You are in your window seat on American Airlines waiting for your plane to head out for the runway. Your pilot goes on the intercom, politely asks for your attention, and then requests that you power OFF your cellphones or place them in “airplane mode.” But why?
Your pilot is clearly concerned about the cumulative electromagnetic interference (EMI) emitted from the cell phones reflecting off of hard surfaces inside the cabin. Even though the plane’s computer systems are encased in metal to protect them from interfering radio waves, there is still a concern that some intrusion onto the computer’s circuitry, which controls the landing gear, could cause an EMI malfunction.
What is EMI?
Electromagnetic interference is the distortion resultant on to:
an existing electromagnetic field, or
a conductor, which is termed the "receptor." A conductor in the electrical context is any wire that can provide a path for current to flow. In general terms, any metallic object or any mass that is retaining moisture is conductive. A human being or animal can also be a receptor of an electromagnetic radiation source that can cause biological interference to healthy functioning cells.
Interference requires:
a noise source (emitter)
a pathway (method of coupling or energy transfer)
a receptor, (a receiver that is susceptible to interference.)
Any Electromagnetic Field Test is ultimately an EMI test in the sense that its purpose is to determine the potential for any specific ambient electromagnetic field of causing interference or obstruction to electrical systems, equipment function, or staff safety.
However, EMI testing goes to even deeper levels whereby we also look at the specific equipment of concern or under test (EUT) down to the circuit boards and the device’s electrical function, emissions, and electromagnetic compatibility (EMC) when deciding on its best location in your facility.
EMI causes the altering of waveforms, electron path and direction, due to the coupling of noise from electromagnetic radiation. How disruptive any interference will be to a receptor depends on its susceptibility and immunity to the interference.
With the exponential increase of wireless technologies around the globe, EMI has become common vernacular. Synonyms are signal-to-noise ratio (SNR), line noise, harmonic transients, dirty electricity, RFI (radio frequency interference), and electromagnetic coupling.
I prefer electrical engineering terms because it is with engineers whom I have to communicate.
We used an oscilloscope with a magnetic sensor probe to measure EMI on an Apple TrackPad II. If there was no EMI on the DUT (device under test), we would have had a smooth sine wave, instead it is quite jagged. Notice the 645.9 kHz fundamental. A smooth sine wave would be 60 Hz. Copyright ©2020 All Rights Reserved.
The Four Types of Electromagnetic Coupling are:
1. Conductive Coupling occurs when the coupling path between the source and the receptor forms a direct electrical contact with a conducting body.
Examples of Conductive Coupling:
when wires cross in an electronic device
when a human touches an active wire
when the system ground wires attached to an active water pipe which is conducting a neutral current
Signal-to-Noise (S/N)
When the S/N ratio appears in phase, in the same direction on both conductors, we call this common impedance.
When the S/N ratio appears out of phase, in the opposite direction on both conductors, we call this differential impedance.
2. Inductive Coupling occurs when a strong electromagnetic force, or EMF, intersects an electrical conductor within a magnetic field, causing the first magnetic field to become distorted. The famous Michael Faraday, who developed what is now termed the Faraday Cage, discovered electromagnetic induction in 1831. James Clerk Maxwell, who preceded Albert Einstein, mathematically described this process as "Faraday's Law of Induction."
3. Capacitive Coupling occurs when two fluctuating electrical fields co-exist between two adjacent conductors, thereby inducing a change in voltage on the receiving conductor, or receptor. Capacitive Coupling is one of the most intriguing and challenging for the new student to grasp. We see this occurring when we turn off the circuit in a room, and the electric field becomes stronger. It is because the electrician strung wires in parallel from different circuits.
4. Radiative Coupling occurs when there is a distance exceeding one wavelength between the source point and the receptor. The source point and receptor both act as antennas whereby the source-point emits or radiates an electromagnetic force which then emits across space in between and is coupled or received by the receptor. An example is a cell transmitter sending an HF signal that couples on to your equipment’s wiring.
Why Be Concerned With EMI?
Electromagnetic interference (EMI) causes latency, malfunction, and sluggish performance to fine electronics such as computers, medical devices and equipment, pace-makers, financial trading platforms, graphic software, recording equipment, and more.
How Do You Know It’s EMI?
An easy way to tell if you have an EMI issue is to observe the presence of any:
overheating of any metal enclosures. Are enclosures very hot to the touch? (Inductive Heating)
motor failures from overheating. (Voltage Drop)
fuses blowing for no apparent reason (Inductive Heating and Overload)
static or interference on sound or voice communication (Harmonic Line Noise)
electronic equipment shutting down for no apparent reason (Voltage Distortion)
computer malfunction or locking up. (Voltage Distortion)
flickering fluorescent or LED lights (Transformer Saturation)
blinking incandescent lights (Transformer Saturation)
flickering or distortion lines and static on screens (Transformer Saturation)
What Are the Additional Benefits of Reducing EMI?
Reduced Electrical Consumption
Cooler Equipment
Longer Lifetime for Equipment
Lowered Utility Bill
EMF Reduction for a Safer and Healthier Environment
Surge Protection for Your Entire Facility
Improved Screen Quality
Improved Audio
Phase Correction Which Improves Efficiency and Performance
Cleaner Power Resulting from Transient Harmonic Attenuation
Improved Health and Wellness
How Does EMI Occur?
Metal, of course, is a conductor for electromagnetism. If you have a strong electromagnetic field nearby a metal wire that has an electrical current and/or voltage, the nearby electromagnetic field will magnetically converge, couple, and ride along with the original current. Imagine a surfer hopping onto his surfboard to ride that perfect wave.
The amount of interference that will occur on an electronic is relative to frequency, the V/m (Volts per meter), and the magnetic flux of the intruding EMF.
The analogy of wind and water wonderfully illustrates the concept of EMI.
If there is a slow and easy breeze moving across the surface of a lake, you will see ripples or small mercurial waves in the water.
When wind velocity and force increase, you will see more turbulent water. This resembles EMI.
EMI is why certain hospital wings will have cell phone-restricted areas.
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