Final answer:
Signals can indeed act over short or long ranges. AM radio signals, with their long wavelengths, can travel long distances and navigate around obstacles. FM signals require a line of sight and make use of elevated antennas.
Step-by-step explanation:
It is true that signals can act over both short range and long range. In the context of electromagnetic waves and radio broadcasting, AM radio signals can travel long distances due to their low frequency and long wavelengths. They are able to go around obstacles such as buildings and hills. On the other hand, signals such as FM radio and TV are best received when there is a direct line of sight between the broadcast antenna and the receiver. These typically operate at higher frequencies and require elevated antennas to transmit over obstacles.
As for antenna length, it cannot be just any length. The length of an antenna is typically related to the wavelength of the signal it is designed to transmit or receive. An antenna designed for long wavelength signals, like those used in AM radio, will generally be larger than those for signals with shorter wavelengths, such as FM, TV, and mobile phone signals. Furthermore, electromagnetic waves used for communication submarines require very long wavelengths, necessitating extremely low frequency signals, due to their ability to penetrate water depths, unlike shorter wavelengths.