The 60 dBµ* contour of an FM broadcast station is defined by the FCC as the linear, radial distance from the transmit site to locations along the surface of the earth where a radiated field of 60 dBµ exists 30 feet above the surface of the earth.
There are an infinite number of combinations of transmitter power, transmit antenna configuration, height of the transmit antenna radiation center above the earth, antenna polarization, frequency, terrain elevations, earth conductivity and other factors that can produce a 60 dBµ field at 30 feet AGL at a given location/distance from a transmit site.
Here is one case for a set of conditions that might be of interest here (NEC4.2 data).
- Frequency = 99.9 MHz
- Transmit Antenna = 1/2-wave, Center-Fed Dipole. Vertically-Polarized, Self-Resonant
- Transmit Antenna Radiation Center = 30 feet AGL
- Transmitter Power at Antenna Feedpoint = 1.25 watts
- Earth Conductivity = 5 mS/m, d.c. 15 (average conditions)
- Level Terrain
- Propagation Paths = Line-of-Sight (no obstructions)
This setup produces a 60 dBµ field at an elevation of 30 feet AGL for a path length of one mile.
For a receive elevation of 5 feet AGL with other things the same, the field drops to 48.3 dBµ, which is about 260 µV/m.
*a field intensity of 1,000 µV/m
I pointed out that modeling tool months ago, I got no responses. http://www.part15.us/forum/part15-forums/transmitter-talk/modeled-part-15-fm-service-contour-map-generation .If I did my math right in that thread, to get that site to model a transmitter putting out the field strength allowed by 15.239 requires putting in the lowest power it will accept 0.00001W, and a gain of -27.3dBi. The 60 dBu contour of a part 15 transmitter right at 15.239 limits is tiny, not even a 100’ radius circle.
I caught WCDX and Rich caught me. The correct term would be 0 dBi where i designates isotropic, though sometimes gain is referenced to a dipole as dBd.
kc8gpd's comments in post 12 are essentially correct and give a good quick view of the topic. An isotropic antenna is a mathematical construct which has precisely defined characteristics but which does not exist in reality and the utility is that other antenna gains can be compared to this.
There is some good technical information in this thread and it illustrates, to me at least, just how puny a 15.239 compliant signal is in terms of usable range.
Neil