There are two kinds of radar jammers, "passive" and "active"
Passive jammers receive a radar signal, mix it with an "FM chirp", whatever that is, and supposedly scramble and "reflect" the signal back to the cop, confusing the gun. The general concensus is that they DO NOT WORK. They dont actually "transmit" a signal so they are not regulated by the FCC. Some states however, have decided that they are illegal anyway including OK MN VA CA NE and DC.
Active jammers on the other hand definately work. They transmit a fairly strong
signal that will overpower the ability of the radar gun to get an accurate reading.
The FCC has authority over these types of units and they are illegal in ALL states in the US. Supposedly cops can tell when their radar is being actively jammed and if you get busted the fines can be HUGE.
Laser jammers work also but are not considered illegal by the feds. Since they transmit light as opposed to radio waves they are not regulated by the FCC.
For some reason they are under the jurisdiction of the FDA, go figure.
Again however, some state governments have banned them including MN VA UT OK NE CA.
Your best defense is to get a good radar detector, learn how to use it wisely, and keep your eyes open. They are legal everywhere but VA and DC. Most come with
laser detection also, but that is pretty much useless. Laser is a very narrow beam
and if you receive a warning that means you, and you alone, are being clocked
right now. You will not receive any advance warning of laser, unlike radar which bounces around, hopefully giving you a heads up of someone ahead of you
being clocked.
I have a Passport 8500 which consistently ranks number one in the tests and
seems to work really well. The Valentine One gets good reviews too, supposedly
falses more but for a hundred dollars more has those really handy directional
arrows that tells you where the radar is coming from.
Good luck, hope this helps.