Why can't a radar jammer jam radar at short distances?

My question is about the products that are sold with the intention of ‘jamming’ police radar.

Even the bigger radar detector manufacturers have begun marketing detectors that do some jamming across one or more ‘bands’…

…but they are always effective up to a certain distance, and never all the way up to and through the trap. Actually, one of the competitive issues between the detectors/jammers is to see whicj product can jam up to the shortest range, giving maximum warning and maximum braking time before being clocked.

For example, one new detector/jammer outperforms alot of the competition by jamming all the way to 100 yards, but can’t jam below that distance. Most can jam up to several hundred yards. Either way, it’s not fool proof. It helps provide and window of opportunity to brake, but it begs the question:

Why can’t we jam police radar at short(er) distances?

I’d plunk down 300-500 bucks for a jammer/detector that would warn me like a good detector and jam the radar right through the trap.

Being as these devices are illegal in all 50 states, I wonder if this forum is the place for this sort of discussion?

Don’t be so quick, because some are legal…ones that take the signal transmitted instead of creating their own are legal, because they are not ‘transmitting’ on the polic bands, but using the signal the police created.

Loopholes are everywhere.

Carryind a radar jammer is like putting a giant sign on your car saying PULL ME OVER, I HAVE A RADAR JAMMER ON BOARD. The jammers themselves are filthy easy to detect.

To answer your question, though, Phil, my experience with jammers is simply that within a certain distance, it becomes likelier the receiver you’re attempting to jam will pick up the signal it’s actually trying to pick up rather than your own. As your car approaches the cop with the radar, his own radar signal is also getting “louder,” since it’s travelling less distance to your car and back. Getting closer, it becomes increasingly likely that his radar, which is a directed radio signal, will get a clear signal back through your jammer, which is probably an omnidirectional transmitter.

You could increase the efficacy of a radar jammer by simply boosting the power, but this simply increases the likelihood that a cop equipped with a jammer detector will catch you. In theory, you can better jam a reciever by using a directional antenna, too, but this would not work in your car, since the cop is moving relative to your car and the moment your directional antenna isn’t pointing at him, the jammer loses all its effectiveness.

You are referring to passive radar jammers, which have been demonstrated to be completely ineffective. Active jammers, to which I was referring, are illegal in all 50 states due to FCC regulations and also state and local laws.

That’s swell, but I believe the question of ‘why’ they are innefective hangs out there, but was helped along by RickJay.

Also, why active jammers are innefective at short ranges - or all ranges (even though they are illegal)- can still be pursued on these boards.

Why passive jammers are ineffective is discussed on the site I provided. Essentially it boils down to the fact that since a passive jammer merely sends back the same signal, perhaps with some modulation but without amplification, your car, being a much larger reflector, has a radar cross-section dozens of times larger than that of the jammer. The police radar receiver will have no trouble at all picking out your car’s reflected signal.

Jamming a radar signal in general isn’t exactly rocket science. Radar is just radio waves. The transmitter sends out a radio wave, then the receiver listens for reflections (in something like police radar the transmitter and receiver are built into the same unit, and share the same antenna). The way you jam them is simply to transmit more radio noise back at the receiver than what it receives. By sending out a carefully modulated signal, the receiver can be tuned to that specific signal, which means that the receiver will naturally reject other signals, like the jamming signal. In order to overcome this, you have to send out a huge honking signal to swamp the receiver.

Sending out that much signal from a car is not only illegal, it’s dangerous. The only difference between a high power radio transmitter and your microwave oven is that your microwave oven tries to keep all of the signal inside the box. High power radar transmitters have been known to cook birds.

Couldn’t you make a bra out of some kind of material that would tend to absorb RF energy and dissipate it as heat. IIRC don’t some aircraft use something like this to reduce radar profile. Even if it wasn’t totally effective many cops I have seen are aiming at cars a couple hundred yards up the street, much closer and the car is long gone by the time the officer gets his vehicle moving. A small reduction in signal strength could make a difference on the fringes of the radar devices effective range.

The radar absorbent bra was around at one time… it was covered by some car mags in the 80’s, but not sure if it is around any more. Too much car (wheels, mirrors, driver, etc) go unprotected. … the radar absorbent bra was even used in conjunction with radar absorbent paint - supposedly. :dubious:

And the overall point is largely moot and getting… er, mooter, as more and more departments switch over to LIDAR laser-based speed guns, which can be more precisely aimed to pick up the 'Vette in the flock of one-ton Fords, don’t have errant (and detectable) backscatter and don’t give inattentive officers testicular cancer.

But hey, if’n you fellers want to keep toying with countermeasures to '80s technology, go right ahead. :smiley: