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Overview of EMF & EMI

This site and the work of our company are focused on issues related to electromagnetics. This section (and much of this site) is directed at those who are seeking a better understanding of these issues but for whom the mathematics and theories of electrical engineering are not well understood.  Accordingly, this is not intended to be a graduate course in physics, but a reasonable distillation of basic principles.

First, Terminology.  Throughout this site, we make use of the terms EMF and EMI.  Unfortunately, in common use, these terms are used interchangeably; but it is important to understand that they are not identical.  We offer the following clarification, in common terminology.

EMF is defined as either “Electromagnetic Field” or “Electric and Magnetic Fields”.  EMF is, under either definition, a thing, an agent, or a force.

EMI is defined as “Electromagnetic Interference”.  EMI is the result of an electric or magnetic field acting on a device, causing it to malfunction.  It is this interference that affects the proper functioning of a device.  EMI is a broad term that covers all interference from all frequencies in the electromagnetic spectrum – DC, Quasi-DC, AC and RF.

RFI is defined as “Electromagnetic Interference” which is caused by an electromagnetic field that is specifically in the “Radio Frequency” band (see Discussion, below).


EMF (Electric and Magnetic Fields or Electromagnetic Fields) can be classified or described in two ways:

  • By Frequency or Frequency Band
  • By Type


The rate at which a field changes per IMG_5284second is its Frequency.  Traditionally, “Cycles per Second” (CPS); now, more commonly, “Hertz (Hz).  Electric power in the U.S. changes at 60 cycles per second, or “60 Hz”.  Magnets (the Earth) produce fields that are static, or zero cycles per second “0 Hz”. Microwaves change millions of times per second “Megahertz” (MHz).  Examples of frequency Bands are ELF (30-300 Hz), VLF (3 kHz – 30 kHz) and UHF (300 MHz – 3 GHz).   Thus, power frequency (60 Hz in the US; 50 Hz in Europe) is in the ELF band.

ionizing radiation

Ionizing radiation is so named because electromagnetic radiation at these high frequencies (above visible light) has sufficient energy to break a chemical bond in a molecule, destabilizing or “ionizing” it. We speak of these bands as different types of “rays”. Included in this range are Ultra-violet (above violet) rays, X-rays, Gamma rays and Cosmic rays.

Non-Ionizing radiation are all those electromagnetic fields at frequencies below visible non-ionizing radiation light. Included are the bands of most interest to us: DC (direct current or static), ELF (including 50/60 Hz or AC fields from commercial power), the VHF and UHF television/radio frequencies (RF), and higher radio frequencies, including microwaves.

 FMS’ business is dedicated and limited to the measurement, characterization and mitigation of non-ionizing fields – those fields and frequencies most associated with communications or power.


Electromagnetic Fields are composed of two different but related fields — electric, known as the “E-field”, and magnetic, called either “H-” or “B-field”.  Electric fields are created by voltage and measured in volts per meter (v/m). The higher the voltage, the stronger the field.  An electric field will exist even when there is no current flowing.  Magnetic fields are created when electric current flows: the greater the current, the stronger the magnetic field. Magnetic fields are measured in Gauss (G) and milliGauss (mG) or Tesla and microTesla (mT). 10 milliGauss equals 1 microTesla.


Earth'sMagnetic-01Electromagnetic fields are present everywhere in our environment.  Electric fields are produced by the build-up of electric charges associated with thunderstorms. The earth’s static DC magnetic field is present everywhere on the planet and far out into space.


sub 1Man-made EMF sources are ubiquitous in any developed society: subway rail systems and medical MRI equipment produce strong DC fields; passing electric distribution lines and building electrical equipment emit AC magnetic fields; broadcasting, cellular and other communication transmitters and radar systems generate powerful RF electromagnetic fields.

Recent Projects


FMS as the EMF/EMI Consultant designed and installed the passive and active mitigation systems for this newly established interdisciplinary organization under the Office of Research of the University of California Irvine (UCI) to house state of the art electron-microscope labs (4 imaging cells). The project required analysis of interference potential of all internal and external emission sources, which included vehicle and facility elevators (DC), broadband electrical system (AC) sources, etc.

Harvard LISE

FMS was engaged as the project’s EMI Consultant by the project architect (Wilson Architects) to conduct an electromagnetic field (EMF) analysis in an existing laboratory (shell space) located in the basement level of the Harvard CNS facility.


Seattle Children’s Research Institute is currently in the construction phase of ”Building Cure”, a 540,000 square foot research building, which will be dedicated to developing therapies for children with diseases such as Cancer, Type 1 Diabetes and Sickle Cell Anemia.

Un. of Illinois Urbana-Champaign Electrical and Computer Engineering Bldg.

FMS was engaged to conduct a thorough assessment of the building’s design for EMI interference threat concerns to sensitive research equipment. A document review and extensive 3-Dimension computer simulation studies were conducted to evaluate EMF emissions from the building’s electrical distribution, mechanical and other systems.


Over its 20 years, FMS has successfully completed hundreds of EMI projects which included a diverse range of consulting and mitigation services.

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