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Reduced ERP from "Overheight" FM Stations

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Rich
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Reduced ERP from "Overheight" FM Stations

Post by Rich » Wed Jul 24, 2019 6:19 am

FM stations with antenna heights greater than the HAAT permitted for their Class of station must reduce their ERP to provide a radius to their 1 mV/m field contours equal to that of a station exactly meeting the maximum ERP and HAAT authorized for that Class.

As an illustration of this, below is a comparison of the FCC F50,50 coverage radius to the 1 mV/m field intensity contours for Class B FM stations on the Willis (Sears) Tower in Chicago and a 50 kW FM at 500 feet HAAT.

The reason for the parity of the two signals is the lower propagation loss resulting from better Fresnel zone clearance for the higher transmit antenna.

Image



CK-722
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Re: Reduced ERP from "Overheight" FM Stations

Post by CK-722 » Wed Jul 24, 2019 9:40 am

Yes, the signal improvement with height increase can be quite dramatic, even for a rimshot move in Class A going from 100 meters to 150 meters HAAT.

The FCC used to use a different ERP taper with HAAT calculation which was a linear log log graph with a slope of about 25 dB per log unit of HAAT. Then they went to the rounded reference contour distance equivalence, which is about 20 dB per log unit of HAAT. That's when a bunch of FMs increased their ERPs. WWJ-FM...WXYT-FM waited a long time before they increased from 12 kW to 15 kW ERP from about 893 feet HAAT. Before the calculation change, the Chicago FMs on the Sears/Willis Tower were only 3 kW as I recall.


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Re: Reduced ERP from "Overheight" FM Stations

Post by k8jd » Sat Jul 27, 2019 1:05 pm

Back in the late 60's I got all the FCC signal level curves and charts for Low antenna high power vs. High towers and low power. I wanted to see what reception would be at different distances and see if there was really parity .
I found that he low power and high antenna signal went a lot longer distances with low but receivable signals than the high power and low antenna did. The low antenna/high power fell off a lot faster as the distance increased.



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Re: Reduced ERP from "Overheight" FM Stations

Post by Radio Chili » Sat Jul 27, 2019 2:48 pm

When I built my Class A FM in Allegan back in 1990, I went for height. 600' HAAT and 860w ERP. Great coverage!



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Re: Reduced ERP from "Overheight" FM Stations

Post by k8jd » Sat Jul 27, 2019 7:37 pm

Not happy with 300 ft and 3000 W ? :D
Was that on 100.9 ? I see 3000 W and 328 ft HAAT listed !!
Was the station licensed to Allegan or just located there ?



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Re: Reduced ERP from "Overheight" FM Stations

Post by ftballfan » Sun Jul 28, 2019 12:39 pm

k8jd wrote:
Sat Jul 27, 2019 7:37 pm
Not happy with 300 ft and 3000 W ? :D
Was that on 100.9 ? I see 3000 W and 328 ft HAAT listed !!
Was the station licensed to Allegan or just located there ?
I think that was what is now WZUU before it moved into Kalamazoo proper



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Re: Reduced ERP from "Overheight" FM Stations

Post by CK-722 » Sun Jul 28, 2019 2:42 pm

It's a Mattawan station.

Directional antennas do strange things.

Theoretically, it puts a 70 dBu signal over all, or nearly all, of Kalamazoo. There's an area on the fccdata.org map which is unclear as to whether it's Kalamazoo or not, and that part is somewhat cut off. It's hard to find accurate city limit maps anymore, nor any database that completely shows annexed areas. Wikipedia is very incomplete about annexations. Annexation is apparently a very controversial topic.

Remember that there are only one or two azimuths of a DA that have the full measured maximum field ratio. All other directions are less than maximum, regardless of the licensed pattern envelope field ratios.
Last edited by CK-722 on Sun Jul 28, 2019 2:54 pm, edited 1 time in total.


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Re: Reduced ERP from "Overheight" FM Stations

Post by Rich » Mon Jul 29, 2019 8:18 am

CK-722 wrote:
Sun Jul 28, 2019 2:42 pm
Remember that there are only one or two azimuths of a DA that have the full measured maximum field ratio. All other directions are less than maximum, regardless of the licensed pattern envelope field ratios.
That is true.

Directional FMs are licensed for the maximum ERP they may radiate in the horizontal plane, and the maximum field(s) they may radiate in the protected azimuth(s).

Non-D FMs are licensed on the ERP of the RMS value of their azimuth radiation patterns (no matter how directional those radiation patterns really are when the antennas are mounted on their supporting structures).

It is possible, legal, and even likely that a non-D FM station is radiating more power toward some azimuth directions than is authorized by their station license.



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Re: Reduced ERP from "Overheight" FM Stations

Post by CK-722 » Mon Jul 29, 2019 8:47 am

Larry Langford, Jr. owns or owned WUBU 106.3 South Bend, which has a DA. Larry once compared FM DA proofing to "sausage making", in that you didn't want to see what goes on on the antenna range.

Also, remember that with nondirectional FM antennas, the ERP can be substantially less than the licensed ERP in some directions, as well as the other directions which exceed it. The ERP of a nondirectional is a type of "Average" ERP in all directions. Canada specifies Average ERP on domestic stations, while the international databases specify the Maximum ERP in any direction. This is often confusing in trying to figure out various records in the database. Unless you need to use the DA data for contour protection with border applications, often the maximum ERP as a nondirectional is used.

In some instances, if you have the resources to buy a complex panel directional antenna, you might get a pattern that is close to what you want in at least several directions, toward several nearby markets. In that case, it might be desirable to be directional. Every case is different.


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Rich
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Re: Reduced ERP from "Overheight" FM Stations

Post by Rich » Mon Jul 29, 2019 9:20 am

The radiation pattern on the left side of the image below shows an extreme example of the pattern distortion than can result from mounting a stock, "omnidirectional" FM antenna on the side of a large-faced tower.

Image



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Re: Reduced ERP from "Overheight" FM Stations

Post by Radio Chili » Tue Jul 30, 2019 2:39 am

k8jd wrote:
Sat Jul 27, 2019 7:37 pm
Not happy with 300 ft and 3000 W ? :D
Was that on 100.9 ? I see 3000 W and 328 ft HAAT listed !!
Was the station licensed to Allegan or just located there ?
Mine was on 92.3 and licensed to Allegan. At that time 100.9 was licensed to Otsego. I put up a tall stick on one of the big hills south of Hopkins. Original calls were WKGH. It eventually found it's way into Brinks hands after the second owner couldn't make any money with it through poor management and irresponsible spending. Brink asked FCC for the okay to move it to Mattawan in 1998 and bumped it up a channel to cover Kalamazoo better. In doing so, Brink had to move 100.9's COL to Allegan to replace taking my former station to Mattawan.

I went for the height with lower ERP arrangement under the mistaken belief that the nearby FCC monitoring station required lowered RF fields from nearby stations. Oh, well. It turned out to be a flamethrower class A from that hill! Even with only 860 watts.



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Re: Reduced ERP from "Overheight" FM Stations

Post by Radio Chili » Tue Jul 30, 2019 3:17 am

The first tower you see NE of Allegan marked 1360' is the one I put up in 1990 for 92.3.

http://vfrmap.com/



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Reduced ERP from "Overheight" FM Stations

Post by CK-722 » Tue Jul 30, 2019 3:26 am

Rich shows the three faced panel antenna licensed as nondirectional. It has a six sided rounded hexagonal pattern. By rotating those basket reflector bays individually, you might get a pattern you can "steer" the nulls and lobes individually, and control lobe size and null depth. Between computer modeling and antenna range experimentation, you might come up with some near ideal patterns for individual station requirements. The three faces can also be varied for power distribution and phase to meet specific station requirements.


Is THAT where they got the idea for the 486-SX?

Same (x, y, z), different (t)

Your bullet missed my trial balloon.

RTN Price. Not guaranteed. As of 12:30, 157.71 Down 0.22.

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Rich
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Re: Reduced ERP from "Overheight" FM Stations

Post by Rich » Tue Jul 30, 2019 6:54 am

Additional: The mounting environment of an FM transmit antenna has a very significant effect on its radiation patterns, as the tower structure and other re-radiators near the antenna aperture become part of the antenna system.

The image below shows these effects for one case by comparing the radiation pattern envelopes of a mounted FM transmit antenna (left side of image) to the free space radiation produced only by the elements, themselves.

Note that the radiation toward ±90° elevation is very significant from the mounted antenna, but (theoretically) is zero when the same antenna is isolated in free space.

These antennas can be predictably directionalized in the horizontal plane to meet FCC requirements by measuring its radiated patterns on a test range while adjusting the mounting location on the tower, the compass bearing of the antenna elements, selecting face vs. leg mounting, and adding parasitic radiators near each of its elements.

Image



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Re: Reduced ERP from "Overheight" FM Stations

Post by CK-722 » Tue Jul 30, 2019 10:52 am

WHNN has a pretty good pattern, considering the requirements. The tower is near Quanicassee. They wanted as large as possible major lobe toward Bay City and Midland (Licensed is ~100 kW) to the West, a shallow null to the SW (Licensed is ~31.6 kW, but toward Saginaw) to protect short spaced WMAX-FM 96.1 Holland, and as large as possible lobe toward Flint (Licensed is ~90 kW) and short spaced WDVD. As I recall, WHNN agreed with coowned Cumulus WDVD to have mutual maximum facilities at some point. I seems like they moved back to the M-15 tower for a while at some point, while they redesigned the directional antenna because they were unhappy with the pattern.

I don't know if any of you know more about the antenna system. It's a 12 Bay. I'll have to look to see if there is more information. It does a pretty good job from my experience. I don't know if the measured pattern could be better still.

https://www.fccdata.org/?facid=&call=WH ... d=&lang=en


Is THAT where they got the idea for the 486-SX?

Same (x, y, z), different (t)

Your bullet missed my trial balloon.

RTN Price. Not guaranteed. As of 12:30, 157.71 Down 0.22.

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