I remember reading this - it was in the magazine in the late 70’s.
They asked the air force what they used to radar-proof missiles, and bought a couple of sheets. Apparently it was quarter-wave foam with embedded metal particles. i.e. the foam thickness was approximately one quarter the wavelength used by typical radar units. (18 to 40GHz, so 1cm wavelength give or take.). Radar makes the most use of flat forward-facing metal objects - which is why police love front license plates. It bounces off these back to the reader, and the doppler shift indicates the velocity of the vehicle. The idea with stealth foam is that the particles in the foam reflect back the radar wavefront, but it reflects from different points at the same time over half a wavelength, so all the reflections cancel each other out.
(I.e. let’s say the wavelength is 1cm. We have 0.25cm and the signal can reflect from many random points within that thickness - so some of the signal travels 0.5 cm more than others, distance into and back from reflection points. The two signals cancel each other out.
I vaguely recall they used a small Porsche, since it had the sloped front that already made it less detectable. The constructed a rough bra of the material - of course, there are other bits of the car front that weren’t covered. The goal was to allow the driver time to recognize the police and slow to an acceptable speed before the radar picked them up from all the rest of the car, by covering the front end as much as possible. this worked. They got it so the radar would not detect the car until it was about 100 to 200 feet from the radar gun.
The article ended with a caution - while they were testing this on a back country road, while the car was about 200 feet away, a big flat-fronted 70’s Cadillac-type vehicle comes around the bend a quarter mile away and instantly registers on the radar. In this scenario, the police would have pulled over the Porsche and attributed the Caddy’s speed to it.
As for the question - I’ve wondered too, perhaps you could rig a reflective antenna behind the car’s grill, attach it to a high speed vibration mechanism, so it reflects an oscillating signal back. The question is - how does police radar work? Does it average out the signal, or does it try to lock onto a single, steady speed signal (so lack of a steady signal confuses it), or does it average? Also note newer units can detect (lock onto?) multiple return signatures (i.e. two or more vehicles) and show the speed and relative signal strength of each.
Also note some police use “instant-on” so the signal won’t be detected until it’s pretty much taken a reading in a matter of a second or so. Other units use infrared laser - another reason why cops love front license plates, they are usually painted with that highly reflective safety paint, so are perfect reflectors for IR speed guns. Also why some radar/IR guns need aim. At typical detector distances - a few hundred feet - the IR beam only spreads about a yard wide.
Radio Electronics magazine about 1986 or so published plans for a “radar gun calibrator” - point it at your typical radar unit and it would send a signal modulated with the IF frequency and a fixed reading of 30 or 60mph (switch selectable). Of course, this was only so you could ensure your sports radar was reading pitching speeds correctly. The electronics can get hot, so couldn’t be run continuously. To ensure it didn’t burn out, rig it to a radar detector to only trigger when it detected an incoming radar beam.
Police now have radar–detector-detectors, which detect illicit microwave units in cars by looking for the leaking IF frequency. Almost all commercial microwave radio devices use the same IF frequency (100Mhz?) which is why the radar calibrator trick worked. It broadcast a signal modulated with that frequency.