On another site I found a link to a page that will plot coverage prediction using Longley Rice. http://lrcov.crc.ca/main/index.php (requires free registration). (If this has been covered before, I am sorry. I searched the forum for “lrcov.crc.ca”, and did not find anything so I think this may be a new topic).
The tool is pretty slick, you can use the map to select the transmitter location and the area to model reception in.
I tried putting in the tiny power used by Part 15 compliant FM transmitters, and the coverage prediction tool complained and failed to run. Then I worked through orders of magnitude until I found the lowest power it would accept, 0.00001W.
I used the first formula on page 29 from the FCC’s oet63rev.pdf to figure out the required antenna gain to get a field strength of 250 uV/m at 3m with a power of 0.00001W. I got a power gain of 0.001875 or -27.3dBi.
So if I did my math right, using the generator linked above, a power of 0.00001W and an antenna gain of -27.3dB with the polarization set to vertical should model a part 15 transmitter, assuming it is putting out the maximum allowed 250 uV/m at 3m using a vertical antenna.
Then I got stuck at the coverage display.
What is a dBµV/m, and how does it relate to dBu used by the FCC for service contours?
In dBµV/m, what would correspond to what a car radio can receive, what a portable radio can receive, a 60dBu service contour, and a 57dBu service contour?
I found a conversion from dBu to mV/m, and I found a conversion from mV/m to dBuV/m, and a dBu is the same as a dBuV/m.
I also found several values for the sensitivity of a car radio, and a consumer radio.
| Radio type | uV/m | dBuV/m | source |
|---|---|---|---|
| car radio | 1.5 | 4 | http://www.part15.us/forum/part15-forums/regulations-law/recent-talk-about-part-15-fm-range-us |
| car radio | 2 | 6 | http://www.part15.us/forum/part15-forums/general-discussion/part-15-fm-transmitter-best-coverage |
| car radio | 5 | 14 | https://answers.yahoo.com/question/index?qid=20060825020057AAfs7Fr |
| consumer radios | 25 | 28 | http://www.part15.us/forum/part15-forums/regulations-law/recent-talk-about-part-15-fm-range-us |
| consumer radios | 50 | 34 | http://www.part15.us/forum/part15-forums/regulations-law/recent-talk-about-part-15-fm-range-us |
So it looks like the weakest contour that reception could be expected would be 4 dBuV/m. A more realistic minimum is probably 14 dBuV/m. Consumer radios should start receiving the part 15 FM signal around (being generous) 28 dBuV/m.
The transmitter height does not do much to help with reception using consumer radios. On the other hand, elevating the transmitter pushes out the car reception range.