How do police transmit through a scanner?

Answer 1

Signals are sent from a police officer's radio to a "Repeater" located on a building somewhere. That repeater will have a signal code. for example... 159.900 is a signal frequency code. When your scanner is set on that code, or any other code, it will receive the transmissions from that code where ever you are that the signal will reach.

Answer 2

A less technical answer of how you hear the police through a scanner is much like how you get to listen to music on your radio. There is a transmission site that broadcasts over an area. Equipment such as your scanner is setup to capture those frequencies and play it through the speaker.

Many times a police department will use equipment to boost their signals. Equipment which is named a repeater. In some instances such as when a police car communicates with another police car without using a repeater the signal may be much weaker than if they were communicating using a repeater depending on your proximity to the cars that are communicating.