The long and short of it, for a quick introduction to the uninitiated, is as follows:
- Ham radio is probably not worth doing in a car, maybe just having a ham handheld (instead of a base station that most ham people use for big power and lots of range and features) to monitor ham channels if you really wanted to do that. You need a ham license to transmit on ham frequencies which requires a test plus a fee. I don't care about this.
- GMRS is used for two-way communications, lots of people use this for offroading or whatever. You need a GMRS license (no test, just a fee) to transmit on GMRS but if you and a driving friend are just talking to eachother and you're not bothering other people you should be fine. I have one of these I use to monitor all kinds of traffic and talk to FRS because GMRS includes the FRS band.
- FRS is what you buy from stores like Cabelas or WalMart, they're easy to identify because unlike the above two (and unlike CB, below) you can't remove the antenna. The antenna is integrated or permanently attached to the radio. You don't need any kind of license to transmit on these. They're also the some of the cheapest and lowest power and range, because of this and because they're usually smaller than Ham/GMRS/CB handhelds.
- CB is a totally different frequency/spectrum. You don't need a license to transmit on CB. This and GMRS are probably the most useful for listening to truckers and other drivers during long road-trips.
With any of the above (other than FRS because you can't fit an aftermarket antenna) you'd want to run an antenna to the middle of your roof. You don't want it right on the edge of your car because antennas work best when there's a ground plane, aka when the base of the antenna is a large continuous flat space.
That being said, I plan on spraying plastidip through a stencil (so that it won't be as boring as just a plain square shape) to create a spot on my roof I can affix magnetic-base antennas without them scratching my paint long-term after a lot of cycles of applying and removing the magnets.
Stay tuned to see my eventual setup! At the end of this summer I'll likely have 3 devices running two antennas with some other random gear in the car, and I plan on doing everything for under $250