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About EMF Shielding

What Kind of Unit is "Ohms per Square"?

Surface resistivity is measured in Ohms per square (Ohm/). Ohms are units of resistance, but what about the square?

Engineers know that you can measure surface resistivity from one point on the surface to another using an ordinary ohm-meter. Then the units would be Ohms per distance between points or Ohm/m. But this method yields very inconsistent results, especially with surfaces that are not perfectly homogeneous.

More consistent results are obtained by measuring the surface resistivity between two bars. For simplicity, two parallel bars of equal length are placed exactly 1 length apart on the surface to be measured. They form two opposite side of a square. It doesn't matter how long the bars are, since longer bars will be placed proportionally further apart. The resistance measured between the two bars in Ohms is the resistivity of the surface in Ohms per square! Surface Resistivity Meter

What is the best way to shield magnetic fields from wiring?

Magnetic pattern around a wireFirst, let's understand that the magnetic fields from a wire emanate from that wire in a pattern that could be described as concentric cylinders. The image at right represents a cross section view of a current carrying wire. Notice the concentric circles of magnetic field lines around the wire. Notice also, that the magnetic field lines are more concentrated near the wire, and less concentrated as the distance to the wire increases.

Shielded AreaNow, 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. First, let's make a shielding cylinder around the wire. 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. However, magnetic field lines at all other radii will not be affected. Net effect: no shielding.

But what happens if we use a flat shield? 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.

Shielded Area

But also, note the following:

  • There is an area near the shield which enjoys LOWER field strength
  • The areas near the edge of the shield show HIGHER field strength
  • The magnetic fields of large radius are unaffected
  • The wider the shield, the larger the shielded area, both in width and depth

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".

In conclusion, flat (or nearly flat) shielding is very effective for fields from wiring in the area adjacent to the shield. The wider the shield, the larger the shielded area. Contact us if you have specific questions about your shield design.

PowerlinesWhat is the safe distance for people from powerlines?

The sad short answer is: there is no such thing as a safe distance.

Here are the reasons:

  1. The magnetic field from a powerline decreases with distance, for sure. 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. It will be higher during peak electricity usage times. 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. We always recommend taking multiple measurements at various times during the day.
  2. There could easily be additional sources of field. They might come from underground wires, ground mounted transformers, or even common sources within the home. They will add to the strength of the field emitted by the powerlines. Either field alone could be within tolerable limits, but could possibly exceed tolerable limits when combined.
  3. The safety or danger of a magnetic field from a powerline depends on more than just the strength of the field. 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.
  4. Sleeping

  5. The time of day that you are exposed may be very important. Some research shows that exposure during sleep may be more harmful than exposure during waking hours as it affects the melatonin balance which is a hormone that, among other things, fights cancers cells.
  6. Whether you are located upwind or downwind of the powerline may also be important. Recent research has shown that the corona field around high tension lines can ionize the air around the lines. This ionized air has been thought to attract and concentrate radio-active particles and automotive pollutants that can be harmful.
  7. There are probably other factors which determine how much EMF your body can tolerate, such as genetic predisposition, how much exposure you receive at work or school, your age, your exposure to harmful chemicals (pesticides, preservatives, etc.) which may be activated by the EMF, your overall health, and so on.
  8. Most important of all, scientists simply do not yet know how much exposure is safe or harmful.

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. There is plenty of evidence to show that biological effects occur at levels well below the standard limits. In the end, we are each left to decide how much exposure we are willing to accept. 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. 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.

You should get a gaussmeter and make some measurements. At least find out if the fields from the powerline exceed the 2.5 mG guideline.

What can I do (short of moving and saddling some other poor person with this problem) to reduce my exposure to a powerline outside my window?

In general, there are 5 ways to reduce your exposure to magnetic fields:

  1. Reduce power to the source (if there is no current, there will be no field)
  2. Apply shielding to the source
  3. Apply shielding to yourself
  4. Increase the distance between yourself and the source
  5. Cancel the incoming field with an equal and opposite field

When it comes to powerlines, the options are limited as you do not have control over the powerlines themselves. 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. Remember that the field will vary according to how much current (not the voltage) if being carried by the powerline. Also, remember that the only relevant readings are those taken where people actually spend time. High readings up close to the powerline are meaningless if the field inside your house is low.

LinemenArmed with this documentation, your next step should be to contact the utility company that owns the powerlines. Explain your concern and ask for their help in reducing your exposure. If the utility company wants to, they can do several things to lower your exposure:

  • Relocate the lines (further from your home or even underground)
  • Reconfigure the lines to achieve better field cancellation between lines
  • Re-route the power to another line so that less current flows through the line near your home.

VaultShould you fail to get assistance for the power company (likely), you may be tempted to consider shielding. Naturally, the most effective shielding approach would be to shield the wires. Unfortunately, this is also impossible as the power company would never permit it. Shielding your home is possible, but not very practical. 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. It would also be very expensive. Placing magnetic shielding material around your body is possible, but again not very practical.

Moving your house further back from the powerlines may be an option, but certainly not a very easy one. 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. Selling your home and moving to another location also comes under this heading. Make sure to use your gaussmeter to survey all homes you are considering, to avoid jumping from the frying pan into the fire.

Finally, there is the possibility of installing an active cancellation system. This is a device which constantly monitors the incoming field and produces an equal and opposite cancellation field. While it is not a do-it-yourself project, it may actually be your most practical solution. For each active cancellation system, an engineer must visit your location and custom design and install a system that takes into account the size, location, and strength of the offending source(s) and the dimensions of the area you want protected. Costs can vary from $10,000 to $50,000 or more. If this option appeals to you, contact us. We will be happy to answer your questions and refer you to a qualified engineer.

What brand of TV or computer monitor is best?

Our experience in measuring monitors of all kinds is that one cannot make generalizations about which type or which brand has higher or lower emissions.

storeA few years ago, we took our meters to a large electronics store to try to settle this question. We measured dozens of different types of TVs and monitors, including CRT, LCD, and plasma. Some where high, some were low. Some were high on E but not M, some were high on M but not E. Size had no correlation either. We found one unit which was the lowest on both. It was a Motorola product. The display model had a black bezel (plastic frame), but we wanted the gray one so we took a boxed unit OF THE SAME MODEL, but with gray bezel. When we got it back to the office, we set it up and tested again. It was worse than the worst unit in the store!!

Furthermore, a few months later, we went back and found that almost all the units available were different models.

From this, we have learned:
1- this month's recommendation will be obsolete in a very short time, as new models appear in the stores very regularly
2- the best advice we can offer is to take your meters to the store and select the best FLOOR MODEL. Then expose yourself to that unit for a short while to determine if YOU have any symptoms to it. Take THAT FLOOR MODEL UNIT home if it passes both of these tests.

Cell TowerHow do I shield my home from cell tower (or RF) radiation?

Compared to magnetic field shielding, shielding a home from cell tower radiation is reasonably straightforward. In theory, you want to create a continuous, highly conductive enclosure around the home. Any areas that are not conductive, even cracks under a door, will allow radiation to leak in. Perfect total shielding requires a perfect total enclosure. However, in a home environment, total radiation elimination may not be required. For example, perhaps 90% reduction is adequate.

There are several materials you can use to create the conductive enclosure, depending on your needs and your budget. Some materials are more appropriate for walls and ceilings, while other are better for windows. The higher the conductivity of the material, the better the shielding it will provide. Keep in mind such additional factors such as: durability, corrosion resistance, toxicity, ease of installation, appearance, and size.

For doors, walls, floors and ceilings, CuPro-Cote or Y-shield conductive paints offer very good shielding and are very convenient. Apply like ordinary paint on interior surfaces. You can paint over the conductive paint with a standard latex paint to achieve the desired color and to protect the conductive surface.

You can also cover the walls with a conductive fabric such as Pure Copper Polyester Taffeta or Soft&Safe. Apply the fabric as you would a wallpaper, remembering to overlap slightly at the seams to avoid leakage. You can cover over the fabric with a standard wallpaper, paneling or drywall.

Cover windows with conductive plastic film such as Clear-Shield or ScotchTint and/or shielded drapes (or drape linings) with conductive fabric such as High Performance Silver Mesh.

Remember to treat openings such as switch plates, outlet covers, dryer vents etc. 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. Also remember to provide proper grounding to each component which is not in contact with the others.

I used an RF shielding product which claims 80 dB attenuation at 2.4 GHz. Why am I only seeing 20 dB?

There is only one important 1 key to successful RF shielding: control leakage. Shade

Remember that the attenuation spec for a shielding material is how much radiation penetrates through the shield. Let’s look at an analogy:

In many ways, RF behaves much like visible light, and RF shielding materials behave much like two sided mirrors. Image that you are outdoors on a sunny day. You set a large mirror on a stand above your head. The attenuation specification for the mirror is very high, perhaps 120 dB or more, so basically no light comes through the mirror. If leakage was not an issue, you would be in total darkness. We all know this is not the case. Leakage from the sides easily illuminates the shaded area. Granted, the amount of illumination is less than standing in full sunlight, but the attenuation is no where near 120 dB. Maybe more like 20 dB. Furthermore, using a shield with even better attenuation will not yield any more benefit. Shade

Now imagine you are in a small room with only one window. Bright sunlight comes in the window and illuminates the room. When you place your mirror shield over the window, you get a dramatic attenuation of the light. But the extent to which leakage of light occurs around the perimeter determines how far from total darkness you will achieve in the room. We have all experienced this when trying to draw a curtain over a bedroom window. You must control the leakage to get it really dark. There is nothing wrong with the shield, light is leaking around the shield.

Because RF shields are reflective on both sides, radiation which does leak in will be reflected by the inner surface of the shield, effectively amplifying the amount of radiation in the room. Even the tiniest leakage at a seam can reduce attenuation by many dB. The obvious solution is to pay serious attention to leakage points. A great how-to book is available which describes materials and methods for controlling leakage in detail.

Why is it so difficult to shield a cellphone signal?

There are several ways to interpret this question.

First, let's look at it from the perspective of the cellphone owner:
For the cellpone to work, it must radiate. The microwave radiation emitted by the phone must reach the cell tower. Furthermore, the emissions from the tower must reach the phone. The trick is to allow this communication to take place, but minimize the amount of radiation that is "wasted" by being absorbed by the user's body. There are several ways to accomplish this.

  • Increase the distance between the phone and the body. By increasing the distance, the intensity of the radiation is decreased... just like the flame from a candle is hotter close up to the flame and cooler as distance increases. headset Using a built in speaker phone is one way to accomplish this. Using a hands-free headset is another. Of course, if you hold the phone in your hand, or in a pocket or purse near your body, you have not reduced your exposure, only transferred it from your head to another part. Naturally, the further the phone is from the body, the less radiation is absorbed by the body. Use an extension cord on the headset if you can, and put the phone down.
  • Place a shield between the phone and your body. There are several styles of cellphone shield which block the emitted radiation on one side of the phone. skin-blok Naturally, you will want that shield on the side of the phone that is between the phone and your body. You can use the shield style that goes right onto the phone, or you can line a pocket or purse with a shielding fabric. The shield should be at least as big as the phone, since the entire phone radiates... not just the antenna.
Notice that these techniques reduce the user's exposure. Because the room is still filled with the microwaves emitted by the phone, exposure is not eliminated, and of course there is no benefit to others in the room.

Now, what's so hard about blocking a cell phone signal completely?
Let's say you don't own a cellphone and want to shield your house completely from external signals. Or maybe you have a secure area (such as a hospital, data center, prison, or movie theater) and you want to prevent incoming or outgoing calls. There are many shielding materials you can put on walls and windows. But there is a big problem: cellphones can operate very nicely with only a very small fraction (less than 1 millionth) of a normal signal.

1- Therefore your shielding materials must provide very high attenuation levels. Typically, 80 to 100 dB attenuation materials are required. RF leakage

2- You must control leakage point VERY carefully. Gaps under doors, joints between shield sections, and even pinholes from sewing shielding material can permit these high frequency signals to penetrate. You need to create a "complete" enclosure. Any part that is not shielded is a leakage point.

A small pouch is not that difficult to make with the proper material. Shielding a whole house, or even a whole room is a more difficult challenge... if you want to completely kill the signal.

I have speakers near my TV (or computer monitor) that are causing distortion (jitter) to the image. How can I shield these speakers?

SpeakersConventional speakers incorporate both a permanent magnet and an AC magnetic field to produce sound. The field from the permanent magnet is present whether the speaker is active or not. 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. 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).

You will have to use magnetic shielding alloys to shield these magnetic fields and you have a choice of several methods. 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.
Note:Unlike the bucking magnet method, these shielding methods do not alter the sound quality of the speaker.

1] Method for maximum aesthetics

To achieve maximum aesthetics you will need to be able to open the speaker cabinet and get access to the back of each speaker. 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.

Because you will be placing the shielding material in close proximity to this strong magnetic field, you will have to take saturation into account. This means using at least 2 layers of shielding.

For the layers closest to the magnet, choose a high saturation material such as MagnetShield. 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. It is very low cost, so 2 or 3 layers are practical.

The outermost layer should be a high permeability material such as Joint-Shield. This outer layer will "absorb" much of the field which has evaded the first layer and yield a very high degree of attenuation. Note that attenuation will be greatest close to the speaker magnet, where the field is strongest (most interfering) anyway.

MagnetShield and Joint-Shield are both offered in a convenient 4 inch wide strip. The material is thick enough to provide good shielding, but still can be cut with a scissors and shaped by hand. For especially strong magnets, you may need more than one layer of each material.

Here is how you do it: Inner layer

Wrap the MagnetShield around the speaker magnet (notice that it is attracted to the magnet) in a cylinder shape. Cut it so that you have about 1" of overlap at the seam. Use duct tape to tape the seam securely. Cut the material which extends backward at several locations so you can bend these "tabs" inward to form the "bottom of the cup" shape. Leave this layer in place.

Outer layerJoint-Shield is provided with a peel-and-stick adhesive on one side. Before removing the adhesive backing, cut and shape the material just like the first layer, but on top of the first layer. Remove the adhesive backing and press the second layer onto the first layer. You are done!

Just be careful not to disturb or allow the shield to touch the electrical contacts on the speaker.

2] The Quick and Easy Way (and Maximum Field Reduction!)

Flat Foil ShieldIf 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.

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. Take this into account when considering where your speakers will be positioned relative to the TV.

Shielding the side of the speaker:
Most times you can get away with one or two layers of Magnetic Shielding Foil. On each speaker, place the foil flat against the side of the speaker cabinet which faces the TV, or place it against the side of the TV. A good, inexpensive way to check for shield performance and the best position for the shield is to use a Pocket Magnetometer. You can get additional attenuation by using multiple layers of foil, especially if you use a spacer (such as cardboard) in between the layers. This may offer some aesthetic challenges, but you don't need much technical expertise.

Shielding the back of the speaker:
A high saturation material of significant dimensions such as 36"x15" MagnetShield Plate is required here. If your speakers cabinets are small, you can cut the 36"x15" sheet in half to get two 18"x15" pieces. 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. Move either towards the speaker or towards the TV from this point and you will loose shielding effectiveness. Therefore be sure to check the position with a Pocket Magnetometer or digital DC Gaussmeter. Naturally, some situations may require shielding of both the back and the sides of the speaker cabinet.

3] The Third Alternative

You can always place the TV or monitor inside a shielded enclosure. This will protect the monitor from external fields produced by the speakers and any other sources.

How can I shield my laptop?

The most direct way to reduce your exposure from a laptop is to increase your distance from the device. Use a remote keyboard (not a wireless type!!) and place the laptop as far away as you can while still being able to view the screen. You can increase the text size on the screen if needed.

Laptops produce at least two types of electromagnetic fields: AC electric fields and AC magnetic fields. You can either shield the laptop (source) or shield yourself.

ClearShieldTo shield the electric fields from the laptop, use ClearShield or VeilShield Fabric to cover the screen. Be sure to attach a ground cord. 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.

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. It is not necessary to ground the foil. laptop Shield 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. The base of the laptop is inserted into this box. The open end of the box faces away from the user.

A SafeGuard Apron worn on the body will help block the electric fields. Shielded Gloves can be used to protect the hands. Another option is to use a remote keyboard and increase the distance from the laptop to your body.

Why can't I just use lead or copper or aluminum foil for magnetic shielding?

In the strictest sense, magnetic shielding is not truly shielding at all. 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. The physical property which allows them to do this is called "permeability".

Magnetic Field Lines in AirUnlike X-rays, sound, light or bullets, magnetic field lines must travel from the North pole of the source and return to the South pole. Under usual circumstances, they will travel through air, which by definition has a permeability of "1". 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.

Here's how the permeabilities of some common materials compare:

Air ........... 1
Copper ...... 1
Aluminum ... 1
Tin ............. 1
Lead .......... 1

Nickel .................. 100
Commercial Iron ... 200
Stainless Steel ....... 200
MagnetShield ........ 4000

Magnetic Shielding Alloys* ....... 20,000+
Annealed MetGlas ................. 1,000,000

* such as Magnetic Shielding Foil, Mag-Stop Plates, and Joint-Shield

Source Inside in TubeNow 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. 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. Then traveling through the shield, they will emerge into the air surrounding the South pole and back to the source. Traveling through the low permeability air outside the shield does not offer any efficiency advantage! (Notice that the diagram to the right is a cut-away view of a tube shaped shield.)

Source Outside TubeSimilarly, 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. 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!

How do I stop the attraction between 2 magnets?

An important consideration when shielding magnets is that the magnets will be attracted to the shielding material. There are no magnetic shielding materials that will not be attracted to a magnet.

Shielding repulsion between 2 magnets is easy:
Use a high saturation alloy like MagnetShield. Simply stack enough layers between the 2 magnets until the attraction to the shield balances the repulsion between the magnets. 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. A little experimentation quickly gets the correct result.

Shielding attraction between 2 magnets requires that each magnet have its own shield. 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. 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. Simply add one layer at a time until the two magnets drop away from eachother.

Finally, a word about shielding just one pole of a magnet:
While this is not technically possible, it is possible to distort the magnetic field lines around one pole of a magnet. Remember to think about the magnetic field lines as travelling through the shield more easily than through air. Therefore, the shield acts as a "conduit" for some of the magnetic field lines. They still must travel from the magnet's N pole to its S pole... but the path they take is what can be manipulated. 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. The effect is the same as if you bend the magnet itself into a different shape.

How Can I Shield High Magnetic Fields in MY Car?

Magnetic Fields in car

Magnetic shielding is the basically the same no matter where you use it:

1] Start by using a gauss meter to determine IF you have high magnetic fields where the people are. In a car, the magnetic field profile will be different at highway speed compared to idling. Check both ways.
2] If you do have high fields, use the gauss meter to determine where the sources are located. In a car, you will have multiple sources. Sweep around with the meter to identify as many sources as you can.
3] Shielding involves applying Magnetic Shielding Foil (0.010 thickness) over the source of the offending field. You must cover an area larger than the source. The closer the shield is to the source, the better the attenuation will be.

Just be aware of a few points:

a) as in all magnetic shielding applications, you need a fairly wide shield to prevent fields from coming around the edge of the shield. In a car, there are many size limitations, so you may not always be able to get as large a piece of shield in place as you would like
b) there are multiple source of EMF in a car, so even if some can be shielded, there will be others in locations which cannot be effectively shielded
c) having said that, there is no way to get 100% reduction in a car. If the majority of the problem comes from sources just under the floor, or behind the firewall, shielding can be quite effective
d) most times, the best approach is to line the floor and up onto the firewall with shielding. Remove the carpet, fix the shielding securely in place, then replace the carpet.
e) if the fields are particularly strong, you can use multiple layers of shielding.
f) always be sure that the shielding will not interfere with driving safely.