An EMI Noise Filter for Power Lines & Cell Phones

Electromagnetic Filters (EMI Noise Filter)


High voltage test lab where sparks produce EMI radiation

An EMI Noise Filter  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 most all radio waves entering and leaving the box.  Boxes can be constructed of steel, wire mesh or contain metalized  surfaces.  Copper metal or mesh works well  attenuating   electromagnetic energy.  Copper mesh is often used to reduce and almost eliminate radio or TV frequencies or interference.  Enclosures 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 nearby 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 to home AC wiring.

Shielding for Home AC Power  “Dirty Electricity”  Interference


Shielding of AC wiring in your home can be very difficult to achieve. Improper installation of wiring and improper electrical grounding elevates the emissions within the home. Fairly large AC 60Hz magnetic fields can be generated in by wiring within the interior walls. Particular AC circuits can be filtered with special power line filters with spade terminals to prevent AC noise interference getting 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. These higher frequency emissions which travel on AC wiring are sometimes referred to as “Dirty Electricity”.  Light bulbs such as low-cost CFL devices, and standard florescent lighting can interfere with other electronics in the the typical household.  A separate AC EMI noise filter filter can be purchased for each individual appliance to mitigate interference.

AC Wiring Filters


An EMI Noise Filter can also be used to reduce AC home wiring noise.  It can be done magnetically by introducing AC isolation by means of an isolation transformer, and 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.

Power Strips Come with a Built in EMI Noise Filter


Individual “Break-Out Boxes”, or AC Power strips can often be purchased with some types of an EMI noise filter 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 electromagnetic interference filter protection, but are quite often useless in filtering out “annoying buzz” and other common interference.

Blocking Cell Phones &  Cell Phone EMI Radiation

It's a Good Idea to Keep Cell Phones at a Distance

It’s a Good Idea to Keep Cell Phones at a Distance

Cell phone radio waves are generated in approximately the same frequency range as microwave ovens.  This is the general area of about 2.0 GHz 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 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. Microwave ovens are heavily shielded and are designed to  externally radiate radiate less than (5 milliwatts/cm squared) of EMI energy.

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 or our voice, other patterns emerge.

Blocking Cell Phone Transmissions


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 most of the the frequencies received or transmitted by a cell phone will make it non-operable for communications.  Cell phone radiation can be blocked if the cell phone is encased in a conductive enclosure, such as a copper wire 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 or transmitted.

Cell Phones Ping (Transmit and Receive) Constantly

Cell phones transmit and receive electrical energy every few seconds.  They do this by  “pinging” , or transmitting, to find the closest cell tower in the area.  When in motion, a cell phone in a car will increase power temporarily while  changing from one cell tower to another.  Unfortunately, scientific 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. The results of the study only relate data from studies that assume all damage to cells is done only by radiation heating. Newer studies with animals have found that microwave radiation in much smaller intensity seems to interfere with human cell growth and may cause damage to human DNA.

The Inverse Square Law acts as the Same as an EMI Noise Filter


If persons are concerned about this cell phone radiation, they can put the phones in a metal cage or metal lined bag. This traps the EMI and eliminates most of the radiation around the device.  Putting the phone in  “airplane mode” will also attenuate the radiated power quite a bit.   In addition, placing the phone at some distance from our bodies is also an effective way of reducing power.   This occurs because of a law of physics, known as the “inverse square  law “.  One might think that power reduction is achieved by increasing the distance of the cell phone from the body, and it is. This law states that the power at a fixed from the phone distance is reduced by dividing that power by the distance to the phone squared!   For example assume the cell phone power is 100  milliwatts (mw). At two feet it will be 1/4 of that value or 25 milliwatts (mw). So placing the cell phone at relatively small distances from our bodies can help reduce the power density quite a bit!

Can Cell Phone Tower Radiation be Blocked into Your Home?


Cell phone towers transmit and receive 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 cell phone users do not seem to be bothered by the electrical energy radio waves generated from cell towers. But a small population of individuals has been shown to be quite sensitive to this energy and and is uncomfortable with these types of emissions.

A Band-Aid for Cell Phone Harmful Transmissions


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 cell signals on the inside, or outside of a building or 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 surface of the home.  Special conductive screens can be placed under the roofing on the house, and act as an EMI noise filter reducing the cell phone energy by a large amount.  Special conductive paints can be applied to the exterior surface 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.

A Close by Cell Tower Can Transmit Dangerously High Fields


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

Should you have Concerns about Cell, 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 wish to reduce this type of 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 as well.  Some 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 being transmitted in each interval, compared to that “normal” internet traffic.

How Do I Test for Harmful EMI/RFI Radiation in my Environment?


Unfortunately  only way to actually know if you have harmful radiation in your home or office environment, is to measure and monitor it. If you wish to monitor EMI radiation in a given area, you will need to use an EMI/RFI power/volt meter  to make these kinds of specific measurements. These measurement devices tend to be fairly expensive, but some are available on a rental basis.