• What Kind of Unit is "Ohms per Square"?
• What is the best way to shield magnetic fields from wiring?
• What is the safe distance for people from powerlines?
• What can I do about powerlines outside my window?
• What brand of TV or computer monitor is best?
• How do I shield my home from cell tower (or RF) radiation?
• Why is it so difficult to shield a cellphone?
• How do I shield my laptop?
• How can I shield high magnetic fields in my car?

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!

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

Magnetic pattern around a wire First, 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.

http://www.lessemf.com/images/wire_pattern.gif

Shielded Area 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. 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.

http://www.lessemf.com/images/cylindrical_shield.gif

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.

http://www.lessemf.com/images/shielded_area.gif

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.

What is the safe distance for people from powerlines?

http://www.lessemf.com/images/faq6.jpg

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 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.
5. 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 particle and automotive pollutants that can be harmful.
6. 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.
7. 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.

Linemen Armed 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:

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

Vault Should 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.

How 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.

Why is it so difficult to shield a cellphone signal?

There are several ways to interpret this question but 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.

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.

ClearShield To 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.

How Can I Shield High Magnetic Fields in MY 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:

• 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
• 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
• 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
• 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.
• if the fields are particularly strong, you can use multiple layers of shielding.
• always be sure that the shielding will not interfere with driving safely.