FM DBU contours

Generally speaking, what's the lowest DBU's (I'm not sure if I'm using this correctly) that are needed to listen to an FM signal clearly? I have an FM station's map that shows the 60 DBU contour...it's about 15 miles from the TX site..how much further should the station be able to be heard?

These contours apply to the FCC F50,50 field intensity 9 meters above the earth, and 60 dBu (1 mV/m) at that elevation is a healthy signal to an FM receiver. But not many FM receivers have antennas located that high (or higher).

The field intensity 6-8 feet above ground for this "60 dBu" field, inside a typical wood frame home in an urban area can be quite a bit less. Even less for other types of structures.

Car radios will work fairly well when the field intensity in rural areas arriving at their receive antenna is about 0.1 mV/m, but with some noise bursts with movement of the vehicle.
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As a dBu, that would be around 40dB to receive a listenable signal.

Most FM radios can produce a reasonably quiet FM stereo signal down to the 35-40 dbu contour-BUT remember that most FM stations are not protected from interference past their 60 dbu contour. This means that generally speaking, signal stregnth isn't the limiting factor in picking up a particular station, interference is.

Most good receivers with external antenna jacks will provide listenable mono reception at 3-5 uv, some better. Adding a yagi antenna will provide good reception at even less signal level. Here in the Indiana flat lands, barring interference, a 6KW 328' Class A can be listened to at better than 50 miles with little effort. That said, there are often two Class A's on the same channel at 71 miles. That severely limits listening range in the real world.

BobOnTheJob said:

Most good receivers with external antenna jacks will provide listenable mono reception at 3-5 uv, some better.

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But isn't that 3-5 µV the input sensitivity rating of the receiver for its input Z, and not the field intensity needed at the antenna?

At 98 MHz it would take an incident field intensity of about 10.6 µV/m on a ~matched 1/2-wave dipole to produce 5 µV across the 75 ohm input Z of a receiver. If the receive antenna is 2 meters above the ground, then the field at 9 meters as calculated by the FCC FM curves would need to be a little over 42 µV/m (~33 dBµV/m).

Usually an FM receiver is not driven very far into limiting at such low values of input voltage. Reliable reception even in mono would be difficult at these low fields, especially in a moving vehicle. Good stereo reception may take 5X the mono r-f input (if the receiver doesn't "blend" toward mono reception at that level).

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For an average portable radio, 80 and above would be clear, and would be good till 60. A sensitive car radio or home stereo with an outdoor antenna can be clear above 40 and staticy above 30. A sensitive portable or home stereo can be clear above 55 and listenable above 40. A crappy radio would be good only till 75 and would need over 90 to be clear.
All this is assuming no overload or interference.

Nick said:

For an average portable radio, 80 and above would be clear, and would be good till 60. A sensitive car radio or home stereo with an outdoor antenna can be clear above 40 and staticy above 30. A sensitive portable or home stereo can be clear above 55 and listenable above 40. A crappy radio would be good only till 75 and would need over 90 to be clear.
All this is assuming no overload or interference.

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Using ZIP code specific data from Arbitron (at home and at work, where 70% of listening takes place) 80% of listening is in the 70 dbu contour, and another 15% in the 64 to 70 contour.

Most portables and clock radios and office desk radios are good to about the 68 to 70 only.

DavidEduardo said:

Using ZIP code specific data from Arbitron (at home and at work, where 70% of listening takes place) 80% of listening is in the 70 dbu contour, and another 15% in the 64 to 70 contour.

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"... and "urban clutter" can reduce the values given in the FCC FM propagation curves for a given tx HAAT and ERP by something like 6 to over 20 dB.

So in a case where a receiver is located at the FCC 60 dBµV/m F(50,50) contour of an FM station the amount of r-f actually reaching its antenna could be down to around 40 dBµV/m, or even less if the receive antenna is near the ground.

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Using ZIP code specific data from Arbitron (at home and at work, where 70% of listening takes place) 80% of listening is in the 70 dbu contour, and another 15% in the 64 to 70 contour.

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David -

Those are very interesting statistics and somewhat akin (though even more extreme) to things I've been saying for many years. But I've never actually seen anything from Arbitron that shows it. Where have you seen this? Did you actually run Maximizer studies? Is it market specific?

Mike

Just look at all those rim-shot stations that have a hard time putting 60 dBuV/m over the target metro. Not many of them in the top 10. Being second & third adjacent to a strong local doesn't help.

Nick said:

For an average portable radio, 80 and above would be clear, and would be good till 60. A sensitive car radio or home stereo with an outdoor antenna can be clear above 40 and staticy above 30. A sensitive portable or home stereo can be clear above 55 and listenable above 40. A crappy radio would be good only till 75 and would need over 90 to be clear.
All this is assuming no overload or interference.

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We've stopped using the 50/50 curves on our FM coverage maps. We have Longley Rice shadowing done. One color is 60 dBu and above which we label Primary Grade A signal and then we have another color showing 48 to 60 dBu. We label this Grade B Signal. We have the study conducted at 2 meters above ground which is about the height of most car antennae.

Our markets are basically rural except for small town urban settings. We get reliable in house listening out to the 60 and most car radios seem to pick up down to the 48 dBu without a problem unless there is co channel interference. The Longley Rice gives you as close to real world reception results that you can get.

While I have not read any data about the percentage of listenership inside the 70 dBu until I read a posting here, it seems logical to me this is the case. The 70 Dbu is typically where the station has dominance or is relative to the audience. I would venture to guess that most of the auto listening out beyond the 70, 60 down to the 48 is by those people who live and work in the 70 dBu of the station.

I do know that if you are trying to serve a market with a Class A that is beyond the 70 dbu, you are always going to have signal complaints with advertisers..even in rural markets.

MikeF said:

Using ZIP code specific data from Arbitron (at home and at work, where 70% of listening takes place) 80% of listening is in the 70 dbu contour, and another 15% in the 64 to 70 contour.

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David -

Those are very interesting statistics and somewhat akin (though even more extreme) to things I've been saying for many years. But I've never actually seen anything from Arbitron that shows it. Where have you seen this? Did you actually run Maximizer studies? Is it market specific?

Mike

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I actually did studies, overlaying contours on MapMaker (from Arbitron) in over 12 markets. MapMaker plotted the diary returns by ZIP for at home and at work (70% of listening) and we came to those conclusions. Exceptions were "passion driven" formats like classical, which people would go out of their way to listen to where you would not expect it.

Separately, Arbitron did some runs when determining a new aspect of ascription in the diary survey 10 or so years ago when networks of stations started appearing in one Top 15 market with considerable overlap. So they ran tons of diaries vs. contours and came to a standard of where the signal strength trumped going to ascription for simulcast networks... such as exist in massive numbers in Puerto Rico or the old Big City combos of Class A stations.

There is no formal report, as I believe that broad conclusions that have exceptions could be cited as reducing the value of some properties. But for those of us who program, knowing where there is a probability of listeners tuning in is a big help...

DavidEduardo said:

I actually did studies, overlaying contours on MapMaker (from Arbitron) in over 12 markets. MapMaker plotted the diary returns by ZIP for at home and at work (70% of listening) and we came to those conclusions. Exceptions were "passion driven" formats like classical, which people would go out of their way to listen to where you would not expect it.

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This is easily explained by the audio experience:
• People dislike listening to static in the radio signal
• Why listen to McRadio format X "over there" when you can get a strong signal of the same content right where you are?

Thus the "passion driven" formats will attract people from way-way out: you can't get that format anywhere but "over there". It isn't only passion driven, but unique.