I would first find out all you can about the systems you want to monitor.
There are a lot of types of systems.
Digtal, yes, but is it 9600 bauds or 3600 bauds, etc...
Check out this site...
http://www.radioreference.com/

There is a lot of info. out there, so do some research before you buy.
Hope this helps some ...


""Don't shake my tree, if you don't like what comes down""
:-) **** Life Is Too Short...*** (-:
****Enjoy The Ride...***