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Explaining dBu

9

#905

Guest
I recently saw an FM station's coverage map and was wondering what the dBu contours mean - 60, 54 and 40. I know it has something to do with coverage, doesn't it? Perhaps Bob on the Job or Chief Engineer or anyone with engineering savvy could explain in easy-to-understand terms. Thanks
 
You might try posting your question on the Engineering board under Professionals.

Yes, dBu is an engineering terminolgy for describing the "theoretical" strength of the radio energy at a given point. The maps are "theoretical". If you have an expensive measuring device you can go to a location and measure the "actual" strength of the signal.

I don't deal with the engineering issues very often so I can't tell you which number represents the amount of signal you expect to have over your city of license and should give good reception to even low cost receivers, and what level of coverage is adequate for rural coverage where the interference may be less, and what level of coverage means you can tune in the station if.... IF you have a superior antenna properly located and hooked up to a superior receiver system.

Try the engineers, and make it clear in your question that you want the answer in layman's terms.
 
First, it is worth noting that dBu is a measure used only for FM stations.

The 60 dBu contour is considered the "city-grade" coverage. Any radio can pick this up (even those having only headphone wire antennas). For the 50 kw stations in Indy, this would extend roughly 35-40 miles from the tower site. Using WFMS as a "for instance", its 60 dBu extends roughly to Danville.
The 54 dBu contour is "fringe" coverage. The signal may be a bit staticy on a good radio. Again, using WFMS, that would be to Crawfordsville.
The 40 dBu contour is the minimum a very good radio needs to be able to pull in your station at all. WFMS gets this almost all of the way to Terre Haute.

On the other hand, a class A station like WNOU (Radio Now ... Now at 100.9!) would get its 60dBu contour to all of Marion County. It would not be receivable at all in Crawfordsville (unless you get lucky or the channel is really blank).
 
Interesting stuff. Thanks for the info PT. I think most of us that have ever worked in a radio station are usually just trained to make sure that the board is on, meters are moving, and station is on the air (and how to get it back on if it gets knocked back off). But it's always nice to have guys like yourself to explain some of the more confusing terms.
 
As a low to medium level ENG.. I can also add, that within the last 15 years, tuners have advanced (especially in auto reception) to the point that most Engineers consider 60dbu as city and 54dbu as a metro signal that 75 to 80% of the tuners can get a decent signal on... 40dbu is really fringe for most tuners and when we look at possible market reach in a regional sense, we consider 45 to 50dbu the measure, depending on terrain.... ;)
 
mV/m = 10(dBu/20) / 1000. dBu = 20 log10 ((mV/m) * 1000.)
The factor of 1000 is used to convert millivolts per meter to microvolts per meter.


dBμ or dBu

dB(μV/m) — electric field strength relative to 1 microvolt per metre.

PT Board Operator...it was a good definition you provided.

dB/u can be used for AM as mV/m can be used for FM because both are measurement of how much energy is in the air at a specific distance from the transmission point. One is just measuring microvolts vs. millivolts. It just doesn't take as much energy to get a signal on AM as it does on FM because of the frequency so they can use mV/m and have an understandable number vs a number with a bunch of .00 in front of it. That's why WSYW's 1k signal is about as good as WNDE's 5k signal in the day. WNDE's 5k signal covers about the same amount of area as does WEXM's Class B (50kw at 500 feet or equivalent) WEXM and WFMS have identical signals.
 
Excellent comparison of AM and FM field strengths.... I always laughed at that night pattern on1260/WNDE.... :p
 
The most inaccurate part of the maps one sees is the height of the antenna. Most FM coverage maps are drawn based on an antenna that is 9.1m above the ground (about 30ft). My car radio has an antenna that is barely 2m at the tip ... headset antennas would rarely be higher than that. Signals are reduced when one goes indoors. Creating a map based mathematically as if you were sitting on the roof of a two story house introduces error.

I've drawn a few using a 2m height for the receive antenna. 60 dBu doesn't go as far (especially in areas with hills). Then again, as noted, a good radio doesn't need 60 dBu. I would love to create some "real" maps to where a car or pedestrian would get a signal, instead of a 30ft tall person. :)
 
70 dbu - City grade for Commercial FM stations (Hear on your teeth fillings)
60 dbu - protected on Class A (No Interference)
54 dbu - protected on Class B and C stations
40 dbu - Interfering contour (Looks good on coverage map when you have little coverage but don't expect to hear it.
 
I love the definition of the 40dbu.. Nice to look at with little coverage.. So true... "Skip"
 
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