Average dBu Gain with FM Mono

[MicrophonesReach]

Is there a rule of thumb for basic dBu gain when a stereo station goes to mono?

(-MR-)

 

[rfry]

Is there a rule of thumb for basic dBu gain when a stereo station goes to mono?

Probably the units here are dB, and not dBu -- which is "shorthand" for dBµV/m. Values expressed as dBu or dBµV/m apply to field strength, which, (other things equal), are the same for a given ERP no matter if an FM station transmits in stereo or monaural.

Theoretically, received FM stereo has about 23 dB less signal-to-noise ratio in the L&R channels than monaural FM -- due to the greater r-f bandwidth needed for stereo, and the triangular noise distribution of the demodulated baseband spectrum.
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[oldrover]

Translate this for an engineering idiot: let's say I'm looking at a radio-locator coverage map of an FM. How much additional coverage could I expect in switching to mono? Would it extend to the 50dbu level shown on their maps?

 

[rfry]

Would it extend to the 50dbu level shown on their (Radio Locator's) maps?

A field strength of 50 dBµV/m is 316 microvolts/meter. Any decent quality FM stereo receiver should give good, low-noise reception at that signal strength, barring local noise and interference. Most will give acceptable stereo performance down to 100 µV/m or so (= 40 dBµV/m). (Car radios usually blend to mono as a function of the received noise level).

A good FM mono receiver gives good quieting with a received field strength of, say 3 µV/m -- which is the same as 9.54 dBµV/m.

For calibration using the FCC's F50,50 curves, 50 kW of FM radiated from a height above average terrain of 500 feet will produce a 50 dbµV/m field 9 meters above a point on the ground about 46 miles away. It will also produce a 9.5 dBµV/m field at 9 meters above the ground over 100 miles away. So those might be considered the limits for stereo and mono reception for that situation.

But longer paths have greater variations in signal strength at various times of day, and in different seasons.

As usual, a simple question doesn't always allow for a simple answer.

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[oldrover]

As usual, a simple question doesn't always allow for a simple answer.

LOL... ain't that the truth. Given your example, as a guesstimate, how far out would that signal be consistent... i.e., sellable? Certainly not 100 miles, right?

 

[rfry]

Given your example, as a guesstimate, how far out would that signal be consistent... i.e., sellable? Certainly not 100 miles, right?

It depends mostly on how reliably receivable it is with respect to other signals in a given market. Local stations are tough to compete with in their own market. It's similar to setting yourself up for most predictable success in a walk-in business: location, location, location.

If your signal doesn't suffer from a lot of multipath distortion and can be heard easily inside homes and offices using normal receivers, AND people like your programming -- then you should compete well in a distant market when compared to the locals there.

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[oldrover]

Understood... programming is key; I'm just trying to figure out if the signal can reach there.

 

[amfmsw]

Also, lack of stereo pilot will allow you to be a stand out with louder, but legal, modulation levels. That will also help cover up some noise.

 

[gtomas]

Take a look at this thread: http://www.radio-info.com/smf/index.php/topic,43658.0.html


Regards,
Goran Tomas

 

[oldrover]

Thanks, Goran. I read that thread... and I'm looking exactly that: taking a rural Class A FM, switching the format to news/talk/local sports and going mono. In the 60dbu coverage area of the station, there is only one decent-sized town. However, in the 50dbu coverage area, there are three towns of 10,000 or more that are underserved re: news/talk, and parts of two other 20,000+ towns in the 40 dbu coverage area. I'm thinking I could go from essentially selling in ONE town to selling in at least FOUR towns, if the signal improves.