next to a 100 kW (or bigger) FM, you might have a field strength that's up in the range of 150-160 dBu. But it falls off quickly as you move away from the antenna, of course.

It's unlikely in the real world that you'll experience more than about 125 dBu from an FM station anywhere that's not a restricted area around a tower.

Does that all make some sense?
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Absolutely, that explanation does indeed shed some light on this topic. You definitely provided some helpful insight on how FCC licensing works, as this is something that I have no experience with.

I am definitely curious how about how the equation to calculate “contribution” of various stations utilizing a tower works. I was always under the impression that you could place as many antennas as you want on a tower, as long as there’s sufficient space. However, it sounds like this isn’t necessarily the case (if there are specific power restrictions that need to be observed). Perhaps this explains why Mount Wilson has so many different towers that are placed at the summit (to ensure that no one tower exceeds the limit). It’s the same situation in my home market (Seattle), where both Cougar and Tiger mountain have multiple towers at the summit and equipment spread across the various towers (I tip my hat to your photo collection of those sites, by the way).

FM is a bit of a unique beast, as someone is not likely to be listening to a radio station right next to the antenna beam (where you’d likely see those huge numbers). There would be some separation where the power would be a bit less concentrated.
 
Another reason you just cant add as many antennas as you want to a tower, even if there is space. You also have to take into account the loading of a tower. Wind loading and physical strength of the tower when adding antennas like multi bay FM antennas.
 
kevtronics said:
What would be the most broadcasting-RF-saturated part of the USA? The NJ Meadowlands? L.A. near Mt. Wilson?
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There is an amount of distance between most AM sites (unless duplexed or triplexed) while you can have many FMs on a common antenna or in a very close zone.

Mt Wilson, South Mountain in PHX, the area with nearly all the FMs and TVs in Miami on the Dade-Broward line, even the peak in ABQ all come to mind, but the expert on antennas is Mr. Fybush!
 
I don't have any really hard data right now to go on, but I suspect the very highest field strengths would be at one of the mountaintop sites David mentioned.

I know that there were issues in the analog TV era on Mount Wilson with "hot spots" that were on publicly accessible roads or driveways where the total RF exceeded the acceptable limits. And I have read RF studies from Sandia Crest above Albuquerque where there are definitely spots that exceed public limits. There's a gate at the entrance to the Sandia road that prevents public access to the line of towers, partially for that reason.

It's less of an issue at most combined sites because they're either on tall buildings or tall towers, and the RF levels at the ground 1000 feet or more down are therefore reduced and safe.

Responding to another point, FM and TV combiners are expensive to build and install, and so you use them when tower space is limited (like at Empire or in Chicago). When you have land to put up multiple towers and antennas, it's usually cheaper to do it that way than to combine, which is why you have the tower farms on Wilson, South Mountain in Phoenix, Sandia and so on.
 
fybush said:
Using the FCC's usual formulas, it turns out WCMF's field strength at the translator site is 112 dBu (that's very strong, by the way, though where I live, even closer to WCMF, I get something like 120 dBu).
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i dont even get that under or near the KSKO antenna... i think the most ive gotten is about 75-80 dbuv
 
fybush said:
I know that there were issues in the analog TV era on Mount Wilson with "hot spots" that were on publicly accessible roads or driveways where the total RF exceeded the acceptable limits.
Click to expand...

What are the acceptable limits anyway?

fybush said:
It's less of an issue at most combined sites because they're either on tall buildings or tall towers, and the RF levels at the ground 1000 feet or more down are therefore reduced and safe.
Click to expand...

What about the RF level where it's not 1,000 feet down? For example, for workers on the top floor of an office building with a 50kw or 100kw ERP FM stick on the roof?

fybush said:
Responding to another point, FM and TV combiners are expensive to build and install, and so you use them when tower space is limited (like at Empire or in Chicago).
Click to expand...

Good example of the question above. Another example is the former CKOI-FM transmitter, which was over 300kw ERP on top of the CIBC office building in downtown Montreal.

Also, what are the acceptable limits for AM? For example, I believe WABC's 50kw tower stands less than 700 ft from some nearby homes. WWKB stands pretty close to some people's backyards too, and that example is directional, plus another 5,000 watts from WGR at the same site. I've always wondered about this.
 
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Theater of My Mind said:
What are the acceptable limits anyway?
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For the general public, on the VHF band (FM and TV) the limit is 200 microwatts per square centimeter. There is a higher limit for trained persons in a restricted area.

Given on page 65 of this document:

Theater of My Mind said:
What about the RF level where it's not 1,000 feet down? For example, for workers on the top floor of an office building with a 50kw or 100kw ERP FM stick on the roof?
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The FCC does require that licensed broadcast stations do not create too high a level of RF radiation for public exposure, which would include employees in an office building. For example, KREV (now KEXC) in San Francisco broadcast from the top of a building (until recently moving to Candlestick Point after a change in ownership). Their application with the FCC indicated that their Class A FM signal, and other RF antennas on the same rooftop, would reach 18% of the above mentioned limit on the roof. Indoors would be less, simply because the building provides some attenuation, and being on the top floor is by definition further from the antenna than the roof.
 


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