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B E H A V I O R

They squawk as you drive down the high- way and occasionally bark protectively to let you know that a radar speed trap awaits you. They detect police radar, hopefully in time for you to slow down.

H A B I TAT

The radar sensor is mounted behind the car’s grill, out of sight. The visible part of the detector is mounted inside the car on or near the dashboard.

H O W I T WO R K S

The word radar comes from radio direction and range, which indicates the initial use for this technology was finding (military) targets, mostly airplanes, and figuring out where they were (direction from the radar station, and distance or range).

Radar uses electromagnetic waves in the radio frequency range. A police unit sends out a radio wave at a particular frequency. The wave reflects off solid objects and a tiny portion of the outgoing wave reflects back to the radar unit. There a radio wave detector will sense the signal.

Of course, it isn’t enough for police to know that your car is there— they need to know how fast you are driving. So rather than report your presence, the radar gun measures how much the returning radio signal frequency has changed from the signal it sent. If your car is parked along the side of the road, the returning signal will have the same frequency as the outgoing signal had (assuming the officer is standing still, too). But if you are driving toward the radar gun, the returning signal will have a higher frequency and the magnitude of the change is related directly to your speed.

Imagine that one pulse of radar reflects off your car and a second later another pulse reflects off your car. By the time the second pulse

reaches your car, the car’s position will have changed (if you are moving). The faster the car is going, the more the position has changed. So although the two radar pulses were sent a second apart, they will be received less than a second apart since the second one traveled a shorter distance. The radar gun measures this change, called the Doppler shift.

Radar detectors are effective in that they can pick up a weak signal, one that might be too weak to reflect off the car and travel back to the radar gun. So the detector can “see” the radar gun before the radar gun can see the car. This assumes that the radar gun is on while you are approaching.

As in all technology battles, both sides constantly improve their hardware. Police now increasingly use laser detectors called LIDAR instead of radar. And speedy drivers use LIDAR detectors, or put coat- ings on their cars (stealth technology) to hide from LIDAR. Unlike radar, LIDAR doesn’t use the Doppler shift. It measures the distance from the gun to the car and compares the distance between successive pulses. Large differences in the distance indicate high speed.

Radio

B E H A V I O R

It brings you acoustical enjoyment, football action, news, weather, and traffic advisories.

H A B I TAT

The radio occupies the center of the dashboard. Located not as close to the driver as the car controls are, the radio is reachable by both driver and front seat passenger. But passengers, don’t touch that dial.

H O W I T WO R K S

Radio stations broadcast in either AM (amplitude modulation) or FM (frequency modulation). When you select a station to listen to you tune your radio to one frequency band. The radio signal you select is vibrat- ing at that frequency. AM stations alter the amplitude of the vibration to carry the sounds of the annoying talk show hosts. FM stations alter the frequency that is superimposed on top of the carrier frequency.

In both cases, the electric signals are amplified and then sent to an antenna. The antenna converts the pulsating electric signals into elec- tromagnetic waves that propagate at nearly the speed of light. Wires carry it into the receiver. The antenna also receives hundreds or thou- sands of other electromagnetic signals from other radio stations, cell phones, and space aliens. The radio receiver filters out all the signals,

leaving only the one frequency you selected. It amplifies the signal and sends it to the speakers where the pulsating electric signal is converted into the sound of Rush’s voice. Turn that off!

I N T E R E S T I N G FA C T S

When car radios were first sold the price was about 25 percent of the cost of a new car. Not like the compact integrated circuit models of today, these early radios were bulky and required a massive rebuild of a car’s dashboard to install, requiring several days of labor. On top of these difficulties, short circuits commonly caused fires.

The first commercially available car radios were available in 1926. The manufacturers faced several technical problems including how to minimize interference from the car’s electrical system, which generated radio static. Isolating the radio from the car’s electrical system improved the reception, but caused other problems until Paul and Joseph Galvin invented a new electric circuit. Their circuit design not only made car radios work well, it also launched their new company, Motorola.

Sales of both home and car radios surged in 1930 when, despite the start of the Depression, people had to have them to catch the new NBC radio show Amos ’n’ Andy. This sales growth led to more innovations that improved the quality of the radio.

Still, drivers in the 1930s had to endure problems that we don’t face today. When you stopped the car, you had to turn the radio off or face the possibility of draining the car battery. Before police had their own two-way radios installed in patrol cars (which started in 1931), they broadcast police calls on public stations. This allowed officers and everyone else, including the people perpetrating crimes, to know what was going on.

FM radio came to cars in 1951, nearly twenty years after the first FM broadcasts. But the largest technological change was the introduction of transistorized radios in 1958.