After a recent article I wrote concerning the increase use of low-power unlicensed transmitters by churches and other organizations, Low-Power Transmitter Guidance From Larry Wilkins.
It garnered some responses concerning additional information about the legal power of these units.
While power is an important item, the commission’s rules in Part 15 seem to center on signal strength. On FM frequencies, these devices are limited to 250µV/meter at 3 meters.
For the nontechnical this is an effective service range of approximately 200 feet (61 meters). This relates to Section 15.239, and the July 24, 1991 Public Notice (still in effect).
On the AM broadcast band (although AM is rarely used) these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR Sections 15.207, 15.209, 15.219, and 15.221. These devices must accept any interference caused by any other operation, which may further limit the effective service range.
15.219 Operation in the [AM] band 510–1705 kHz:
- The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100 milliwatts.
- The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.
This will render (according to the July 1991 notice) an approximate maximum AM coverage radius of 200 feet.
As we stated in the original article these devices must be registered by the FCC and the proper use of the recommended antenna system to comply with the rules.