Bob1370 said:
If you're aiming at a highly urbanized area you want at least a 10 mV signal for an AM to cut through the noise from industrial and office equipment, and all the steel and concrete construction.
The main sources of noise today in residential areas are CFLs, conventional fluorescents and anything with a microprocessor, including tablets, computers, "computerized" home security devices, etc., etc.
That means... and the Arbitron data that was compiled when at work and at home ZIPs were both available... that, except for truly rural parts of a market, 10 mV/m is pretty much the minimum. Thus, that is where 95% of the listening to AMs in the markets studied took place.
If we are talking about significantly populated areas, there is not going to be any significant (redundancy intentional) listening outside the areas with that strength or greater.
When you're talking about a more suburbanized area without the same density of development where the stores and offices are brick and the homes largely frame dwellings, like in most suburbs (and even a lot of neighborhoods in the outer boroughs of NYC), 5 is plenty and even 2.5 can be adequate--it's more than enough to cut through the noise for most car radios outside a typical urban downtown.
Two problems... actual tabulations don't support that statement, and, further, in the NYC metro, only about 25% of listening was in the car when we had this data more fully broken out. If anything, noise levels are increasing, making the minimum signal issue even more significant.
A 5 kW signal will get you that much field strength at least 30 or 40 miles out from the center of town, and carry further than that if it's in the lower half of the band.
I live less than 20 miles, as the carrier flies, from a non-directional 10 kw station on 1140. In my home, it is unlistenable, and in the car on the street in front of my home it is noisy still. This station has a simulcast with a 5 kw AM on 970 at "the other end" of the market". That station, less than 10 miles from me, offers good reception but is lost 10 miles in the other direction. So it takes a 10 kw station on 1140 and a 5 kw station on 970 to cover a valley that is 45 miles long and 15 miles wide, on average... and they added an FM translator to fill in the gaps in coverage.
There are 8 AM stations in the market, and none cover more than about half of the market in the daytime. At night, it is vastly worse. Compared to a decade ago, the noise levels are considerably higher. And, additionally, the area is non-industrial with very little high density commercial or residential property. But it sure is noisy!
As noise levels increase (remember, incandescent bulbs are being phased out via legislation) due to electronic devices, poor maintenance of power lines, etc., etc., AM's usable coverage will decrease unless you live in an almost entirely rural environment and don't have modern electronics in the home.
