Service Contours on FCC site

[rdalton]

Hello!

I was wondering why, on the FCC's website, when you go to look at an FM station's 60 dBu service contour, a non-directional pattern changes when it hits water (such as an ocean or swamp). The coverage area gets larger.

The explainations say that terrain has no impact on these maps.

Thanks!

Richard<P ID="signature">______________
www.RichardJDalton.com</P>

 

[ironbear]

> The explainations say that terrain has no impact on these
> maps.

The terrain around water is considered, however, and it tends to be lower around it (down to sea level in the direction of ocean, for instance).

 

[Andy_Brown]

> Hello!
>
> I was wondering why, on the FCC's website, when you go to
> look at an FM station's 60 dBu service contour, a
> non-directional pattern changes when it hits water (such as
> an ocean or swamp). The coverage area gets larger.
>
> The explainations say that terrain has no impact on these
> maps.
>
> Thanks!
>
> Richard
>

Electromagnetic ground waves have better propagation over sea or brackish water
because the ground conductivity of salt water is better than anything else. The better the conductivity, the less the attenuation. This is what results in the 60 dBu contour pushing out in a non d pattern.

<P ID="signature">______________
Electricity is really just organized lightning.
~George Carlin</P>

 

[radiofish]

It could be several reasons but probably just the data technique. The FCC's FM propagation models DO NOT consider effects of conductivity.

1) along a radial, the FCC model requires input of HAAT (height above average terrain) and ERP (effective radiated power) only. The HAAT is calculated by sampling terrain elevation at points along each radial from 3km out to 16km. This gives an average terrain height and the antennas center of radiation (height above ground level) is used to calculate HAAT for that radial. That said, the HAAT and ERP go into the model and you get distance to the countour as output. No effects of conductivity or anything else go into it, though in reality they would have some effect.

2) the FCC's rules have instructions on excluding radials that go over water, so some radials are not used in some cases.

3) some of the 30 arc second and 3 arc second terrain databases used do not have data for over water. With no data, the FCC model can't run.

The bottom line is that you can't trust model output when you don't know how the programmer has set it up. It becomes a "black box" and you are at the model's mercy to trust its accuracy.

For accurate effects of conductivity and terrain shadowing, use the Longley Rice methods for more real results. The "FCC method" models are mainly useful for FCC regulatory and spacing purposes, not for real world coverages.



> Hello!
>
> I was wondering why, on the FCC's website, when you go to
> look at an FM station's 60 dBu service contour, a
> non-directional pattern changes when it hits water (such as
> an ocean or swamp). The coverage area gets larger.
>
> The explainations say that terrain has no impact on these
> maps.
>
> Thanks!
>
> Richard
>

 

[BarryATL]

> > The explainations say that terrain has no impact on these
> > maps.
>
> The terrain around water is considered, however, and it
> tends to be lower around it (down to sea level in the
> direction of ocean, for instance).
>

Any idea why the FCC site does not have countours for the AM side?

 

[SML]

> Electromagnetic ground waves have better propagation over
> sea or brackish water
> because the ground conductivity of salt water is better than
> anything else. The better the conductivity, the less the
> attenuation. This is what results in the 60 dBu contour
> pushing out in a non d pattern.

Ground conductivity at VHF is not a factor. Obstructions are. There are no obstructions on the open ocean, hence the signal goes until the theoretical limit of the 60 dbu service contour is reached.

Ground conductivity is very much a factor at medium wave frequencies, however.

 

[tpt]

No data, in many cases.

Or confusing data.

For example, the FCC would have data on the 630 station I built which shows measured contours in directions where we had to show no overlap, but in other directions the data would be based on the FCC's ground conductivity map (in our area, a higher value than the measured value.

On many older stations there are no measurements on file with the FCC.

FM contours can be calculated based on terrain databases, so projected contours can be shown even for un-built stations.

 

[tpt]

Longley-Rice can be argued for various purposes in preparing applications, but the commisison does not need to believe it.
The Commission folks have also used Longley-Rice when they don't want to do something. Guv'mt at work.

However, officially, if the issue is contour overlap for non-com allocation purposes, or for a Section 73.215 app. (where the tower is short-spaced but reduced power or a DA is being used to protect prior allocations), then they must use the propagation curves in the rules.

This is what is used to predict the service contours (coupled with HAAT/ERP and a terrain database) to produce the service contours.

The service contours are then used by the non-technical folks screening apps.
The program will also generate interfering contours as well as 70 dbu contours (for multiple-ownership purposes), they have closed this feature off to public access.

At least for drawing curves. Another program allows you to enter HAAT/ERP and radial and it will spit out the distance to any reasonable contour you name.

 

[rdalton]

FYI

> Any idea why the FCC site does not have countours for the AM
> side?

FYI, patterns are now shown, if that helps at all:

From http://www.fcc.gov/mb/audio :

"June 1, 2005
AM Directional Pattern Polar Plots are now available through the AM Query. Use the AM Query (detailed + cdbs links) output to retrieve station data, then use the links under "Horizontal Pattern at 1 km radius" to retrieve the patterns and pattern data. Polar plots are saved as PDF files; printing at 11" by 17" or larger will give better resolution."<P ID="signature">______________
http://www.RichardJDalton.com</P>

 

[rdalton]

> However, officially, if the issue is contour overlap for
> non-com allocation purposes, or for a Section 73.215 app.
> (where the tower is short-spaced but reduced power or a DA
> is being used to protect prior allocations), then they must
> use the propagation curves in the rules.

So does that mean that the "service contour maps" really are not showing the service contour, as outlined in the rules, since service contours for FM are not suppossed to reflect terrain or conductivity (from my understanding)?

Thanks!

Richard<P ID="signature">______________
http://www.RichardJDalton.com</P>

 

[rdalton]

> 1) along a radial, the FCC model requires input of HAAT
> (height above average terrain) and ERP (effective radiated
> power) only. The HAAT is calculated by sampling terrain
> elevation at points along each radial from 3km out to 16km.
> This gives an average terrain height and the antennas center
> of radiation (height above ground level) is used to
> calculate HAAT for that radial. That said, the HAAT and ERP
> go into the model and you get distance to the countour as
> output. No effects of conductivity or anything else go into
> it, though in reality they would have some effect.

Ahhh-- ok.

I wonder if, since the ocean is always at sea level, the HAAT is manipulated for those maps...

Say a Non-D station has 500 feet HAAT, and the tower site is 150 feet above sea level. One-third of the coverage area is over water. I wonder if (suppossedly in error for a "service contour") those maps are, with only the radials over water, drawing the contour for (500 + 150) HAAT.<P ID="signature">______________
http://www.RichardJDalton.com</P>

 

[rdalton]

> The terrain around water is considered, however, and it
> tends to be lower around it (down to sea level in the
> direction of ocean, for instance).

It sure looks like it's considered, or at least the elevation!

Now an actual service contour as defined in the rules doesn't consider the terrain/elevation near water, correct?

Thanks!<P ID="signature">______________
http://www.RichardJDalton.com</P>

 

[tpt]

They are service contours as far as the FCC is concerned. Reasonably good for most purposes, anyway.

Remember the propoagation charts are called the 50/50 and 50/10 charts for a reason.

That's 50% of the receiving locations 50% of the time (or 10%) where the receive antenna is a dipole 30 feet above ground.

Theoretical ground. 60 dbu is quite adequate coverage in flat parts of the country, even 54 dbu (.5/mv/m). Where I am at, in hill country, 70 dbu is required for a reliable signal. Because everyone lives in the valleys.