I tried a few, no luck with any, until.....I found the Phantom. I think this is the one a couple people above are talking about.
Back when I used it, it saved my bacon countless times!
What they claimed in the ad when I bought one years ago was.....
The ad said that it was against the law to carry a device in your vehicle that emits a signal that would interfere with the radar signal. (paraphrased, not exact wording)
But they claimed the Phantom device was legal because it did not emit a constant interfering signal. But it did take in the signal, scramble it, and send it back. Since it was sending back the radar guns own signal, and it did not emit a signal of it's own, they said it "technically" it fell within the legal guide lines.
They also said it had a "break through" zone. Meaning that once you got within a certain distance, I think they said 25 yards, that it would not protect you. They claimed once you were that close the mass, due to the size of your vehicle, the radar signal would correct itself, or get a good reading, I don't remember the exact ad copy, but this is all pretty close.
I haven't used it in a couple of years, I think I could find it in the garage, I just assumed since it's about 6 years old that it is old technology so I've left it on the shelf.
I once heard if you mount a fish finder in your vehicle, grill, it will jam radar. That story is about 15 years old.
Who knows. I'm sure if you look hard enough you can find a unit that will do the job, legal or illegal.
Edit:
I went to the garage and found two devices I used to use. My old Cincinnati Microwave "Passport" radar detector. Also the Phantom Jammer, however it looks nothing like the one in the link above. I'm not even sure if they are the same after seeing the link pic's and my old one. I plan on plugging them both in on my way in, in the morning and see if they still work!