By Norm Ryder.
All electrical and electronic equipment emit Electromagnetic Radiation (EMR), fortunately most devices do not emit high levels of EMR. The ones of prime concern are those that emit radio signals. These include but not limited to: Cell Phones, Cordless Phones, Laptop, Tablets, Baby Wireless Monitors, Wireless Routers. The laptop this is being written on emits approximately 25 milivolts per meter (Electric Field) when turned on, 25 more when the charger is plugged in and regularly over 1000 milivolts when on Wifi. To determine the Power Density would would also need to measure the amps per meter (Magnetic Field). Few radiation meters measure the Amps per meter. These regular meters calculate the amp reading based on established physics when the measurements are in the far field or at a distance from the source.
Yes this is a topic that few of us are aware of or understand on first reading. More explanation of the physics of Radio Frequency is in Cell Tower Siting Considerations. An excerpt on Near Field Radiation is in the pdf accompanying this article. The full booklet is available on request by contacting firstname.lastname@example.org .
The radiation field around a transmitting device is divided into 3 primary zones: the Near, Far and a zone that is called the Intermediate or Transition zone – a blend of the other 2.
In the near field Spectacular peaks and Valleys of radiation intensity can occur, the radiation pattern is considered to be difficult to model if it is possible to model it. In the Radio Frequency Tool Kit For Environmental Health Practitioners by the BC Centre for Disease Control (BCCCDC) the near field zone is referred to as chaotic. In his zone both Industry Canada (IC) and the BCCDC specify both the Electric Field and Magnetic Fields must be measured in several locations to obtain the true power density readings.
The extent of the danger zones around an emitting device is determined by a series of calculations based on the frequency (wavelength) and size of the antenna. Additional confusions can be introduced by interference from other sources, reflected energy, stored energy and antenna design and imperfections to name a few of the other sources of complexity that may possibly exist.
The Danger zone around a cell phone can be consider to be in the order of 0.1 m and greater if the phone is using wifi. A cell phone antenna – even ones on roof tops over 12 meters to the extreme danger limit and 50 meters to the high danger zone. Larger antennas or higher frequencies could extend these distances considerably. A home wifi router the minimum distance to the extremely dangerous zone limit 0.1 and to the highly dangerous zone 0.3 meters (1 ft). For a laptop assume distance in excess of 1 meter and 4 meters respectively.