Enclosures that are not perfectly conducting are still good Faraday cages as long as the charges can redistribute themselves fast enough to cancel the internal fields.
For static electric fields, almost any material will look like a conductor, since the free charge can slowly reposition itself. Because there are no free magnetic charges, it is not possible to terminate lines of magnetic flux on a shield. At frequencies where the term under the radical is negative, the propagation constant is imaginary and fields do not propagate. Shielding principle "low-frequency alternating electrical fieldsLow-frequency alternating electrical fields dispose of a source, an energised cable for example, and of a field-ending, usually surfaces with grounding potential (0V). Shielding principle high-frequency electromagnetic waves (HF)“High-frequency electromagnetic waves can be shielded either by reflection at highly electrically conducting surfaces (mostly metallic) or by absorption in adequate materials (mostly building material). You can cover over your shielding materials with almost any decorative medium that you like. Starting with the area with the highest field, place the meter in a location which you can find again after shielding is installed.
Should you fail to get assistance for the power company (likely), you may be tempted to consider shielding.
You will have to use magnetic shielding alloys to shield these magnetic fields and you have a choice of several methods.
Unlike X-rays, sound, light or bullets, magnetic field lines must travel from the North pole of the source and return to the South pole. Now it is easier to see why a magnetic shield in the shape of an enclosure (sphere, box, tube, etc.) offers much better shielding than a flat shape or partial enclosure. Similarly, if the source of the field is outside of the enclosure, the magnetic field lines will travel through the material of the enclosure on their way back to the source, never finding it more efficient to permeate the air space inside the enclosure. An important consideration when shielding magnets is that the magnets will be attracted to the shielding material. The proper number of layers will depend on the strength of the magnets, the distance between them, and the size and shape of the shield. While this is not technically possible, it is possible to distort the magnetic field lines around one pole of a magnet. 1] Start by using a gauss meter to determine IF you have high magnetic fields where the people are.
A single unshielded, unfiltered wire penetrating a shielded enclosure can completely eliminate any shielding benefit that the enclosure otherwise provided.
Fields at any frequency can penetrate the opening in the TEM mode, so there is no benefit to using a thick aperture if a wire penetrates the aperture. Thus the field is regional When field producing circuits are layed in walls, ceilings and floorings and measurements concerning installation and circuit are not applicable there is still the possibility to shield affected areas by laying electrically conducting wallpapers, fleeces and emulsion paintings and professional grounding.
But the magnetic field from a powerline varies from moment to moment depending on how much current is flowing in the wire at the time. Make sure you carefully survey every proposed location for your house to make sure the fields are actually sufficiently lower at the new location that you are considering.
But because shielding materials are conductive, be very careful to avoid allowing them to come into contact with electric wires to avoid a shock hazard. Keep in mind that with magnetic fields, you can either shield the source of the offending field, or shield the thing(s) that you wish to protect. There, you will find a donut shaped magnet, proportional in size to the size of the speaker, over which you will place a cup shaped shield. This outer layer will "absorb" much of the field which has evaded the first layer and yield a very high degree of attenuation.
Unlike the way a lead shield stops X-rays, magnetic shielding materials create an area of lower magnetic field in their vicinity by attracting the magnetic field lines to themselves. Remember to think about the magnetic field lines as travelling through the shield more easily than through air. In a car, the magnetic field profile will be different at highway speed compared to idling. It is important that the meter position be repeated in exactly the same position (before and after shielding), as changing the meter position will change the readings. The AC magnetic field is only present when the speaker is activated, and varies in frequency and strength with the pitch and volume of the sound produced. Note that attenuation will be greatest close to the speaker magnet, where the field is strongest (most interfering) anyway. But if a material with a higher permeability is nearby, the magnetic field lines, efficient creatures that they are, will travel the path of least resistance (through the higher permeability material), leaving less magnetic field in the surrounding air.
The best shielding strategy in any given application depends on a number of factors including the electrical characteristics of the circuit or system being shielded, physical constraints (e.g. 3(b) shows how grounding a heatsink attenuates the electric field between the heatsink and a printed circuit board ground plane.
Notice also, that the magnetic field lines are more concentrated near the wire, and less concentrated as the distance to the wire increases. Either field alone could be within tolerable limits, but could possibly exceed tolerable limits when combined.
One rule of thumb that is used by some experts is that you should limit your exposure to 60 Hz magnetic fields which are in excess of 2.5 mG. Therefore be sure to check the position with a Pocket Magnetometer or digital DC Gaussmeter. 2(b)] can substantially reduce these potentials by altering the path of the electric field lines and preventing the stronger field lines from reaching the victim circuit. Note that the eddy currents cause the magnetic flux to be diverted around the plate and significantly reduce the coupling to the circuit. Note that a shielding enclosure can be very effective even when it has a significant amount of open area as long as each individual aperture is much smaller than a wavelength.
If the field is time varying, there will be a current on the surface of the conductor as these charges move back and forth. Key concepts for practical electric field shielding are choosing a location that will intercept the stronger field lines and choosing a suitably conductive shield material.
Intercepting electric field lines with a conductive shield is primarily a matter of visualizing the field lines that are potentially responsible for the unwanted coupling and positioning the shield so that it blocks these fields.
Currents induced in a conducting material by a time-varying magnetic field in this manner are called eddy currents. In order to divert a magnetic field with a conductive plate, it is important to be able to develop sustained eddy currents. Note that it is important for the magnetic material shield to divert the magnetic flux all the way around the object being shielded.


At high frequencies it is possible for the currents induced on the shield to radiate as well as (or possibly much better than) the original source of the fields.
A seam that optically appears to be well sealed can often disrupt the flow of surface currents significantly causing a major breach in the shielding enclosure.
This model does not account for how the field was set up at one end of the opening or how efficiently it is radiated from the other end. Shielding principle low-frequency alternating magnetic fields (NF)Low-frequency magnetic fields result from flowing electricity in cables, circuits and electrical consumers.
Shielding due to reflectionThe arriving electromagnetic wave is reflected by highly electrically conducting materials such as paints, fleeces, fabrics and wallpapers  according to the laws of optic. Next, for each “hot spot”, sweep around that immediate area looking for the source of the high field. Complicated shapes and multiple barriers in a vehicle may not permit the placement of shielding in all locations that need it. Compared to magnetic field shielding, shielding a home from cell tower radiation is reasonably straightforward.
Note:Unlike the bucking magnet method, these shielding methods do not alter the sound quality of the speaker. Again, the proper number of layers will depend on the strength of the magnets, the distance between them, and the size and shape of the shield.
2] If you do have high fields, use the gauss meter to determine where the sources are located.
In large enclosures with very stringent shielding and thermal requirements, it may be necessary to further reduce the amount of energy that can escape through any given aperture. Gaps under doors, joints between shield sections, and even pinholes from sewing shielding material can permit these high frequency signals to penetrate. Some research has shown that harmonics (higher frequency fields), radio-frequency signals in the line, and power spikes may have more to do with health effects than just the normal 60 Hz magnetic field.
This is a device which constantly monitors the incoming field and produces an equal and opposite cancellation field. This material has the ability to "absorb" the initial blast of the field without saturating and becoming useless, but it will only give a limited attenuation. The material is thick enough to provide good shielding, but still can be cut with a scissors and shaped by hand. A source within the shield will produce field lines which will travel through the air immediately surrounding the North pole until they reach the shield. It is convenient to divide enclosure or component shields into 3 categories: electric-field shields, magnetic-field shields and shielded enclosures.
Remember that the field will vary according to how much current (not the voltage) if being carried by the powerline. A good, inexpensive way to check for shield performance and the best position for the shield is to use a Pocket Magnetometer.
As long as charges are able to move freely enough to reorient themselves as fast as the field changes, cancellation of the external fields will be achieved. For this reason, conductive materials are generally poor magnetic shields at low frequencies (e.g.
So the only way to know how strong the field is at a given distance, AT ANY PARTICULAR MOMENT, is to measure it with a gaussmeter. The magnetic field from the two sources can deflect the electron beam in a cathode ray tube monitor (TV) causing distortion of the image, sometimes called jitter (and possible damage to the equipment). Conductive magnetic shields are also ineffective if they have slots or gaps that interrupt the flow of the eddy currents.
The answer relates to the fact that there are different kinds of shielding for different applications.
Most metallic enclosures without significant seams or apertures provide excellent electric field shielding over a wide range of frequencies. 1(a) field lines may terminate on other conductors resulting in potential differences between these conductors as indicated in Fig.
However for high-frequency electric fields, the conductivity of the shield material must be high enough to allow the charge to move quickly back and forth. However any electric field on the surface of a good conductor will cause currents to flow in that conductor. Both the incident field and the magnetic field created by the eddy currents are shown in Fig. Since the eddy currents are driven by time-varying fields, a conductive plate cannot divert a static magnetic field.
Since these materials have a reluctance much less than air, magnetic field lines can effectively be rerouted by providing an alternative path through a permeable material such as steel or mu-metal.
A plate of magnetic material above or below the circuit board would provide no shielding at all.
In order for the enclosure to provide shielding, currents must be able to flow on the surface unimpeded. Therefore, by itself, it cannot be used to determine the shielding effectiveness of any particular shield. The concept of radiofrequency (RF) shielding is simple: simply put a barrier between the source of the radiation and the area you want to protect.
Keep adding shielding until either you are satisfied with the results, or you find that adding more shielding does not yield any further decrease in readings. For doors, walls, floors and ceilings, CuPro-Cote or Y-shield conductive paints offer very good shielding and are very convenient.
Because you will be placing the shielding material in close proximity to this strong magnetic field, you will have to take saturation into account. 3] Shielding involves applying Magnetic Shielding Foil (0.010 thickness) or Giron over the source of the offending field. The subfloor can be painted, or a shielding fabric layer can be applied before the final floor surface is installed.
Recent research has shown that the corona field around high tension lines can ionize the air around the lines.
This will protect the monitor from external fields produced by the speakers and any other sources. For these reasons, enclosing either the source of the field, or the thing(s) that you wish to protect from the field, offers the most effective use of the shielding material, and is usually the most cost efficient as well! Electric fields generated within the volume either terminate on objects within the enclosure or on the inner surface of the enclosure wall as illustrated in Fig.
Note: There will be a position somewhere between the back of the speaker magnet and the front of the TV which will yield "near perfect" shielding.


You can paint over the shielding paint with any color of latex paint to achieve any appearance you like. Move either towards the speaker or towards the TV from this point and you will loose shielding effectiveness. Naturally, some situations may require shielding of both the back and the sides of the speaker cabinet. A vertical magnetic field from an electric motor couples to a small circuit board resulting in interference.
Even if the field is slowly varying, losses in the conducting plate will cause the eddy currents to dissipate allowing the magnetic flux to penetrate the plate. The attenuation calculated in (38) should be added to the shielding effectiveness that would be obtained from the same aperture configuration in a thin shield. Equation (38) cannot be used to calculate the shielding effectiveness directly; and there is no reason to reduce the Equation (38) value for multiple thick apertures if you are starting from the attenuation provided by multiple thin apertures. First, let's understand that the magnetic fields from a single conductor wire emanate from that wire in a pattern that could be described as concentric cylinders.
Remember that the attenuation spec for a shielding material is how much radiation penetrates through the shield. For the layers closest to the magnet, choose a high saturation material such as MagnetShield. It can have very high shielding performance, and in general should be grounded for peak efficiency.
The first step should always be to record readings of the magnetic field strength over a period of a few days using a reliable AC Gaussmeter to find out if you truly have a problem. Simply stack enough layers between the 2 magnets until the attraction to the shield balances the repulsion between the magnets. Covering the keyboard area of the laptop with a shielding fabric such as High Performance Silver Mesh will reduce electric fields from these areas while still allowing you to see the keyboard. Every vehicle will have multiple sources of magnetic field, some of which may be in areas that are difficult to access for shielding.
This means that if you "shield" one pole of a magnet, you are basically relocating the place where those magnetic field lines emerge into air. Note that field lines terminating on a conductor imply there is a negative charge induced at that point.
Now, understanding that magnetic shielding "works" because it is a better "conductor" of magnetic field lines than air or just about any other material, let's see what happens with 2 different shield designs. In many ways, RF behaves much like visible light, and RF shielding materials behave much like two sided mirrors.
Wrap the MagnetShield around the speaker magnet (notice that it is attracted to the magnet) in a cylinder shape.
This will not help you determine the “hot spots” and therefore the location of the offending sources of the field.
The shield does not need to be in contact with its respective magnet, but it must be held fixed in position relative to its magnet.
Free charge on the enclosure relocates itself as needed to exactly cancel the fields within or external to the enclosure.
If your largest piece of shielding is not wide enough, you can place pieces next to each other, with 1-2 inches of overlap where they meet. In the cross section image at right, we see that the magnetic field lines that would have occurred at the radius of the shield will exist INSIDE the shield. You can use the shield style that goes right onto the phone, or you can line a pocket or purse with a shielding fabric. If you need maximum field reduction, or cannot open the speaker cabinet, or you simply want to take the easy route, you can simply place flat magnetic shielding alloy between the speaker and the TV. As you can see from the image below, the magnetic field lines which intersect the flat shield will be compressed into the shield, leaving less magnetic field on either side of the flat shield. You could also use a shielding fabric such as Nickel Copper RipStop or AL60 behind the wall. Placing magnetic shielding material around your body is possible, but again not very practical. If the edges of the shield are bent slightly TOWARDS the source, the high field area at the edge of the shield will move further away from the "shielded area". Conventional speakers incorporate both a permanent magnet and an AC magnetic field to produce sound.
The magnetic fields at the side of the speaker magnet have different characteristics compared to at the back of the magnet, and different shielding materials are required.
Use screws, pop rivets, tape, glue or any other mechanism you can think of to keep the shielding material in place. In conclusion, for net current, flat (or nearly flat) shielding is more effective for fields from wiring in the area adjacent to the shield.
A high saturation material of significant dimensions such as 36"x15" MagnetShield Plate is required here.
If the floor in already installed, you can lay shielding fabric, and cover it by a large area rug or sheet linoleum. There is not a lot of scientific evidence to support this recommendation, but it is based on the Swedish recommendation for exposure to ELF fields from computer monitors. The safety or danger of a magnetic field from a powerline depends on more than just the strength of the field. Laptops produce at least two types of electromagnetic fields: AC electric fields and AC magnetic fields. High readings up close to the powerline are meaningless if the field inside your house is low. While there are official standards for exposure to electric and magnetic fields, they are based on the amount of field needed to cause immediate harm.
To shield the electric fields from the laptop, use ClearShield or VeilShield Fabric to cover the screen. To achieve a reasonable degree of shielding, you would have to create a metal vault around your house, using thick metal plates with no windows. To shield the magnetic fields we recommend that you form a tray under the laptop with Magnetic Shielding Foil if you will have the laptop near you.
If you are using a remote keyboard, you can achieve much higher reduction of magnetic field by making a 5-sided box from Magnetic Shielding Foil.




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