Author Topic: Doppler shift vs velocity  (Read 7989 times)

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Offline CirclotronTopic starter

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Doppler shift vs velocity
« on: February 08, 2024, 02:13:01 am »
Say there was a radio transmitter some distance away that is transmitting on 300MHz. No modulation. Light travels at ~300 million metres per second and electromagnetic waves are coming toward me at 300 million Hz with a wavelength of 1 metre. If I walk toward the transmitter at 1 m/s will I now measure an apparent frequency of 1Hz greater than if I remained stationary?
 

Offline CatalinaWOW

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Re: Doppler shift vs velocity
« Reply #1 on: February 08, 2024, 02:17:18 am »
Close (to many digits) asI remember the physics from several decades ago.  At this speed you can ignore time dilation.
 

Offline ejeffrey

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Re: Doppler shift vs velocity
« Reply #2 on: February 08, 2024, 05:45:57 am »
Yep.  The way you described it is a little hard to measure, you need frequency accuracy of a few parts per billion.  You can do that with an OCXO or atomic standard, but that is bulky and power hungry.  Instead if the transmitter also has a receiver it can monitor the signal reflection and it will see a shift of 2 Hz.  Now it's comparing against itself and the 2 Hz shift is easily resolvable.  The beat note is pretty low frequency.  You can still resolve it but that can make for a slow measurement and be limited by 1/f noise in a simple homodyne receiver. Bump your transmit frequency up to 30 GHz and now the signal is at 200 Hz, and now you have a radar speed detector.
 

Offline jbeng

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Re: Doppler shift vs velocity
« Reply #3 on: February 11, 2024, 03:28:58 pm »
The Wiki page on Doppler radar may be of some help here in determining the difference in frequency of an RF signal which has been reflected from a moving object.  Look in the "Frequency Variation" section a short way down the page for some formulae on this.

Wiki link ( https://en.wikipedia.org/wiki/Doppler_radar )
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Offline m k

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Re: Doppler shift vs velocity
« Reply #4 on: February 11, 2024, 04:35:58 pm »
Depends what you mean with apparent.
There in no other frequency that is more real.

We know that energy has strength and we can measure it, even photon.
But energy has also a shape of some sort and measuring that we must combine it with time.

We can also measure no energy from a point where we empirically know the energy should be.
Is our measuring apparatus then failing.
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Offline NiHaoMike

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Re: Doppler shift vs velocity
« Reply #5 on: February 11, 2024, 06:22:44 pm »
Yep.  The way you described it is a little hard to measure, you need frequency accuracy of a few parts per billion.  You can do that with an OCXO or atomic standard, but that is bulky and power hungry.  Instead if the transmitter also has a receiver it can monitor the signal reflection and it will see a shift of 2 Hz.  Now it's comparing against itself and the 2 Hz shift is easily resolvable.  The beat note is pretty low frequency.  You can still resolve it but that can make for a slow measurement and be limited by 1/f noise in a simple homodyne receiver. Bump your transmit frequency up to 30 GHz and now the signal is at 200 Hz, and now you have a radar speed detector.
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