Which of the following is NOT a limit included in FCC rules?

The correct choice is the one that indicates a characteristic that is not explicitly limited by FCC rules. In the context of the FCC regulations, signal strength refers to the power of the signal as it is received, which can be affected by a variety of factors including distance from the transmitter, environmental conditions, and interference. While the FCC may have regulations related to signal levels for interference purposes, it typically does not impose direct limits specifically on signal strength itself across all situations.

In contrast, the other elements are subject to specific regulations. Radio power is directly regulated to prevent interference and ensure that broadcasts are made efficiently. Frequency use is strictly outlined to avoid overlap and interference between various communication bands. Antenna height is also regulated to manage the coverage of different signals and minimize potential interference with other communication systems. Thus, while signal strength is an important concept in communications, it is not a defined limit in the same way that the other factors are explicitly governed by FCC rules.