There is a station I hear that promotes itself as a 50 KW FM, but the ERP on the FCC website is listed as 6.24 KW. I understand what the station puts out of their transmitter is based on the height of their antenna so in the end it is equivalent to a 50 KW station. Since there is no mention of the 50 KW on any FCC records for the station, it is a little confusing to me. I guess what I am asking is, how do you determine what a station is putting out. Is there a way to calculate by looking at the FCC website? If a station is saying they are 50 KW, what do you call that power? You don't call it their ERP because in this case that is 6.24 KW.
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Question about ERP
- Thread starter shreveville
- Start date
ERP is EFFECTIVE radiated power isn't it? So if lower power at "height above average terrain" is higher than the maximum allowed at 50 Kw, then is the signal the EFFECTIVE RADIATED POWER of 50 Kw?
No..."effective radiated power" has a very specific meaning, defined as (and here I'm oversimplifying a bit) transmitter power output, less transmission-line loss, multiplied by antenna gain. But the FCC sets maximum height above average terrain for each station class, and mandates that stations operating at heights exceeding that maximum must reduce ERP so that they don't cover any greater area than a station operating at class maximum for ERP and height.
If it's working, this link will lead you to an on-line calculator at the FCC site that will do the calculations:
http://transition.fcc.gov/mb/audio/bickel/fmpower.html
The phrase I think the original poster is looking for is "class-equivalent power" - presumably 6240 watts at the height the station in question uses would be equivalent to a standard class C2, 50 kW ERP at 150 meters.
If it's working, this link will lead you to an on-line calculator at the FCC site that will do the calculations:
http://transition.fcc.gov/mb/audio/bickel/fmpower.html
The phrase I think the original poster is looking for is "class-equivalent power" - presumably 6240 watts at the height the station in question uses would be equivalent to a standard class C2, 50 kW ERP at 150 meters.
Thanks Scott. Class-equivalent power. Now I have found a name. The link you suggested referred to it as, "Reference ERP."
Scott is exactly right on the money.
Stations operating above maximum height often use their class power instead of listing their actual power. The coverage is, theoretically, identical between the two. It's confusing to advertisers to have to explain why 6.2kW is actually the same coverage as 50kW. Some simply won't believe it.
In practice, the higher HAAT can actually be more effective in some situations, particularly where the area to be served is relatively vast and the structure density is low.
Stations operating above maximum height often use their class power instead of listing their actual power. The coverage is, theoretically, identical between the two. It's confusing to advertisers to have to explain why 6.2kW is actually the same coverage as 50kW. Some simply won't believe it.
In practice, the higher HAAT can actually be more effective in some situations, particularly where the area to be served is relatively vast and the structure density is low.
Within reason in MOST situations it's a better deal (maybe not in the mountains.. donno)
Kmagrill said:
The ERP of an FM station whose antenna radiation center exceeds the HAAT permitted for their Class must be reduced so that the distance to its 60 dB(µV/m) field intensity is the same as produced by a station using the maximum power for that class from the maximum HAAT defined for that Class and power.
But there are differences in the signal levels of the two installations.
The link below leads to a graphic showing that the signal strength from the system with reduced ERP is not quite as high from the antenna site out to about 5 miles (in this case) as one with full ERP. This can be an issue for building penetration where the antenna site is in the center of a major city.
But the mitigation for that is that an FM station with reduced ERP, and suitable radiation patterns in the vertical plane will produce less multipath distortion in analog FM receivers in that zone than a full-power station located away from the city center.
This was the case for the FMs on Sears (Willis) Tower in Chicago, who all use single-bay Harris CBR antennas having no nulls in their elevation patterns except at +/- 90 degrees.
In the graphic it can be seen that the close-in field of the 6-bay antenna is starting to drop at 1 mile, and between the antenna site and 1 mile it would contain 5 nulls, or zones where signals would be very low, and susceptible to multipath interference.
http://i62.photobucket.com/albums/h85/rfry-100/CoverageCompare.gif
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Fascinating comparison...it looks like (in this case) that there is no location where the higher antenna has an advantage but close in & far out the lower height has a clear advantage, but in the area of interest, it's pretty much a tie. Wonder if one of those studies exists for other heights such at 12.5KW @ 984' vs 50KW @ 492'? Thanks for sharing this!R. Fry said:
BobOnTheJob said:
When it comes to providing analog FM coverage with the least multipath zones/distortion in a forest of high-rise buildings in a major city, then an antenna with no nulls in the elevation pattern mounted as high as possible at the city center has the advantage -- even if that means reduced ERP.
In the comparison I posted, even the lower ERP produces fields ranging from 300 mV/m at 1 mile to 40 mV/m at 5 miles from the site -- very healthy signals. Probably more signal than that is not really necessary in that zone.
The antenna configurations would need to known for each of those to make the best comparison, but it will be similar to the graphic linked earlier.
The lower power as mentioned in the case of the "ahem" tall building in Chicago does have another result for fringe listeners.
More flutter and dropouts in the car.
Mobile reception of higher powered but lower height stations always seemed to be stronger to me and more solid in fringe
reception than those perched at the hundred-somethingth floor.
BUT, I'm only counting the listening area I grew up in, which is pretty much flat as a
pool table, with some slight rises like the dunes and morraine hills at the southern tip of Lake Michigan.
Throw in a few real hills or a ridge or meaningful elevation change and it don't think that would have been my experience
at all.
Where I live now I'm 7 miles from downtown, get multipathing from nearly continuous overhead jets on approach to
3 parallel runways. I'm not sure more or less power would change that.
Are there any FM antenna designs that only radiate in a hemispehere, and then mount facing downward?
In other words, is there any design that eliminates upward radiation, so it wouldn't be bouncing off aircraft
(nearly as much)?
More flutter and dropouts in the car.
Mobile reception of higher powered but lower height stations always seemed to be stronger to me and more solid in fringe
reception than those perched at the hundred-somethingth floor.
BUT, I'm only counting the listening area I grew up in, which is pretty much flat as a
pool table, with some slight rises like the dunes and morraine hills at the southern tip of Lake Michigan.
Throw in a few real hills or a ridge or meaningful elevation change and it don't think that would have been my experience
at all.
Where I live now I'm 7 miles from downtown, get multipathing from nearly continuous overhead jets on approach to
3 parallel runways. I'm not sure more or less power would change that.
Are there any FM antenna designs that only radiate in a hemispehere, and then mount facing downward?
In other words, is there any design that eliminates upward radiation, so it wouldn't be bouncing off aircraft
(nearly as much)?
The aircraft really aren't "up" in relation to the broadcast antenna. Let's assume the antenna in Chicago is 1,500ft for the sake of the argument here. If the plane is 5 miles away from the antenna, flying at 2,500 ft on the runway approach, then it isn't very far above the "horizon" for the transmitting antenna. I haven't run the math, but it should only be a few degrees at most. The plane, being metal and rather cylindrical, will take that signal and spray it around, forcing a lot of it downward.
A 6-bay, 1-wave spaced transmit antenna is a common configuration for a Class B FM running 50 kW from 492 feet HAAT. The elevation gain of that antenna is down 3 dB at an angle of 4.25 degrees above and below above the horizontal.
So, if to reduce radiation above the horizontal plane, that antenna was installed with its radiation center 1,500 feet above level terrain and radiated a maximum ERP of 4.5 kW in the horizontal plane, it would radiate 2.25 kW toward an angle of +4.25 degrees. At a distance of 7 miles that radiation would cross an elevation about 4,500 feet above the earth. No doubt this is above the elevation used by aircraft in a landing approach for O'Hare Field.
Field strength at sites close to the earth at a distance of 7 miles N of the Chicago loop is reduced by obstruction loss. So a reflection from the metal surface of a plane from a line-of-sight, obstruction-free path radiated toward +4.25 degrees can be about as strong as the direct signal from the transmit antenna -- causing multipath distortion and signal "fluttering" as the plane flies over.
It isn't possible in the FM broadcast band to generate a radiation pattern having very low radiation at all angles above the horizontal, while still adequately serving receivers located near the earth. It could be possible with a lot of engineering to design an FM transmit antenna with a null centered on the horizontal plane (or some other elevation angle), but its ability to serve FM receivers near the ground at all distances would not be as good as a "conventional" FM antenna.
One way to reduce the airplane flutter is to use an analog FM receiver with a very good capture ratio such as a Blaupunkt "Digiceiver," which tends to reject reflected signals that are not almost identical in amplitude with the desired signal.
So, if to reduce radiation above the horizontal plane, that antenna was installed with its radiation center 1,500 feet above level terrain and radiated a maximum ERP of 4.5 kW in the horizontal plane, it would radiate 2.25 kW toward an angle of +4.25 degrees. At a distance of 7 miles that radiation would cross an elevation about 4,500 feet above the earth. No doubt this is above the elevation used by aircraft in a landing approach for O'Hare Field.
Field strength at sites close to the earth at a distance of 7 miles N of the Chicago loop is reduced by obstruction loss. So a reflection from the metal surface of a plane from a line-of-sight, obstruction-free path radiated toward +4.25 degrees can be about as strong as the direct signal from the transmit antenna -- causing multipath distortion and signal "fluttering" as the plane flies over.
It isn't possible in the FM broadcast band to generate a radiation pattern having very low radiation at all angles above the horizontal, while still adequately serving receivers located near the earth. It could be possible with a lot of engineering to design an FM transmit antenna with a null centered on the horizontal plane (or some other elevation angle), but its ability to serve FM receivers near the ground at all distances would not be as good as a "conventional" FM antenna.
One way to reduce the airplane flutter is to use an analog FM receiver with a very good capture ratio such as a Blaupunkt "Digiceiver," which tends to reject reflected signals that are not almost identical in amplitude with the desired signal.
N
necrat123
Guest
shreveville said:
It is not uncommon for radio stations to use their class maximum as an "advertising gimmick" when their actual ERP is much less.
In example:
"Hot Hits WXYZ-FM transmitting 50,000 watts from the top of the Anytown building in downtown Fort Wayne" when in fact "WXYZ" has a height reduced ERP of 15,000 watts. It's simply a promoting gimmick. And it's been done for years on FM stations.
I do like the WNCI legal with "transmitting 175,000 watts from the top of -- building, this is WNCI Columbus". (The name escapes me)
If memory serves me correctly, WBBQ-FM, in Augusta, used to plug their FM power as 137kW, for 100kW ERP in the horizontal plane and 37kW in the vertical plane.
It's only a gimmick if you run less than allowable. To say, for example, that a station is operating at a class equivalent powere of 50 Kw, when the ERP may be 14 kw from a higher elevation, is not gimmicky, but a valid comparison to the station down the street operating with a class equivalent running a class equivalent of 6 kw. And an accurate coverage map will reflect the difference.
On the other hand, find me listeners who decide to stick with a given station because of this comparison...
On the other hand, find me listeners who decide to stick with a given station because of this comparison...
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