RF communication frequencies vary between 3 kHz to 300 GHz, although are commonly between 300 MHz to 6 GHz. The wavelength and frequency of the signal affect how the signal propagates through open air, obstacles (e.g. walls and trees) and the type of radio required to capture them. Information transmitted over a wireless medium may be captured in-transit whether the sniffing device is the intended destination or not. This technique may be particularly useful to an adversary when the communications are not encrypted. In the 2017 Dallas Siren incident, it is suspected that adversaries likely captured wireless command message broadcasts on a 700 MHz frequency during a regular test of the system.”]
Source: https://collaborate.mitre.org/attackics/index.php/Technique/T0887