At the February meeting, The topic of the night was MPSCS radio operation.

Operation of these radios is a lot different than the typical amateur radio. 

The first major difference, these radios do not transmit on the exact frequency each time you make a transmission.  The radios share a pool of frequencies, and all users are able to access these channels as needed.  This is called Trunked radio. 

If you remember the old phone trunks of 50-1000 or more pairs of wires that ran between cities, counties and states... the concept is exactly the same.  This is where the term "trunked radio" comes from.

Everyone is a city chares those pairs to talk to someone in the next city or state. Sometimes when you called someone in the next state, you heard a 'fast busy signal'... that told you all the lines in the trunk were in use, and to try your call again later. (that differs in the normal busy tone that told you the person you were talking to was already using their phone)

What happens in a trunked radio environment when you push the PTT switch, the radio sends a message to the radio system requesting an available radio channel.  It grants you a channel, and tells your radio and all other radios in the same group you want to talk to to switch to that channel for communications.  The radio tells you its ready to carry your message by "chirping" at you... and you can talk to all the other users.

The radios do all this communications within a second or less.

Seeing as all radios share a pool of frequencies... you are no longer talking on a specific frequency to talk to your team mates, you are using a "talk group".

A talk group is a collection of people that you want to talk to.  A good example of this is how we break down some of our nets during a big event.  Lets use the Saint Clair Shores Memorial day parade as an example.

At the parade, there is a net for staging, another net for lineup changes and a net for security information along the course.  These are all different talk groups.

It is the same with public safety agencies and private entities that have trunked systems. 

I have attached several references to this short article.  there is much more information in these documents about the Macomb County Public Service Communications system, and the statewide version, the Michigan Public Safety Communications system.