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Canceling one electromagnetic wave with another for a part of the radius

  1. Sep 3, 2012 #1
    I was curious about electromagnetic waves and whether or not they can be cancelled in the fashion I require. First I will state a few principles that I am considering in the paragraph below and the paragraph after will pose the question in full.
    A Wireless router produces a signal that emanates through all points in space and the signal gets weaker as the radius (from the point of origin to a laptop) increases. From my understanding, the electromagnetic field of a straight wire with an AC current has this same inverse relationship between field strength and radius from the signal generated.
    Let's assume that I require a magnetic field at (4 ft.< Radius < 5 ft.) from the origin of the electromagnetic signal. Meaning, I want a field that has a 1 foot depth and I want to generate it from a single point but cancel the electromagnetic wave from r=1ft to r=4ft.Now for the question.
    Let's assume I have 2 electromagnetic field generators producing opposing fields to one another. One generator, generator "A", produces a strong field (relative to generator B) allowing for the max electromagnetic propagation radius of 5 ft. A second generator, generator "B", is placed next to generator "A" and it projects an opposing signal to "A" but only up to a radius of 4ft.

    Will the canceling of the magnetic flux from generator "A" BY generator "B" for the first 4 feet of the wave have any AFFECT on the strength of the wave between radius 4ft-5ft (remember this 1ft depth is the remaining portion of the field that was not cancelled). Can this even be done? If so, can this type of wave canceling allow for a field that looks somewhat like an asteroid belt in 2 dimensions? (What I mean is that if the field was only in the x and y plane...would there be an inner radius lacking a field with a rim of field density on the outside...similar to an astroid belt that has a space between the belt and the planet. This is merely and example of shape.)
  2. jcsd
  3. Sep 3, 2012 #2


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    Hi jnels005
    welcome to PF

    You cannot control a EM field with any sort of accuracy like that The EM field will continue outwards from the antenna ( or other generator) and will attenuate according to the inverse square law. The only other things that will affect the strength of an EM field is absorption by objects in the path of the field.

    You will NEVER get an EM wave to radiate to 5ft 25ft 1000ft and stop. it doesnt work like that.
    You also dont seem to realise what an EM wave / field is. Electro-magnetic radiation, say a radio signal as in your WiFi router transmitter, are just packets of photons being radiated from a source. And because photons are chargeless, they are not affected by a magnetic field. And as far as I'm aware, because an EM field is packets of photons, you cannot separate the electric and magnetic fields from each other.

    As both generators are RF transmitters, they both will be generating an EM field. If the 2 radio transmitters are on the same frequency, then you can get interference patterns forming by the interaction of the 2 waves which can produce peaks and nulls of the 2 waves.
    if the frequencies are different then you can get mixing products of the 2 frequencies ( waves)
    This can be a major problem in multi station radio transmitter sites like I used to work on. And the site designers have to be careful what frequencies to use to avoid those mixed products. The 2 common terms for that process is called 1 - heterodyning or 2 - intermodulation

  4. Sep 3, 2012 #3
    Thank you for your reply. I actually was reiterating what a professor was mentioning about magnetic shielding on research buildings, however the only type of shielding that I found was material based not interference based. Also, it's obvious that a wave will not move a certain amount of distance and just stop...that is not the goal. The goal was to shield the material that was closest to the wire with interference while allowing the rest of the "B" field to continue as normal. Maybe I wasn't specific enough. I have not revised this material in quite a while and it is for that reason I ask.
  5. Sep 3, 2012 #4
    I heard that radio signals could be jammed and i thought the same principle may have applied to this
  6. Sep 4, 2012 #5


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    What you can do is to produce a region in which a signal is 'nulled out'. This can be done 'passively' with a suitably placed reflector or 'actively' by receiving and re-transmitting the signal. Sound cancelling headphones work on the same principle but they have a very easy job because they only need to produce a null in a very small region of space (right in the ear canal). Cancelling RF signals over a large region is not possible because the requirement is that the two signals need to be equal in amplitude and in exact antiphase (a delicate balancing act).
    Aamof, this is, in principle, what directional antennae do when they eliminate unwanted signals from a direction different from that of the wanted signal.

    Referring to your OP, you could, in principle, produce a region of low field strength in a nearby 'wanted' place (by some complicated transmitting antenna design or cable layout) but still transmit power in that direction to more distant objects. The details would be crucial before you could be sure it would work in any particular case.
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