# A car with a radio?

1. Oct 20, 2004

### Anzas

if a car with a radio is moving at some speed and it receives radio waves shouldn't these waves get Doppler shifted and the radio signal be disrupted?

2. Oct 20, 2004

### Tide

Yes, the radio waves are doppler shifted but the shift depends on the ratio of v/c which you can see leads to a VERY tiny shift (put some numbers in to convince yourself!) The shift is entirely negligible compared with the bandwidth of any radio signal.

3. Oct 20, 2004

### mathman

In theory yes. However car speeds are around 2 - 3 meters per sec, while the speed of light is 3x108 meters per sec. FM signal is typically 108 cm. A 2 - 3 cm shift would not be noticed.

4. Oct 20, 2004

### sal

For AM broadcasts you can be 'way off in the frequency and still demodulate the signal acceptably.

For FM broadcasts, it's so hard to keep a typical receiver on frequency that nearly all of them "lock onto" the signal and then follow it if the frequency shifts. Typical radios, particularly car radios, will follow the signal far from its "home frequency" before they give up on it. (Actually, this behavior probably falls out of the use of a phase locked loop to demodulate the signal -- once it's locked onto a particular signal it tends to stay there.)

So, combined with the fact that the shifts are very, very tiny, as other posters have already pointed out, the effect will be totally undetectable.

5. Oct 20, 2004

### Gonzolo

A car actually goes 20-30 meters per second mathman (65 mph is roughly 30 m/s).

6. Oct 21, 2004

### NoTime

The shift is enough to get you a traffic ticket as this is how radar works.