Interference is the superposition of two or more waves in which the amplitude of the resulting wave depends on the phase difference of the original waves at a given point in space. If waves with the same phase are superimposed, the amplitude of the resulting wave increases, and if the phases are opposite, the amplitude decreases (down to zero). In real conditions, due to reflections from various obstacles, multiple waves with different phase shifts can arrive at the reception point, causing the resulting signal to vary randomly.
When medium and short radio waves propagate, interference occurs if waves traveling directly from the transmitter and those reflected from the ionosphere arrive at the same point, or if waves are reflected from different parts of the ionosphere. For ultra-short radio waves, interference often results from waves that have traveled different paths through the troposphere or have been reflected from local objects.
The phenomenon of radio wave interference is used in the design of antennas with a narrow radiation pattern, achieved by placing multiple vibrators one after another at distances of a quarter wavelength. By studying the interference pattern of the fields from two transmitters, the distance between them can be measured. Radio wave interference can also be simulated in receiving equipment by combining signals received at different points in space (see Aperture Synthesis). Radio interferometers, consisting of two linked radio telescopes, are used to observe sources of cosmic radio waves.
Related articles: Aperture antenna, Directivity factor, DCF, Interference noise.