Police use radar guns to determine how fast you are going as you travel down the highway in your car. The gun sends an electromagnetic wave toward your car. Some of this wave reflects from your car and returns to the gun. The gun then receives this reflected signal and analyzes it to determine how fast you were going. Ever since police started using radar guns, people have been trying to figure out how to elude them.
1. Some motorists use radar detectors to find out if police are checking their speed with a radar gun. What do you need to know about the radar gun to tune your detector so that it sees the gun's signal?
Answer: You need to know the frequency of the electromagnetic waves that the radar gun uses.
Why: Electromagnet waves are used to transmit an enormous variety of signals (radio, television, cellular telephone, …). These signals are each assigned specific frequencies at which to transmit information. In order to distinguish the various signals you need a detector that resonates at the frequency you are trying to detect.
2. An extreme approach to avoiding the police radar gun is to put a radar jammer in your car. This is actually a transmitter that sends a confusing electromagnetic signal back to the police radar gun. If the police radar gun is operating at a frequency of 10 GHz (10,000,000,000 cycles per second), what is the wavelength of the electromagnetic signal?
Answer: About 0.03 meters or 3 centimeters (just over an inch).
Why: The distance between wave crests (wavelength) is just the speed at which the wave travels divided by the number of crests that pass each second (the frequency). A "crest" can be any reference point along the wave, say the point at which the electric field is strongest and pointing upward.
3. The most passive approach to avoiding radar detection by the police is to put a special covering on your car that absorbs the electromagnetic waves used by the radar gun. Although this covering does not detect or transmit electromagnetic waves it can effectively keep the radar gun from recording your speed. How?
Answer: The radar gun analyzes the electromagnetic waves that are reflected off your car to determine your speed. If none of these waves are reflected, it has nothing to analyze.
Comment: In researching this problem I saw advertisements for a "Stealth Bra" that covers the front of a car and a special radar absorbing wax. The effectiveness of radar absorbing was seems a bit suspicious to me. You should also be aware that many of these devices are illegal and I am in no way advocating their use.
An oscilloscope is a device used to visualize electric signals (currents or voltages). We have used oscilloscopes several times in the demonstrations during class lectures. Old oscilloscopes contain the same kind of picture tube that is in old television sets (an evacuated tube in which an electron beam is scanned across a phosphor-coated screen). The electron beam is typically swept horizontally across the screen in synchronization with the signal you are interested in looking at. The beam is displaced vertically in proportion to the electrical current you send into the oscilloscope. This can be done by sending the current you are interested in visualizing directly to the vertical deflection coils of the picture tube, but often this signal is amplified first. When you are not sending any current into the oscilloscope the electron beam sweeps out a horizontal line across the middle of the screen causing the phosphor along this line to light up. Positive current makes the electron beam move up, above the center of the screen, and negative current makes the beam move down, below center.
4. If you try to use the oscilloscope near a large magnet, what will happen to the trace on the scope? Why?
Answer: The trace will be distorted. The magnetic field will deflect the electron beam in a way that is not related to the signal you are trying to display.
5. Suppose you send a positive, steady (direct) current into the oscilloscope. Describe the trace that will appear on the oscilloscope.
Answer: The trace will be a horizontal line above the center of the screen.
Why: A positive current through the vertical deflection coil will cause the electron beam to move up, above the center of the screen. Since it is a steady current, the beam will remain above center as it is swept horizontally by the internal circuitry.
6. Now suppose you send the signal received on your AM radio antenna into the oscilloscope. Describe the trace on the oscilloscope. Assume that the AM radio station is playing a continuous audible tone.
Answer: The trace will oscillate up and down rapidly at the frequency of the radio transmitter, but the height of these oscillation will vary at the frequency of the audible tone.
Why: An AM radio station transmits information about the sound to be produced by changing the strength of the electromagnetic signal it is sending.
7. Describe the trace that would be observed if you looked at the signal that is sent to the radio's speakers. Again assume that the radio station is playing a continuous audible tone.
Answer: This signal would oscillate up and down at the frequency of the audible tone.
Why: The receiver separates the sound information encoded on the electromagnetic wave from the carrier wave and sends this information to the speakers to reproduce the sound.
8. In modern oscilloscopes the signal is often sampled at very short time intervals and the voltage or current of the signal is assigned a digital value for each of the sample intervals. Describe in general the difference between representing a current of 4.37 amps using an analog representation versus a digital representation.
Answer: An analog representation of 4.37 amps would encode this number directly in some physical quantity (4.37 amps might deflect the electron beam by 4.37 mm) and this quantity can vary continuously as the current being represented changes. A digital representation of this current would break it into pieces and represent each piece using a limited number of discrete values.
Depending on the number of bits used to represent the number you might only be able to deflect the electron beam by 4.37 mm or 4.38 mm, but not 4.375 mm. One advantage to using a digital representation of a trace in an oscilloscope is that the signal can be processed using the same techniques that are used in digital computers.