Electromagnetic pulse

An electromagnetic pulse (EMP), also sometimes called a transient electromagnetic disturbance, is a short burst of electromagnetic energy. Such a pulse's origination may be a natural occurrence or man-made and can occur as a radiated, electric, or magnetic field or a conducted electric current, depending on the source.

EMP interference is generally disruptive or damaging to electronic equipment, and at higher energy levels a powerful EMP event such as a lightning strike can damage physical objects such as buildings and aircraft structures. The management of EMP effects is an important branch of electromagnetic compatibility (EMC) engineering.

Weapons have been developed to deliver the damaging effects of high-energy EMP.

General characteristics

An electromagnetic pulse is a short burst of electromagnetic energy. Its short duration means that it will be spread over a range of frequencies. Pulses are typically characterized by:

  • The type of energy (radiated, electric, magnetic or conducted).
  • The range or spectrum of frequencies present.
  • Pulse waveform: shape, duration and amplitude.

The last two of these, the frequency spectrum and the pulse waveform, are interrelated via the Fourier transform and may be seen as two different ways of describing the same pulse.

Types of energy

EMP energy may be transferred in any of four forms:

Due to Maxwell's equations, a pulse of any one form of electromagnetic energy will always be accompanied by the other forms, however in a typical pulse one form will dominate.

In general, only radiation acts over long distances, with the others acting over short distances. There are a few exceptions, such as a solar magnetic flare.

Frequency ranges

A pulse of electromagnetic energy typically comprises many frequencies from DC (zero Hz) to some upper limit depending on the source. The range defined as EMP, sometimes referred to as "DC to daylight", excludes the highest frequencies comprising the optical (infrared, visible, ultraviolet) and ionizing (X and gamma rays) ranges.

Some types of EMP events can leave an optical trail, such as lightning and sparks, but these are side effects of the current flow through the air and are not part of the EMP itself.

Pulse waveforms

The waveform of a pulse describes how its instantaneous amplitude (field strength or current) changes over time. Real pulses tend to be quite complicated, so simplified models are often used. Such a model is typically described either in a diagram or as a mathematical equation.

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Rectangular pulse
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Double exponential pulse
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Damped sinewave pulse

Most electromagnetic pulses have a very sharp leading edge, building up quickly to their maximum level. The classic model is a double-exponential curve which climbs steeply, quickly reaches a peak and then decays more slowly. However, pulses from a controlled switching circuit often approximate the form of a rectangular or "square" pulse.

EMP events usually induce a corresponding signal in the victim equipment, due to coupling between the source and victim. Coupling usually occurs most strongly over a relatively narrow frequency band, leading to a characteristic damped sine wave signal in the victim. Visually it is shown as a high frequency sine wave growing and decaying within the longer-lived envelope of the double-exponential curve. A damped sinewave typically has much lower energy and a narrower frequency spread than the original pulse, due to the transfer characteristic of the coupling mode. In practice, EMP test equipment often injects these damped sinewaves directly rather than attempting to recreate the high-energy threat pulses.

In a pulse train, such as from a digital clock circuit, the waveform is repeated at regular intervals. A single complete pulse cycle is sufficient to characterise such a regular, repetitive train.

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