The difference between EMF and EMI
EMF and EMI are common terms in the electronics industry and are very often used interchangeably. But it can lead to a bit of confusion, especially when it comes to engineering for, or requesting a solution for a project.
Electromagnetic fields (EMF) are everywhere electricity or communication signals are present. It is stray or sometimes intentional energy, escaping the confines of the cables, wires and components in waves, or wavelengths which vary depending on the source.
In our technological world full of electronic devices and components, it is common for multiple electromagnetic energies to compete for space - each seeking to overpower the other. The resulting collision of these energies can cause serious problems or what is called electromagnetic interference (EMI). With the modern world dependent on electronics for everything, from medical devices to GPS systems and communications, problems produced by EMI can bring disastrous results. Systems quit; signals go astray, lives are put at risk. If you want electromagnetic fields to be uninhibited, EMI is the enemy.
However, sometimes EMI is purposeful, such as in military warfare. Knowing what you are trying to promote, capture or protect becomes the important distinction between EMF and EMI.
Not all electromagnetic fields are created equally
There is not a 'one size fits all' approach to EMI and EMF. Various sources and applications emit different wavelengths of electromagnetic energy:
- Radio waves for data and voice operate at a particular frequency with longer wavelengths.
- Microwaves for heat, communications and radar.
- Infrared for thermal imaging driven by heat, some medical applications and, of course, heat itself.
These are just a few examples, without mentioning gamma, ultraviolet or x-rays.
Every one of these has its own frequency and wavelength. High frequencies will produce shorter wavelengths and vice versa. All of this must be taken into account, plus the location and purpose, when engineering for EMF or EMI shielding. For example, openings in a hardened enclosure must take into account the wavelength that you're trying to disrupt. They must block signals during a wavelength so the interference cannot get through.
Designing your EMF/EMI shielding
The wavelength, in addition to the purpose you’re trying to achieve, determines how your EMI, EMF shielding or RF (radio frequency) shielding should be used.
- Are you building a hardened enclosure which needs to 'blanket' the sensitive components inside from outside interference?
- Are you protecting individual components in the interior of an electronic box?
- Are you trying to protect nearby equipment from EMFs escaping from your equipment?
- Or, is purposeful emittance your goal, as in the case of some medical or military equipment?
On top of this, the effectiveness desired must be calculated. Commercial and military projects can call for specifications, known as 'SE' or shielding effectiveness. Various SE levels can be obtained by calculating the frequency vs SE desired using calculations. Knowing the output of these calculations, you can then determine the metals, rubbers or needed coatings in shielding materials.
However, whichever metal, conductive coating, gasket or ventilation filter you choose, projects get derailed in the details by communicating with the wrong definition. EMI and EMF can easily get twisted.