When we go in a car and tune to an FM radio station, why doesn't our motion disturb the frequency?
Like the Doppler effect?
Electromagnetic Waves – Why a Car’s Motion Doesn’t Affect Radio Station Frequency
doppler effectelectromagnetic-radiationwaves
Related Solutions
(I'm not exactly sure of this, comments greatly appreciated)
Radio waves have a longer wavelength (3-4 m) than cell phone waves(fractions of a meter). Thus these are easy to block. They can travel around buildings &c, but can't enter cars because the windows are tiny. Cell phone waves are small enough to enter the windows of a car.
Radio waves have large antennae due to their wavelength. So reorienting them makes a difference as the effective size of the antenna facing the wave changes drastically. A cellphone jas a tiny antenna. Reorienting it makes little difference. Imagine making the antenna shorter and shorter till it becomes a ball. Reorienting that makes no difference at all. And a cellphone does change quality on rotation, but its less evident. There may be some smoothing circuitry involved.
Edit: Antennae need to be with the same order of magnitude as the radiation as they need to resonate. See http://enwp.org/Antenna_(radio)#Resonant_antennas . A short antenna cannot effectively acheive resonant frequency of a long wave.
The doppler shift causes a shift in wavelength at the origin of the wave (the frequency of the source never changes). This results in a shift in frequency for the observer.
In the link below you can see the emission of the wave for a moving source causes the wavelength to be shorter in front and longer behind. The actual source isn't changing in frequency. So it's a matter of relativity. To the traveling observer (in the source), only the wavelength is changing, to the stationary observer (experiencing the doppler shift) both frequency and wavelength have changed.
Lookang, Wikimedia commons. More simulations and applets here.
Best Answer
It does! However it doesn't change the frequency enough to matter.
An FM transmission is not a precise frequency. Instead it spans a range of about 100 or 200 kHz depending on which country you are in. So your FM radio actually accepts a range of frequencies either side of the central frequency.
Let's suppose you're travelling at the maximum speed permitted in the UK, which is 70 mph or just over 30 m/s. This will Doppler shift the frequency of the FM station by a factor of about 1.0000001. In the UK the FM frequency is around 100 MHz, so the shift in frequency is about 10 Hz. This is only 0.01 % of the range of frequencies the transmission uses, so the frequency shift does not affect reception.
To seriously affect reception you'd need to be travelling at around 100 000 miles per hour.
For completeness I should probably add that modern radios auto tune, and would automatically compensate for a change of frequency due to the Doppler shift.