Vibrating a loop with current - will it radiate?

[Rjay78]

Hi, I am new here so hopefully this is the right place to ask a question like this. Forgive me if I have trampled a rule. If I have, please let me know what I should do to ask this question.

If you take a wire with a flowing DC current and vibrate it, can it radiate as a radio transmission? For instance, if you vibrate a conductor carrying a small current, at a frequency of 1MHz, will it radiate a 1MHz transmission that can be received by an antenna?

This is of course different from a 1MHz oscillating current in the wire, which would clearly radiate a 1MHz E & B Field.

I have read and read on this but I cannot seem to get a clear answer (at least one that I can glean from my reading). I would very much appreciate your thoughts on this.

Best Regards,

Rjay

 

[mfb]

In general yes, but the emission will be negligible. There will be some vibration mode that doesn't radiate if you hit it perfectly.

Nearly everything radiates, but unless you have a suitable design it is negligible.

 

[berkeman]

Hi, I am new here so hopefully this is the right place to ask a question like this. Forgive me if I have trampled a rule. If I have, please let me know what I should do to ask this question.

If you take a wire with a flowing DC current and vibrate it, can it radiate as a radio transmission? For instance, if you vibrate a conductor carrying a small current, at a frequency of 1MHz, will it radiate a 1MHz transmission that can be received by an antenna?

This is of course different from a 1MHz oscillating current in the wire, which would clearly radiate a 1MHz E & B Field.

I have read and read on this but I cannot seem to get a clear answer (at least one that I can glean from my reading). I would very much appreciate your thoughts on this.

Best Regards,

Rjay

Welcome to the PF.

Yes, it will radiate, but not very efficiently. Accelerating charges radiate EM, but the less the acceleration, the less the EM radiation launched.

Actually, I may need to amend my answer -- electron drift is pretty small at low currents, so mechanical displacement may be much larger at higher frequencies. Let me do some calcs...

 

[berkeman]

Upon further review...

The mechanical vibrations will be at low audio frequencies, which will not couple well to EM radiation.
Very interesting question, though...

 

[mfb]

Actually, I may need to amend my answer -- electron drift is pretty small at low currents, so mechanical displacement may be much larger at higher frequencies.

You displace electrons and protons in the same way. The only radiating part is the changing magnetic field, a higher order effect.

AOMs driven by piezos can vibrate with MHz frequencies easily, you can let a cable vibrate in the same way.

 

[atyy]

A current loop is called a magnetic dipole. You can find approximate formulas for the radiation from a rotating dipole eg.
http://www.physicspages.com/2015/01/20/radiation-from-a-rotating-dipole/
http://www.cv.nrao.edu/course/astr534/Pulsars.html

I don't think it is practical to use it for radio transmission, but I recently saw http://www.darpa.mil/attachments/AMEBAProposersDay_FINAL.PDF (I don't understand how they got those numbers).

 

[tech99]

My understanding is that radiation arises from the acceleration of charges. Usually this is the acceleration of the free electrons near the surface of a metal. I also understand that a magnetic pole will radiate if it is accelerated, and in your case I assume you will vibrate the loop mechanically at 1 MHz. But I have not seen proof of this and no one has managed to do the experiment so far as I know.
The difficulty is that if we synthesise the moving magnet by using varying currents in a loop, the radiation effect can also be explained from the acceleration of the electrons.

 

[Rjay78]

A current loop is called a magnetic dipole. You can find approximate formulas for the radiation from a rotating dipole eg.
http://www.physicspages.com/2015/01/20/radiation-from-a-rotating-dipole/
http://www.cv.nrao.edu/course/astr534/Pulsars.html

I don't think it is practical to use it for radio transmission, but I recently saw http://www.darpa.mil/attachments/AMEBAProposersDay_FINAL.PDF (I don't understand how they got those numbers).

atyy, Thank you for these sites. This really helps! - Rjay

 

[Rjay78]

My understanding is that radiation arises from the acceleration of charges. Usually this is the acceleration of the free electrons near the surface of a metal. I also understand that a magnetic pole will radiate if it is accelerated, and in your case I assume you will vibrate the loop mechanically at 1 MHz. But I have not seen proof of this and no one has managed to do the experiment so far as I know.
The difficulty is that if we synthesise the moving magnet by using varying currents in a loop, the radiation effect can also be explained from the acceleration of the electrons.

Welcome to the PF.

Yes, it will radiate, but not very efficiently. Accelerating charges radiate EM, but the less the acceleration, the less the EM radiation launched.

Actually, I may need to amend my answer -- electron drift is pretty small at low currents, so mechanical displacement may be much larger at higher frequencies. Let me do some calcs...

Thank you berkeman.

 

[Rjay78]

My understanding is that radiation arises from the acceleration of charges. Usually this is the acceleration of the free electrons near the surface of a metal. I also understand that a magnetic pole will radiate if it is accelerated, and in your case I assume you will vibrate the loop mechanically at 1 MHz. But I have not seen proof of this and no one has managed to do the experiment so far as I know.
The difficulty is that if we synthesise the moving magnet by using varying currents in a loop, the radiation effect can also be explained from the acceleration of the electrons.

Thank you tech 99

 

[Rjay78]

You displace electrons and protons in the same way. The only radiating part is the changing magnetic field, a higher order effect.

AOMs driven by piezos can vibrate with MHz frequencies easily, you can let a cable vibrate in the same way.

Yes, this is a great way to mechanically move a mirror and change light. I can think of a dozen ways to actuate it but I am struggling with how to detect it. The current is small e.g. pA.

 

[Rjay78]

My understanding is that radiation arises from the acceleration of charges. Usually this is the acceleration of the free electrons near the surface of a metal. I also understand that a magnetic pole will radiate if it is accelerated, and in your case I assume you will vibrate the loop mechanically at 1 MHz. But I have not seen proof of this and no one has managed to do the experiment so far as I know.
The difficulty is that if we synthesise the moving magnet by using varying currents in a loop, the radiation effect can also be explained from the acceleration of the electrons.

Hence, the trouble at finding some good literature. Although, I can see from atyy's comments that once I found his links, then there is world of stuff written on this.

 

[Rjay78]

In general yes, but the emission will be negligible. There will be some vibration mode that doesn't radiate if you hit it perfectly.

Nearly everything radiates, but unless you have a suitable design it is negligible.

Thank you mfb. -Rjay78

 

[tech99]

atyy, Thank you for these sites. This really helps! - Rjay

In Post No. 8, the first reference relates to a crossed pair of ordinary eg half wave, dipoles, and is not just a rotating magnetic field. As I have mentioned before, we cannot get pure rotating magnet action by using fluctuating currents, as that involves acceleration of charges.
The second reference relating to pulsars seems to attribute the radiation to charges being accelerated into the centre. So although it possesses powerful magnetism, this is not the source of radiation.