This is a good article about radar jammers that I found on the web:

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RADAR JAMMER TEST

We test the latest in radar and laser countermeasures

Denver-There's no denying the allure of a gadget able to make police radar or laser guns dummy up. Place the countermeasure in your vehicle, and, with an electronic shield now protecting against incoming microwaves or light beams, you can stop fretting about speed traps. A considerable number of drivers are succumbing to this tempting premise-so many that, whereas three years ago we found only a single company and two products claiming to jam radar, today it's tough to pick up a mail-order catalog or an auto enthusiast magazine without noticing a blizzard of ads for competing devices.

An effective countermeasure performs a role similar to that of an air force standoff anti-radar missile. After launching it from beyond the range of defensive weaponry, the attacking aircraft retires to safety while its missile homes in on the enemy's fire-control radar. When missile meets radar, well, you can predict what happens next.

A police radar or laser can likewise be countered, fortunately with somewhat more subtlety. The countermeasure will preferably employ a detector to tip you to the danger well before you enter target range. Then, while you industriously lock the brakes, it should be chopping the target range of the radar or laser to a few hundred feet at most, preferably much less. Shaving off a few hundred yards of maximum range won't usually help, since few ambushes occur much beyond 250 yards. A countermeasure's mission is to keep the threat at bay until the driver can react.

But do the products really work?

To find out, we gathered every jammer in four categories we could lay our hands on: passive radar jammers that mix noise with the incoming radar signal and reflect it back at the radar; active jammers that transmit a signal to confuse the radar or laser; license plate covers to foil lasers or photo radar; plus a few gadgets that use none of these techniques but still claim success in fooling Smokey's speed-measuring electronics.

We set up two series of typical ambushes in rural eastern Colorado, one each for radar and laser. For radar, we equipped our Chevrolet Corvette target car with a test mount affixed solidly to its dash. Placing one jammer at a time in this fixture, we drove slowly toward our radar vehicle-a Chevrolet Suburban K2500 LS modified for police use. Using one radar at a time, first we made five runs with no countermeasures in use to check the range at which each radar could clock the Vette. Measuring the distance was easy; our Kustom Signals ProLaser II can measure distances with an accuracy tolerance of plus or minus one foot. The average of the five runs determined typical target range.

With one of the countermeasures powered up, we next made five more runs toward each of our five radar units in turn, beginning at 2500 feet-well beyond target range-and ending when the Vette's speed was clocked. The range at which the active jammers first spotted the radar was also recorded.

To check laser countermeasures, we traveled to an industrial park blessed with die-straight, four-lane roads and no traffic. Starting at 2760 feet, our three target cars were driven one at a time toward the Suburban, parked on the shoulder at a right angle to traffic, the driver taking aim out of his lowered window. After recording the average of five passes to establish a baseline, we installed one product and made five more runs each against the two lasers. After a week of field tests, we've returned with some very enlightening results.

In 1993, we were first to report on a new device called a passive radar jammer ["The Little Radar Jammer That Didn't," June 1993]. Invented by Denver-area entrepreneur Mike Churchman and sold through his company, Rocky Mountain Radar, its theory of operation is simple: Receive the radar beam, add a bit of white noise, then reflect the signal back to the radar, reportedly so confusing it that no speed at all is displayed.

The U.S. Air Force has successfully employed radar jamming for decades, but its jamming transmitters pump out microwave energy at power levels sufficient to bake a Thanksgiving turkey in perhaps nine seconds. In contrast, a passive jammer transmits nothing but is somehow expected to jam a sophisticated radar with a minuscule amount of reflected energy. With that in mind, here's how the passive jammers performed.

PATRIOT ($95). A new model offered by Rocky Mountain Radar (a.k.a. RMR Distributing), the Patriot is identical to RMR's Illusion model and-save for the addition of a second front power jack-to the Spirit, tested in our 1993 story and found worthless. None of the radars was affected in the least by the Patriot.

PHAZER ($199). A new model from Rocky Mountain Radar, the Phazer is claimed to counter radar and lasers. Unlike previous models, this one has a translucent end panel that permits a glimpse inside at a pair of LEDs and a cheap, chromed plastic antenna. It's also larger and weighs four ounces more-up to 6.5 ounces. Placing it on the dash actually increased the ProLaser II's maximum target range by eight percent. The Marksman radar unit benefited even more: Instead of clocking the target car at 1954 feet, we could reach out and pop it at 2327 feet. We expect this will meet with approval among police officers.

THE WATCHFUL EYE ($279). Manufactured by Phantom Technology (a version called the Mirage 2001 is offered by Jammers and others), this peculiarly shaped device guarantees immunity to any type of radar. A "temporary holographic-type computer virus" accompanies the radar's return signal, supposedly afflicting it with some sort of electronic Alzheimer's. We've never heard of such a virus, nor have any of the microwave engineers we queried about it. In our tests, the Watchful Eye failed to do any jamming but did produce a measurable effect on target range. Against four of our five radars, its presence on the dash obligingly increased target range by an average of eleven percent, no doubt because its very large antenna efficiently receives the incoming radar beam and sends it directly back to the waiting radar. The fifth radar ignored the Eye.

ANTI-RADAR RADAR ABSORBING MODULAR ANTENNA ($59.95). Ads from the maker, MicroGuard, recommend using a minimum of two antennas, so we ordered a pair, sending a cashier's check for the promised second-day delivery. Five weeks and one angry phone call later, two mailing tubes arrived. We found the antenna to consist of a 28-inch-long, one-and-three-quarter-inch-diameter plastic tube. There was no operator manual, only a sticker explaining: "The ARA-2 module is a nonpowered, nontransmitting, self excited [sic] unit. No power is necessary to operate."

Large decals warn "Warranty void if seals are broken," but we couldn't resist looking inside. What we found was a 120-volt, 50-watt fluorescent light bulb with red electrical wire attached to one of its contact prongs and then wrapped tightly around the bulb, covering it down to the opposite end. That's it.

Testing the MicroGuard antennas was a cinch. Positioning was unimportant ("May be mounted in any position using plastic clips"), so we placed them on the dash of a Chevrolet Tahoe and made several passes against half a dozen radars. None was aware of all that supposed microwave-absorbing action inside the plastic tubes, courtesy of those three-dollar light bulbs.

An active jammer combines a detector and a transmitter. When it hears radar, an internal microprocessor quickly analyzes the signal, then fires back a modulated signal at precisely the correct frequency. The radar accepts the spurious return signal but is unable to decipher it, going into an endless loop trying to make sense of it. Net result: No target speed is displayed. This is far superior to inducing radar to read a preselected speed, say 55 mph, since any experienced traffic officer can visually estimate vehicle speeds within a few mph. Worse, if he's monitoring the radar's audio Doppler, he'll hear the telltale screeching of the modified return signal and know instantly that he's being jammed.

We offer two caveats about active jammers. First, only X- and K-band are covered at present. Run into Ka-band radar, a rapidly growing threat, and you're on your own. Also keep in mind that operating such a jammer is a violation of Federal Communications Commission regulations and, in Oklahoma, Iowa, and Kansas, it's also illegal under state law.

The first truly effective active jammer was the Stealth/VRCD, a large, heavy metal box stuffed with circuit boards, two large metal waveguides, and a pair of varactor-tuned Gunn oscillators that produce a powerful, modulated signal. All this was controlled by a microprocessor programmed with fiendishly complex algorithms. It came out in early 1993, and there was no question that it worked; in several tests it proved capable of making a Mack conventional-cab tractor disappear from five of our six radars at almost point-blank range.

The Stealth is gone, the victim of intense FCC scrutiny. The design has been acquired by Phantom Technology and renamed the Phantom RCD. Aware of its prowess, we were hoping to see our two active radar jammers weigh in with Stealthlike performances.

INTERCEPTOR ($595). From Advanced Radar Components in San Diego, the Interceptor (now called the Evader) is a complex unit that depends on two oscillators (one each for X- and K-band), a sixteen-bit microprocessor, and two large and efficient-looking cast waveguide antennas.

The Interceptor's performance was highly erratic. Against the Kustom Signals KR-10-SP K-band and the X-band MPH K-55, both widely used radar, the jammer achieved an average reduction in maximum target range of 37 percent and 21 percent, respectively. Yet in sixty passes, it completely blanked radar only once. And these numbers don't tell the whole story.

The K-55, for example, began clocking the target car at 921 feet in one test, yet the Interceptor failed to detect the radar for another 151 feet. Once transmitting, it delivered a useful reduction in range, but far too late.

The Interceptor was adept at sniffing out the KR-10 at upwards of triple the radar's target range, yet against the Kustom Signals HR-12, another popular K-band model, a warning sounded barely 140 feet before the radar locked on. It also sometimes failed to detect radar at all and exhibited a sporadic tendency to pump up the radars' target range. In one test, it increased the K-55 X-band radar's range by 20 percent and bolstered that of the K-band MPH Python by 42 percent.

True, in an encounter where the radar is transmitting continuously, an ability to chop target range could buy enough time to spike the brakes. But when we used the instant-on feature, keeping the radar silent until the target was well within range, the ARC Interceptor rarely reacted before a target speed had already been displayed.

PHANTOM RCD ($595). Because it's identical down to the last screw to the Stealth/VRCD, we expected similar performance from the RCD. Although it totally jammed the K-55 on one occasion and chopped target range up to 81 percent in some other tests, it inexplicably increased the HR-12's target range by eight percent. Worse, it occasionally failed to detect radar at all and other times showed an alarming tendency to inflate target speeds by as much as 90 mph. Try explaining that to the arresting officer.

Inventor David Sullivan told us our test unit had not been properly tuned. We believe him. We'll wait to see if Phantom Technology refines this jammer and will let you know.

K40 DEFUSER ($199.95). The only active laser jammer available, made by K40 Electronics. The Defuser is a black Lexan license plate frame with an upper section housing a row of infrared emitters behind a smoked plastic lens. Mounted over the front license plate, it's hard-wired to the vehicle ignition and continuously transmits infrared light at 904 nanometers, the police-laser frequency. An LED in the cockpit verifies that the K40 Defuser is operating.

The Defuser takes advantage of the laser's affinity for vertical, reflective, and light-colored surfaces. For this reason, officers are trained to aim first at the front license plate. By placing multiple, powerful emitters exactly where the laser is aimed, K40 says the reflected signal returns so polluted that the laser has difficulty picking out its own signal from the clutter.

There are two major weaknesses in this approach to laser jamming. First, it's useless if your state doesn't issue front plates. Second, it assumes the officer will continue targeting the front plate even when his or her first few attempts fail to produce a speed. Trouble is, at that point most officers will try for a headlight or a bumper instead. We did likewise and found both lasers instantly showed a target speed, Defuser or no.

Since the shape and size of a vehicle's frontal area have a significant effect on target range, three test cars were used: a 1990 Honda CRX, a 1995 Chevrolet Tahoe, and a 1996 Infiniti I30. With each, we first made five baseline runs to determine average target range with the Defuser switched off and then, with the Defuser switched on, made ten more runs against each laser. We drove at varying speeds and looked for any changes in target range.

Against the Marksman radar unit, the Defuser trimmed maximum target range from an average of 1954 feet to 1436 feet. It decreased the ProLaser II's target range from 2673 feet to 2099 feet. Target speeds appeared continually down to about 750 feet, at which time both lasers began suffering from the Defuser's infrared emitters, sometimes refusing to display speeds. Yet on other occasions the lasers ignored the Defuser entirely and clocked the targets almost to point-blank range. All the while, the Marksman's anti-jamming circuitry enabled it to detect the K40 Defuser and sound an alert. This same circuitry was probably responsible for this laser unit's frequent ability to shrug off the Defuser's jamming signal.

So does the Defuser offer some protection? Well, sort of. Since typical laser ambush range is perhaps 700 feet, the Defuser's ability to begin hindering target acquisition at this distance means that, used with a good detector, it might gain you enough time for a few seconds of heavy braking. Conclusion: A potential driver's license protector, but no guarantees.

In another attempt to capitalize on the laser operator's preferred aiming point, these plastic sheets that mount over the front plate claim to absorb and attenuate the laser beam, weakening its return signal and limiting target range.

THE T3 ALPHA LASER LICENSE PLATE COVER ($49.95) from T3 Technologies is a piece of green plastic that, when installed over our highly reflective Georgia plate, cut the ProLaser II's maximum range from 2589 feet to an average of 2320 feet. Still well over three times the typical ambush distance, this is a meaningless reduction. It slimmed down the Marksman's maximum range from 1954 feet to 1193 feet, a useful decrease if you're targeted at extreme range but no help in the average short-range ambush.

THE LASER PLATE ($19.95), manufactured by Laser Stealth Technologies and sold through catalogs and convenience stores, actually increased the ProLaser II's average target range from 2584 feet to 2754 feet. It slightly hampered the Marksman's range, shortening it from 1954 feet to 1636 feet. Feel free to buy one if you're one of the few U.S. motorists pestered by officers popping you at 500-plus yards.

THE LASER-GUARD FROM TAYLOR-BELL TECHNOLOGIES ($39.95) also increased the ProLaser II's maximum range, this time from 2584 feet to 2599 feet. It trimmed the Marksman's range from 1954 to 1571 feet. Entrepreneur Tom Bell suggested we use a target car with retractable headlights, claiming the Laser-Guard would perform better. Having tested against such vehicles, we know it would. But not enough to save you, retractable headlights or not.

Two plate covers are offered as antidotes to photo radar. The Shadow ($89.95), from Chimera, allows the plate to be read from straight ahead but makes it unreadable when viewed from the side at the 22-to-24-degree angle used by photo radar. One downside: The Shadow's dark tinting makes it easy to spot. Since cities new to photo radar invariably outlaw such covers, the possible vehicle-code infraction must be weighed against the cost of a ticket. We'd take the plate cover.

The other license plate cover is manufactured by a Canadian outfit, Focus Auto Design, and sold as the Eliminator ($29.95) by Redline, an Ontario company, and as the Chameleon ($49.95) by Jammers, Defense One, and others. We found it does mask the numbers, but only when viewed at about 45 degrees or more from the side. That's about twice the proper angle, making it helpless to ward off photo radar.

Still lusting for a magical counter-measure? Take our word for it-aside from the few straight-up companies like K40 and Chimera, this industry is teeming with basement inventors, engineering wannabes, and outright charlatans and con men.

We'd have tested two other jammers, but after we spent $400 to acquire them, both companies simply disappeared. Don't be shocked if some of the firms whose products failed to perform in our tests likewise disappear, although some are bound to resurface with different identities and new product names. Regardless, their appropriate exit line might be: "Take the money and run." Our advice: If bona fide experts haven't testified to the product's efficacy, run in the opposite direction, fast.

-Craig Peterson