What's the difference between RF and magnetic radiation?

Discussion in 'Wireless & RF Design' started by zero_coke, May 29, 2010.

  1. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
    294
    1
    Question is the title of the thread.
     
  2. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    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.
     
  3. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
    294
    1
    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??
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
  5. Papabravo

    Expert

    Feb 24, 2006
    10,148
    1,791
    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.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    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.
     
  7. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
    294
    1
    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!
     
  8. loosewire

    AAC Fanatic!

    Apr 25, 2008
    1,584
    435
    M.i.t could have permissin,you don,t. M.I.T. Is Involved with
    government projects.
     
  9. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
    294
    1
    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...
     
  10. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    I didn't know the FED's writ ran to Toronto?
     
  11. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    A PM exchange between us before this post...

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

    Thread Starter Active Member

    Apr 22, 2009
    294
    1
    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.
     
  13. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
    287
    The main difference is a magnetic field doesn't radiate. It's just there.


    eric
     
  14. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    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.
     
  15. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
    294
    1
    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.
     
  16. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    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.
     
  17. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
    294
    1
    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!
     
  18. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    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.
     
  19. zero_coke

    Thread Starter Active Member

    Apr 22, 2009
    294
    1
    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 ...
     
  20. Ghar

    Active Member

    Mar 8, 2010
    655
    72
    Can you link us to an example?
     
Loading...