Electromagnetic Filters for Power Lines & Cell Phones

Electromagnetic Filters (EMI Filters)

Electromagnetic Filters can come in many shapes and sizes.  The simplest one is a metal cage.  You can put a device into it and it will block all radio waves leaving the box.  Boxes can be all steel, wire mesh or contain metalized  surfaces.  Copper metal or mesh works well to attenuate the electromagnetic energy.  Copper mesh is quite often used to reduce and almost eliminate radio or TV frequencies or interference.  Boxes can be made of copper or aluminum screens and these will due a very good job of eliminating radio waves (electromagnetic waves) from interfering with other separated electronic devices. Exercise care to overlap the screen or metal so no gaps exist in the screen itself.  Copper based screen cages are commonly called “Faraday Cages” and can reduce or attenuate EMI   (electromagnetic interference) quite well.

Noisy AC Wiring – AC Wiring  Interference

Alternating Current (AC)  wiring in your home can also be the source of unwanted electrical noise.  AC wiring in the home is influenced by all the AC devices plugged into the home wall plugs and circuits.  Many devices around the home have the potential to generate unwanted spikes and other forms of noise into the AC Wiring.  Motors can be a significant source of interference in the home, as well as Light Dimmers , which “chop up” the AC voltage into an appliance to control the speed of motors or lights, and can induce significant spikes onto the AC lines.  These spikes will be induced into other unprotected electronics devices, such as radios, TV’s or other controls  connected  home AC wiring.

Shielding of AC wiring in your home can be very difficult to achieve. Improper wiring and electrical grounding elevates the emissions within the home. Fairly large AC 60Hz magnetic fields can be generated near home AC wiring within the interior walls. Particular AC circuits can be filtered with special power line filters with spade terminals to prevent AC noise interference into sensitive electronics.

Normal home AC wiring can also act as a giant antenna for some higher frequencies, such as those generated by  AM Radio station towers, or other communications towers such as cell phone transmitting towers. These higher frequencies  end up “riding over” the home AC internal wiring and can add additional EMI interference into Hi-Fi  tuners, TV’s and AM radios, and other electronic devices in your household.

Light bulbs such as low-cost CFL devices, and standard florescent lighting can interfere with other electronics in the the typical household.  AC interference filters can be purchased for each appliance to mitigate this interference if it is on the AC wiring.

AC Wiring Filters

AC wiring noise can be reduced considerably by means of filtering.  It can be done magnetically by introducing AC isolation by means of an isolation transformer, or by other  methods.  Special Filters using magnetic toroids  can be very effective in reducing AC noise and are usually put around AC wires to reduce  60HZ and other lower frequency signals.   Signals can also be reduced using special filters that use inductors and capacitors to create filters that reduce noise in special equipment such as TV’s, and high end audio and video equipment. They will not however reduce interference from the AC wiring in the walls.

Individual “Break-Out Boxes”, or AC Power strips can be often purchased with some types of AC filtering.  Not all of these power strips are effective. Some only block high voltage spikes, which may permanently damage the electronics. They can be useful for this type of protection, but are quite often useless in filtering out “annoying buzz” and other common interference.

Blocking Cell Phones & Possible Concerns with Cell Phone EMI radiation

Cell phone radio waves are generated in approximately the same frequency range as microwave ovens.  This is the general area of about 2.0GHz to 5 GHz (2000 Megahertz – 5000 Megahertz).  The radio energy emitted from a cell phone is not a steady frequency, such as that in a microwave oven.  The microwave oven puts out a somewhat constant wave when it is in operation. 

Power output from a Microwave Oven is very much greater than a cell phone.  Microwave ovens can put out up to 1000 watts, and cell phones are typically under one watt of radiated power.

The pattern of waves out of a cell phone is dependent upon the mode of operation that the cell phone is in at the time.  Various cell phone carriers have different types of signaling that they employ.  Modes of the phone will vary also.  When the phone is idle, it sends out a series of pulses to keep in touch with the nearest cell phone tower.  When it is transmitting text our voice , other patterns emerge.

If we wish to block cell transmissions we must use a blocking method that will eliminate possible interference with other devices in the area.

Unfortunately  blocking the frequencies of the cell phone will make it non-operable and non-working as in normal operation.  Cell phones radiation can be blocked if the cell phone is encased in an conductive enclosure  such as a copper cage.  Other materials will also greatly inhibit cell phone radiation such as wrapping the phone in aluminum foil, or placing it inside a large metal box.  Trying to partially block the cell phone signals are generally not effective, as either the phone or the cell tower will increase their radiated power to make up for the weak signals received.

Cell phones transmit and receive electrical energy.  They do this in kind of a random manner, but generally they are always doing something such as “pinging” one another and generating some type of emitted energy.  When in motion such as in car, a cell phone will increase power temperately while changing cell towers as it moves along.

Unfortunately, studies of cell phone radiation effects on the human body are not very easy to find, but some do exist.  It might be of some benefit to us all, if more studies were conducted attempting to relate the possible side effects of cell phone radiation at close proximity to the human body and head. If you need more info, look no further than the users guide that usually accompanies your cell phone purchase. It usually sites FCC or FDA information and warnings relating to some modes of use.

If persons are concerned about this cell phone radiation, they can put the phones in a metal cage  or metal lined bag, and thus eliminate most of the radiation.  Setting the phone into the “airplane mode” will also eliminate most of the problem radiation.  In addition, placing the phone at some distance from our bodies is also an effective way of reducing power close to the body.  This is because of a law of physics known as the “inverse square  law” .  One might think that power reduction is achieved the greater the distance of the cell phone from the body, and it is . This law states that the power is reduced by dividing the power by the distance to the phone squared!   So at one foot distance the power might be 100  milliwatts. At two feet it will be 1/4 of that or 25 milliwatts (mw). So placing the cell phone at relatively small distances from our bodies can reduce the power quite a bit!

Can Cell Phone Tower Radiation be Blocked into Your Home?

Cell phone towers transmit and received signals from all the phones connected to them in a local area (hence the name cell).  These signals can be relatively strong depending upon the distance from the tower to your home.  Most persons do not seem to be bothered by the electrical waves generated from cell towers. But a small population of individuals has been shown to be quite sensitive and uncomfortable with these emissions.

What can be done to reduce the effect of these waves for sensitive persons?  It seems that all that can be done is to provide materials that will block the signals on the inside, or outside of the dwelling.  For various reasons, shielding in the interior of a home can be quite tricky and also very expensive.  One approach, is to block signals on the exterior of the home.  Special conductive screens can be placed under the roofing on the house and will reduce these cell phone waves by a large amount.  Special conductive paints can be applied to the exterior of the home to reduce the radiated energy. These conductive paints are colored black or grey, and normal decorative paints can be applied over them for a more pleasing home color.

As an example, some physically close cell towers may produce up to 2000 micro-volts/meter energy exterior to and inside the home.  If we apply   the above methods, the EMI wave energy inside the home can be reduced down from 2000 to 20-40 micro-volts/meter.  This may result in a reduction of 50-100 times the original energy, and may allow EMI sensitive persons to occupy  their homes without too much discomfort.

What about Cell and Wi-Fi Microwave Radiation close to Children and Infants?

The World Health Organization (WHO), has indicated that closely placed microwave radiation from Cell Phones and Computer Devices such as Routers, Gateways, and other Wi-Fi type devices may present health issues for developing infants and children.  If at all possible you may reduce this type exposure by locating these devices at greater distances from children’s play and sleep areas.

Often commercial baby monitors use high-frequency communications and these  radio waves may present a hazard.  Some of these monitoring units use Wi-Fi cameras and/or  microphones to monitor a given area.  The placement of these devices nearby to children may possibly pose a hazard to their health, because Wi-Fi devices use frequencies that are similar and almost equal to cell phone transmissions.  In addition, the constant streaming of data or pictures may increase the “duty cycle”  and result in much more EMI power over a given second or minute interval than that of normal internet traffic.

How do I test for Harmful EMI/RFI Radiation in my Environment?

The only way to know if you have harmful radiation in your home or office environment is to measure it. If you wish to monitor EMI radiation in a given area, use a quality EMI/RFI Testing Meter to make these measurements.









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