If there is one thing that is often missing from the commonly shown contents of Batman's utility belt:
A simple police scanner!
Now I suppose in the high-techie sort of portrail of Batman there is probably a nano-scanner or some such thing built in his mantel. And of course, he is often shown with a high-tech communications system for communicating with his allies.
But a low scale, independent Batman doesn't need those things. What he needs is a handheld scanner radio. In fact, there's a pretty good argument why Batman would go low-tech for this kind of thing: If it's just a reciever and not an emitter no one should be able to track him. Scanners are completely passive.
So what is a scanner radio? A scanner is a radio receiver that scans multiple frequencies looking for transmitions.
The idea is simple but some of the details can get confusing unless they are explained chronologically rather than as they come up. Back in the 1930s people like the police would broadcast through AM radio stations so that anyone with a radio could hear. In order to have their own channel, they would change to a less public frequency. Of course, then and now anybody could just follow them and keep listening as long as they could tune into that channel. Each channel required a specific quartz-crystal which so that your radio could tune in. So the crystal almost acted like a key.
It wasn't until the 1970s that programmable scanner radios allowed you to type in any frequency (in the scanner's range) and listen in.
It turned out that some channels tended to get way more traffic than others. Just imagine say an taxi company. Each of the cars maybe has its own car-to-car channel with every other car. Most of the time these aren't being used. But imagine being the dispatcher. Every car is calling the dispatcher all the time! The dispatcher's channel is busy!
So to alleviate this all the frequencies are pooled together and a “site controller” gives each channel to a frequency. Then after a while he rotates them. The scanner radios then automatically follow because hidden in the transmittion you are currently listening to is the next frequency that the channel will be using. This is all called trunking. A trunk tracker scanner radio can follow this automatically.
Of course, today even radio communication of this sort is heavily digital but we won't go there.
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