The bars labeled E and F in the figures are found between 450 and 470 MHz. This shows that the signals come from a public safety organization, such as a police or fire department dispatch. We can’t tell which specific group is sending these signals, but we know they come from trunked radio systems because they are always active. Trunked radio systems are used by organizations that have many users. They work by giving a channel to a user when needed instead of giving each user a channel all the time. Because the channels are shared, the frequencies are almost always in use.