Licensed FM Broadcast stations and their listeners have the right to unobstructed enjoyment and use of the FM airwaves. Unlicensed use has LIMITED privileges in regard to locally unused frequencies. Your unlicensed use has no absloute rights over the rights of licensed broadcast stations, or the rights of their listeners. If your unlicensed operation interfeeres with any other licensed transmission, your only choice is to CEASE operation or change the operating frequency of your unlicensed equipment.
Unlicensed operation of small transmitting devices in the US is covered in Part 15 of the FCC rules (47CFR15). It is best to obtain a copy from the Government Printing Office, or by contacting the FCC directly, via telephone or INTERNET. You can read more about low-power operation from the FCC location at This Specified FCC Website
If you want to locate the technical document that explains un-licensed operation, check out FCC BULLETIN # 63.
In 1989, the FCC revised its rules specifying a maximum field strength of 250 microvolts per meter at a measured distance of 3 meters.
The field strength of a transmitted signal decreases in direct proportion to the distance away from the radiating antenna. This means that every time you double the distance from your transmitting antenna, your receiver will only receive 1/2 of the signal voltage. However, when you are discussing power, it decreases by the square of the distance; for every doubling in distance, the received signal is quartered. Using the voltage theory, it is possible to create a chart to show the performance of your unlicensed transmissions under the Legal maximum field strength. The theoretical figures assume a simple 1 meter long receiving antenna, and do not take into account that reception can be increased with gain antennas, and/or preamplifiers at the receiving end. In the chart below, the field strength gets theoretically stronger as you move from the edges circular boundaries torward your antenna system.
3 9.5 250 304 ft.
6 19.7 125 1216 ft.
12 39.4 62.5 4865 ft.
24 78.7 31.25 19458 ft.
48 157.4 15.625 1.8 acres
96 314.9 7.81 7.15 acres
192 629.8 3.91 28.6 acres
384 1260 1.95 114.3 acres
768 2519 0.98 457 acres
1536 5039 0.49 1831 acresBy restricing the maximum field strength at a specific distance from your antenna, the FCC plans for your signal to fade out at a somewhat predictable distance from your antenna. On the other hand, the FCC standards do make it legal and possible for you to broadcast on a school campus, campground, or local neighborhood, as long as you do not cause interfeerence to licensed broadast trasmissions.
How about "leaky cable"?
The FCC allows you to use leaky cable (radiating coaxial cable, usually buried underground to limit the maximum radiation), as long as you comply with the maximum field strength.
PLEASE NOTE THE FCC HAS APPROVED LOW-POWER BROADCASTING!
To see more on this topic visit FCC LPFM WEBSITE
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