I don't think carrying a scanner will get you an "impersonating" a police officer charge any more than carrying a gun will. Cops don't carry scanners, anyway. Probably the only things you can carry that will get charged as impersonating is a police badge or other emblem (or flashing a junior g-man toy badge quickly), and maybe police lights on your vehicle (and using them.) Just telling someone that you're a cop-- when you're not-- will certainly suffice. I believe the statute says something about "intentionally misleading others into believing" that one is a peace officer. Overall it is your behavior much more than your equipment which will bring an impersonating charge. There's no law against owning a police badge or even a full uniform. But flashing a badge of any sort at somebody and motioning them to pull over is certainly a no-no.
I have considered carrying a scanner, but never have while OCing for a couple of reasons-- one, it's just too much equipment; two, it's as if you're anticipating or planning an unfriendly encounter with the police. My primary interest in a scanner would not be to provide a warning that police are coming, but rather to get an accurate idea of the information given to the officers by the dispatcher.
In Madison, and probably other larger urban areas, there is so much radio traffic that there's a good chance you'd miss the call regarding you. In more rural areas, that's less of a risk, particularly if the law enforcement agency uses only a single frequency. If you're carrying a gun, and even if you're not, you shouldn't be walking around in condition white. The arrival of other persons openly carrying a firearms (the police) should not come as a surprise to you if you're aware of your surroundings. Five minutes warning, or five seconds warning, shouldn't matter to you if you're awared and prepared.
A couple of weeks ago I was watching one of the Madison cops trudging down State Street with so much crap on her belt that there was no room for another item. I noted the holster/gun, ammo pouch, handcuff pouch, a large flashlight, taser, I think a glove holder, and a radio and who knows what else. I doubt she'd manage more than a jog if she needed to move quickly. We are not police, and there's no reason to overload ourselves with equipment.
I have a couple voice recorders, all are small, yet I haven't carried them. The best thing I've found to use as a voice recorder is my little Sansa Clip 4 gig MP3 player. I doubt it weighs an ounce. It's 2 inches by an inch and a quarter, and barely more than a quarter of an inch thick. In my tests it has excellent voice recording capability built in, not to mention it holds songs and has a built-in radio. It is inconspicuous and does not look like a voice recorder.