Question is the title of the thread.

 

Pretty vague but I would think RF refers to electromagnetic wave while magnetic refers to magnetic field.

The difference is that an electromagnetic wave is a propagating combination of electric and magnetic components (with a ratio of E/B = 377 which is the 377 ohm free space impedance), also known as the "far field".

Magnetic field is "near field" with little to none electric field (ratio much less than 377, or B >> E).

How you shield against them is very different because magnetic field is only affected by magnetic materials like steel. Aluminum and copper do almost nothing to stop it but work pretty well against an EM wave.

 

Ah, thanks for the reply. I asked the question because I am trying to replicate the MIT witricity experiment but some people are warning me that I might get into trouble for using some frequencies that are illegal to radiate. However, If I'm not mistaken, the MIT team didn't use radio frequency radiation, but they used magnetic coupling...how is this possible??

 

Think induction. Try the article and some of the links - http://en.wikipedia.org/wiki/Inductive_coupling

 

As a side note, people seldom get into trouble with the FCC unless one of two thing happens.

1. They willfully ignore written communications from the FCC
2. They continue the offending activity after being warned.

A classic case of this behavior was Jack Gerritsen formerly KG6IRO. Read the story of what happened to him for an idea of how hard you have to try and how far you have to go to get into real trouble.

 

Coming up on a backup freq just before a shuttle launch gets a man with a DF rig at the door right away. The offending transmitter was naval, and it was a valid data link being established.

The FCC was there within 30 minutes. It may have to do with the perceived significance of the interference.

 

Okay, so my next question is shaped by your input and it is as follows:

How can I create a magnetic frield without making a significant electric field so that I don't get into trouble with the FCC? I heard that you cannot get into trouble for magnetic field radiation, but electric field you can. Can someone explain the reason why please.

Thank you!

 

M.i.t could have permissin,you don,t. M.I.T. Is Involved with
government projects.

 

yeah but MIT didn't get permission to do the project which was just near-field magnetic radiation, they got permission because they had to step up the power so much that the room was filled with power so they can transfer the electricity over 2 meters away. I'm wondering how you can only propagate one of the fields while leaving the other field very minimal in terms of its radiation...

 

I didn't know the FED's writ ran to Toronto?

 

A PM exchange between us before this post...

That is great sir! Thank you for the reply. Wookie's signature also noted. Thanks again.

Hey Bill,

I was wondering why the Feds would show up at your door if you play around with some frequencies? I saw you mention it somewhere to be careful when doing such experiments, but can you please tell me more about it? I'm working on the witricity project, but I don't think I will be using radio frequency radiation, but its more like using magnetic fields and magnetic field radiation. Please correct me if I'm wrong. My current circuit consists of a

signal generator,
two coils,
two capacitors,
1 LED

I have the signal generator in parallel with the coil and the capacitor, and from the formula I found out their resonant frequency (f = 1/2PI*root(LC)), which was around 10 MHz. Could I get into trouble for this?

Thanks Bill!

Wookie's signature puts it best...

General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input.

I will say this, as long as it is magnetic coupling you are OK, but if you broadcast as RF you will be running into trouble. It is usually very hard to create one without the other, but it can be done.

FCC specs state you may not broadcast more than 10 mw RF power or have an antenna greater than 10 feet. I don't know what Canadian specs are, or the governing authority they use, but I'll bet it is something similar.

Like I said, Canada has to have their own governing body, but I have no idea who it is or what their name is.

 

Ah, well now that I know that this experiment is not power radiation but magnetic coupling, I shouldn't be too worried. The problem now is to couple two coils almost entirely through magnetic coupling with minimal current input and not through radio frequency radiation.

 

Question is the title of the thread.

The main difference is a magnetic field doesn't radiate. It's just there.


eric

 

The main difference is a magnetic field doesn't radiate. It's just there.


eric

Even fluxing fields? I've been curious exactly how you could separate the two.

There are other ways to transmit wireless. For example, lasers and solar cells. Even tight beamed microwaves (though you could run afoul the requirements). Interesting thing about microwaves, they can be very efficient on transmit and receive. The problem with that scenario is they are also dangerous.

 

Yeah, they can cook you alive. Aren't they planning on setting routers that will make a Wide Area Network (WAN) in big cities like New York and Toronto with the WiMAX technology? They're going to use microwaves to transmit the data and I was wondering how that will work.

 

I don't remember the modulation name, but if it is what I think it is they are very narrow microwave pulses, they have excellent range but no real power.

 

Yeah they're estimating about 20-25 km with Non-line-of-sight, and about 40-50 km with line-of-sight in the WiMax technology. This will be great! Internet everywhere in the city ... awesome!

 

Even fluxing fields? I've been curious exactly how you could separate the two.

From what I understand you don't.
By Maxwell's equations at any point where you have a time varying E field you get a B field and vice versa and hence you will have a propagating wave.

The fields vary depending on distance from the source, where nearby you will have predominantly B or E and as you move further away it turns into a plane wave (by appropriate attenuation of the stronger field, the plane wave component should still exist even near by)

It becomes a question of radiating efficiency I guess, where to radiate effectively the structure needs to be approaching a wave length in size.

 

Okay, so I don't understand what the difference between a series LC and a parallel LC circuit is. The only thing I understand is that in a series LC circuit, you get zero impedance at resonance, and in a parallel LC circuit you get infinite impedance at resonance.

My question is: Why did MIT and a bunch of other groups I've seen use a parallel LC circuit to magnetically couple two coils and power their loads from 2-3 feets away? Doesn't a series LC circuit provide the highest current and voltage because of its near-zero impedance? Wouldn't a really high current induce a much stronger magnetic field into the secondary coil? I'm confused as to why they all used a parallel LC circuit ...

 

Can you link us to an example?