Interference of Radio Waves
Interference of Radio Waves
a phenomenon that plays an important role in the processes of emission and propagation of radio waves. Upon emission from complex antenna arrays, consisting of several emitters (dipoles or slits), the radio waves from the various emitters interfere with each other. The amplitude of the resulting wave is different in different directions, which determines the directivity pattern of the antenna. For example, radio-wave interference from two dipoles D1 and D2, which are separated by a distance equal to several wavelengths and are fed by currents of identical amplitude, phase, and frequency, gives rise to the multilobe directivity pattern. The maximums in the pattern correspond to the coincidence of the phases of waves from the various emitters, whereas the amplitudes of the electrical and the magnetic fields E1 and H1 add at these points: E = 2E and H = 2H1. The energy flux in the direction of the maximums is proportional to the product 2E1⋅2H1, that is, it is four times greater than the energy flux for the radiation of each dipole in the absence of the others. In the direction of the minimum, however, two dipoles together do not emit at all, since in these directions the sum of the fields is equal to zero: E = 0 and H = 0. By varying the number of the dipoles and the distance between them, antennas with the desired directivity patterns may be built.
Interference of radio waves takes place during the propagation of radio waves mainly as a result of their reflection by the earth’s surface. This causes the arrival at any point above the earth of two waves—the direct and reflected waves, which interfere with each other. In connection with this, the directivity pattern of the receiving antenna shows the appearance of additional lobes, which increase in number with increasing height of the antenna above the earth and with decreasing wavelength. Interference during the propagation of medium and short radio waves takes place if waves coming directly from the transmitter and the waves reflected by the ionosphere, or waves reflected by different regions of the ionosphere, arrive at the same point in space. In the case of ultrashort radiowaves, interference is frequently caused by the arrival at the same point of waves that have traversed different paths in the troposphere or by the reflection of waves from local objects.
Radio engineering frequently makes possible direct determination of the phase differences of interfering oscillations, and since the interference pattern exhibits distribution of phase differences arising from the relative position of the transmitter and receiver, the measurements may be used for the determination of the location of the radio-wave receiver relative to the transmitter. This principle is the basis for a number of radio navigational systems of the phase type.
In contrast to optics, radio technology makes possible direct measurement of the wavelength of the emitted waves. For this reason, the study of the interference pattern of the field of two transmitters makes possible the determination of the distance between the transmitters. Conversely, if this distance is known, the rate of propagation of radio waves under given conditions can be calculated with a high degree of accuracy. There are a number of interference methods for the determination of distances and radio-wave velocities.