How Do Helmholtz Coils Affect Electromagnetic Beam Transmission?

[A6Bnito]

What would the behavior of an electromagnetic beam (transmission) be if directed through a Helmhotz coil? Is there a different result with a 4 or 6 coil configuration (all orthogonal)? Is there a direct response depending of frequency of the EM transmission (10kHz - 10 GHz) and current being run through the coils? Is there a different response with the B field of the beam being additive (constructive interference) to the coil field as opposed (destructive interference) to being contrary to the field coil?

Would the transmission simply be deflected, interfered, or potentially blocked?
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[Baluncore]

Welcome to PF.

There are two effects here.
Firstly, the space is linear, so the magnetic field and the transmission do not interact.
Secondly, the coils are conductive loops, so they will act as scatterers, that is they will receive and retransmit the transmissions in other directions. The wavelength of the transmissions and the dimensions of the coils will interact in a way that could be modelled given the dimensions and the wavelength of the transmission.

 

[hutchphd]

What is an "electromagnetic beam"?
The answer to all of your questions is yes, I believe, but why do you ask?

 

[A6Bnito]

By electromagnetic beam I am referring to an EM wave (can be a radio or radar transmission, a strong RF impulse as in a Marx generator (electronic noise/pulse). I am asking to determine if a helmholz coil (if large enough), may act as a barrier entry for a faraday cage. Depending on current flow in the coil(s) there could be either constructive or destrucive interference, would that be strong enough to impact an EM pulse, or is the frequency range too narrow (from the coil's perspective) to offer a broad range solution.

With regard to space being linear, if a transmission is focused on an area of space that contains 6 orthogonal helmholz coils, what, if any, impact would result in the associated B fields (hence the E field) of the transmission or the coild fields. If at the same frequency would there not be some effect?

 

[A6Bnito]

Welcome to PF.

There are two effects here.
Firstly, the space is linear, so the magnetic field and the transmission do not interact.
Secondly, the coils are conductive loops, so they will act as scatterers, that is they will receive and retransmit the transmissions in other directions. The wavelength of the transmissions and the dimensions of the coils will interact in a way that could be modelled given the dimensions and the wavelength of the transmission.

Thank you. So, results could be modeled, but would likely result in a chaotic dispersion as a result of the interaction is what I gather from your reply. Results would depend on subjected frequency, coil frequency, and current flow in the coils as I read it.

 

[hutchphd]

You do not need Helmholtz coils.
Anything that contains electric charges will affect the result. The details will likely not require Helmholz coils: they are often employed to make a volume of uniform field. Not particularly relevant.

 

[Baluncore]

I am asking to determine if a helmholz coil (if large enough), may act as a barrier entry for a faraday cage.

That is most unlikely. The Helmholtz coil becomes an aperture, with induced peripheral currents. Those peripheral currents flow along edges, like a slot antenna, and so guide the external energy into the Faraday cage.

If at the same frequency would there not be some effect?

There will be some effect, but you will not like it. The result will be the sum of the combined fields, at that one frequency, with interference, good and bad.

You would do better by filling the hole with a double layer of metal mesh or sheet.

 

[A6Bnito]

That is most unlikely. The Helmholtz coil becomes an aperture, with induced peripheral currents. Those peripheral currents flow along edges, like a slot antenna, and so guide the external energy into the Faraday cage.There will be some effect, but you will not like it. The result will be the sum of the combined fields, at that one frequency, with interference, good and bad.

You would do better by filling the hole with a double layer of metal mesh or sheet.

Thanks appreciate the insight.

 

[A6Bnito]

You do not need Helmholtz coils.
Anything that contains electric charges will affect the result. The details will likely not require Helmholz coils: they are often employed to make a volume of uniform field. Not particularly relevant.

Thanks appreciate your taking the time and your response.

 

[DaveE]

I am asking to determine if a helmholz coil (if large enough), may act as a barrier entry for a faraday cage.

No

Depending on current flow in the coil(s) there could be either constructive or destrucive interference

Yes, but only for a particular transmission field that matches the fields from the coil at every location, at every instant. This isn't really possible. At any specific location you can (maybe?) cancel the field with destructive interference, but it would also have to work at other locations. The "shape" of your blocking field would have to match the "shape" of the incident field exactly (but out of phase). Plus you would have to know a priori exactly how to drive that coil at each instant in time to match.

But... Here's what you can do. Make billions of tiny coils that cover the aperture at each point and have each cancel the externally imposed fields at their location. The mini-coil next door would cancel the field at that place. In fact, you could have that "coil" excited by the incident radiation so that it would always be synchronized perfectly, without you have to know in advance what to do. This a crude description of the metallic shields that faraday cages are made of. Each of those "coils" is sort of like the metal atom(s). It's kind of magic, IMO. And essentially impossible for you to replicate with one big coil driven from your electronics.

 

[sophiecentaur]

Thank you. So, results could be modeled, but would likely result in a chaotic dispersion as a result of the interaction is what I gather from your reply. Results would depend on subjected frequency, coil frequency, and current flow in the coils as I read it.

If, by the "coil frequency" you mean the frequency of the power being fed through the coil terminals, then no. There is no interaction. If you mean the frequency of any natural resonance in the coil then it could affect the 'beam.

That is most unlikely. The Helmholtz coil becomes an aperture, with induced peripheral currents. Those peripheral currents flow along edges, like a slot antenna, and so guide the external energy into the Faraday cage.

Depending on the geometry of the coil, the induced currents could either reflect or transmit the beam. A slot antenna of the appropriate frequency design can act as a reflector or director (as with the wire elements of a Yagi array. The slot could actually behave is if it wasn't there - just a part of the metal sheet it's cut out of.

 

[A6Bnito]

NoYes, but only for a particular transmission field that matches the fields from the coil at every location, at every instant. This isn't really possible. At any specific location you can (maybe?) cancel the field with destructive interference, but it would also have to work at other locations. The "shape" of your blocking field would have to match the "shape" of the incident field exactly (but out of phase). Plus you would have to know a priori exactly how to drive that coil at each instant in time to match.

But... Here's what you can do. Make billions of tiny coils that cover the aperture at each point and have each cancel the externally imposed fields at their location. The mini-coil next door would cancel the field at that place. In fact, you could have that "coil" excited by the incident radiation so that it would always be synchronized perfectly, without you have to know in advance what to do. This a crude description of the metallic shields that faraday cages are made of. Each of those "coils" is sort of like the metal atom(s). It's kind of magic, IMO. And essentially impossible for you to replicate with one big coil driven from your electronics.

So, a phased array of tiny Helmholtz coils... well that would be something. Nearly impossible to control, so I guess sticking with a standard shielding approach is the best option. Appreciate your thoughts and insight.

 

[sophiecentaur]

So, a phased array of tiny Helmholtz coils... well that would be something. Nearly impossible to control, so I guess sticking with a standard shielding approach is the best option. Appreciate your thoughts and insight.

Would there be any point in doing this? (I'm not sure what you mean by a phased array.) You may be taking a jump too far, here. The array elements would have to be 'tuned' , to match their size with the wavelength involved. Would that be any better than choosing holes of the appropriate size in a continuous sheet of metal (like the window of a microwave oven, which (if ti were big enough) would act like a mirror for a wide bandwidth of EM radiations.
What, exactly would you be trying to achieve? Or has this train of thought just got a bit undirected?

 

[A6Bnito]

Would there be any point in doing this? (I'm not sure what you mean by a phased array.) You may be taking a jump too far, here. The array elements would have to be 'tuned' , to match their size with the wavelength involved. Would that be any better than choosing holes of the appropriate size in a continuous sheet of metal (like the window of a microwave oven, which (if ti were big enough) would act like a mirror for a wide bandwidth of EM radiations.
What, exactly would you be trying to achieve? Or has this train of thought just got a bit undirected?

I have received the feedback and insight I was looking for, thank you. As per "phased array" using coils in a simmilar manner to a phased array radar, (DaveE's suggestion). The reason I said nearly impossible to control is the tuning aspect, not only for a simple array, but for the myriad of frequncies that could be encountered; hence my closing comment, - better to stick with traditional shielding methods. Thanks.

 

[sophiecentaur]

I have received the feedback and insight I was looking for, thank you. As per "phased array" using coils in a simmilar manner to a phased array radar, (DaveE's suggestion). The reason I said nearly impossible to control is the tuning aspect, not only for a simple array, but for the myriad of frequncies that could be encountered; hence my closing comment, - better to stick with traditional shielding methods. Thanks.

Woah there. A 'phased array' is for directing a transmitting or receiving array. Where would be the application to screening? This is a totally different rabbit hole but it is, of course, possible to direct a null in a given receive pattern. That wouldn't really be called "screening" because it's directional and would refer to a resulting signal emerging from a multiplex of feeders and would not / could not give a screened enclosure.

 

[Baluncore]

The reason I said nearly impossible to control is the tuning aspect, not only for a simple array, but for the myriad of frequncies that could be encountered; hence my closing comment, - better to stick with traditional shielding methods. Thanks.

I agree. The best cancellation array is a continuous sheet of perfect conductor. Any incident field will generate a counter current in the external surface, which perfectly cancels the incident fields into the conductive surface. That shields the inside space, by reflecting the incident energy.

 

[hutchphd]

Looked at in that "light" the whole process it quite remarkable. Isn't it lovely.

 

[Baluncore]

Isn't it lovely.

Pulchritudinous and elegant.