Introduction
To recognise, describe and influence signals, you have to be able to characterise an EM wave. Four characteristics define a wave completely: frequency (or wavelength), amplitude, phase and polarisation. This section covers the first three; polarisation returns in detail with radar (Polarisation of the electromagnetic wave).
These characteristics are not academic: every form of information transfer through the EMS works by varying (modulating) one or more of them, and every form of electronic warfare works by measuring, imitating or disrupting them.
Characteristics of the electromagnetic wave
The four basic characteristics of an EM wave:
- Wavelength (λ, lambda): the distance in metres between two successive wave crests.
- Frequency (f): the number of wave crests passing a fixed point per second, in Hz. Wavelength and frequency are linked by c = f × λ — know one and you know the other.
- Amplitude: the maximum displacement of the wave, a measure of the strength (power) of the signal. The larger the amplitude, the stronger the signal.
- Phase (φ): where in its cycle the wave is at a given moment, expressed in degrees (0–360°) or radians. One complete cycle is 360°.
Frequency determines what the wave is (radar, IR, light) and how it behaves in the atmosphere. Amplitude determines how far the signal reaches. Phase becomes important as soon as waves interact or are compared — the basis of interference, monopulse tracking and phased-array antennas.
EM waves in phase, in antiphase and out of phase
Two waves that are in step reinforce each other; if they are exactly out of step, they cancel each other out. Compare it with pushing a swing: at the right moment it goes higher, at the wrong moment it slows down.
In practiceIn a town or a mountain valley a radio signal arrives by several paths: directly and via building walls or rock faces. Where those waves cancel each other out, the link drops — often simply moving the radio a few metres helps. The same effect makes a low-flying target over the sea fade in and out on a radar display.
When two waves of the same frequency meet, their displacements add up at every moment: superposition. The result depends on the phase difference between the waves:
- In phase (0° difference): crests fall on crests. The waves reinforce each other to the maximum — constructive interference. The resulting amplitude is the sum of both amplitudes.
- In antiphase (180° difference): crests fall on troughs. The waves cancel each other out — destructive interference. With equal amplitudes the result is zero.
- Out of phase (any other difference): partial reinforcement or weakening, somewhere between the two extremes.
This principle has direct military applications:
- Phased-array antennas steer their beam electronically by varying the phase of hundreds of small transmitters — the main beam forms where the contributions are in phase (Antenna characteristics).
- Multipath: a radar signal reflected off the earth's surface can arrive in antiphase with the direct signal and weaken detection (fading).
- Coherent processing: Doppler radars compare the phase of successive echoes to measure velocity (Pulse-Doppler radar and Moving Target Indicator); active jammers can send signals back in antiphase to suppress effects.