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How accurate are FCC service contours for FM?

pclover

pclover

How accurate are FCC service contours for FM?

For example there is several LPFMs or low power translators 30 miles away and yet I can still receive them with a decent signal. This despite the service contour putting me in a deep fringe or nothing at all. A recent example, 104.7 KSGG that recently appeared that is nowhere near me and only 250W has a good signal here.

Even with a modest portable radio it's still possible to receive them.

It seems what these actually estimate vs reality don't exactly match by a pretty big margin.
 
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pclover said:
How accurate are FCC service contours for FM?
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"Predicted Contours" are more relevant to licensing than to actual coverage. Those calculated contours are based on a perfect environment and a perfect antenna. In the real world, terrain, buildings, the side of the tower the antenna is on and other factors may change that a great deal.

"Deep Fringe" is "catch us if you can".
 
I came up with a formula many years ago for computing contours, but I forget what it was. It was a very, very simple formula though.

Later, I found a more complicated formula on some website, but I haven't seen it in years.
 
NoWayNoCC said:
I came up with a formula many years ago for computing contours, but I forget what it was. It was a very, very simple formula though.

Later, I found a more complicated formula on some website, but I haven't seen it in years.
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I think I remember my formula now. I think it might have been:

((HAAT*W)^.25)/2.2
 
To add to David a little bit, the FCC rules are dated. They were designed for the tuners that would have been available for purchase in the early 1960s.

What might have been fringe to impossible back then is doable with a handheld now.

The algorithm for predicting curves is actually pretty good, but it predicts field strength at a given distance with an ideal receive antenna, not how well your individual tuner in your individual home might receive it.

Not only does the quality of your tuner matter, but also the local terrain, the construction type of your home (some building materials block more RF than others), the weather (to a small extent), and the local RF environment.

For example, if you live in the Loop in Chicago, you'll probably have a harder time tuning in a suburban fringe signal than you would if you lived by Wrigley Field, because the RF from Sears will deafen most tuners a bit.
 
pclover said:
For example there is several LPFMs or low power translators 30 miles away and yet I can still receive them with a decent signal. This despite the service contour putting me in a deep fringe or nothing at all. A recent example, 104.7 KSGG that recently appeared that is nowhere near me and only 250W has a good signal here.
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That is a Class A, not a LP or Translator.
 
Michi's Rule #19: "Broadcast contours can be deceiving."
Contours are based on a very old formula that looks at the height above average terrain in up to 360 different directions along with the operating power in those 360 directions. Since it depends on HAAT calculations, it only looks at the terrain that is located within 2 to 10 miles when doing the calculation. It does not consider close-by terrain within 2 miles (see KWSV-LP to see how this helps them out). For lower-powered translators and LPFM stations, the actual coverage will vary greatly from the contours. You also need to take into consideration that when calculating HAAT, any value below 30 meters is considered 30 meters for the determination of operating power (for LPFM) and the development of the contour.

The only true way to see coverage is through a terrain-based propagation study such as Longley/Rice.
 
Michi said:
Michi's Rule #19: "Broadcast contours can be deceiving."
Contours are based on a very old formula that looks at the height above average terrain in up to 360 different directions along with the operating power in those 360 directions. Since it depends on HAAT calculations, it only looks at the terrain that is located within 2 to 10 miles when doing the calculation. It does not consider close-by terrain within 2 miles (see KWSV-LP to see how this helps them out). For lower-powered translators and LPFM stations, the actual coverage will vary greatly from the contours. You also need to take into consideration that when calculating HAAT, any value below 30 meters is considered 30 meters for the determination of operating power (for LPFM) and the development of the contour.

The only true way to see coverage is through a terrain-based propagation study such as Longley/Rice.
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If we had a "permanent" board here for all-pervasive gems, this would be the first entry.
 
47 CFR Section 73.333, Figure 1 and Section 73.699, Figure 9 Estimated Field Strength Exceeded at 50 percent of the potential receiver locations 50 percent of the time, at a receiving antenna height of 9 meters (just short of 30').

You mean you don't drive around with a 30' mast on your car???? I have. It was used for making FM field strength measurements, dipole on top, FIM-71 and spectrum analyzer at the bottom. It was fun until you got over about 5mph, and hard to deal with wind.

IIR, the 30' was chosen because of the average height of trees, which absorb a lot of RF making field strength below 30' erratic. But I might be mistaken. I've been mistaken before.

The theoretical antenna was a tuned horizontal dipole, I believe. They don't specify required field strength for specified receiver quieting, or what degree of quieting was considered receiveable, though I'm sure it was factored in. Doubtful that's changed much on average. Seems to me receivers want about 20dB more signal for the same quieting in stereo.

There's also up to 20dB variance over a 100' path with typical multipath, so at the fringe, there's your 50/50.
 
So, just a few additional points: First, as Michi pointed out, LPFM stations are forced to use the 3-15km terrain model, even though they don't generally reach anywhere close to 15 km and terrain within 1-3 km has much more effect than anything past 6km. Second, LPFM stations notwithstanding, the contours are reasonably accurate for most stations within the model limitations. After all, the laws of physics haven't changed in the 75 years since the FRC/FCC developed those curves. However, all models have their limits and the FCC model is designed to be simple so that it can be calculated by hand using slide rules and adding machines. Although the FCC method allows the contours to bend in and out to account for varying terrain, a huge limitation is that this method doesn't allow for pockets of higher or lower signal levels to exist within or beyond the various contours. For example, if you are near the edge of your FCC 50dBu contour, you might move to the top of a hill and suddenly find yourself within an area that has a 70dBu+ signal level.

Far better models that more closely approximate the real world exist. In broadcasting, one such model is Longley-Rice. This model allows for higher and lower signal levels at specific places rather than just the general contours of the FCC model. It also allows for propagation adjustments for receiving antenna height, the type of terrain and vegetation to be encountered. There are other models as well, most of which do a better job of predicting real world coverage than the FCC model. That said, most of the time, when Longley Rice and the FCC model are compared using the same parameters, the core signal coverage is usually surprisingly close. The Longley-Rice model is computationally intensive. For a high resolution plot with a 50km radius it can take more than an hour on a fast PC. It would probably have taken a team of engineers a month to render such a model by hand before modern computers, so that's why the FCC model was made so simple.

All this aside, another possibility for your LPFM is that they are operating at a higher power level than authorized. Within the last 24 hours, I have seen two LPFM stations operating at power levels in excess of 800% and 1000% of their authorized level. While I wouldn't classify this as a routine event, it is actually more common than it should be, so this can explain unusually large coverage areas.
 
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