Reradiated Wave Research Articles

The frequency dependence of sound radiation from mobile dislocation walls

Macroscopic re-radiated plane waves from microscopic dislocation sources oscillating in a narrow slip band have been produced and detected in LiF and NaCl crystals in the 5–110 MHz frequency range. The observed waves were shown to depend upon the dislocation motion through their dependence on the orientation of the exciting waves and their dependence on the number of pinning points on the dislocation lines during γ-irradiation of LiF and NaCl. The quantitative values of the re-radiated wave amplitudes were shown to be in good agreement with the values given by the model of the vibrating string for the dislocation displacements, both in the low frequency or frequency independent region and in the high frequency or ω −1 frequency region of the displacement. Since the measured re-radiated amplitudes were as large as is permitted by the vibrating string model, dislocation-dislocation interactions could not play an important role in limiting the dislocation motion for this configuration of dislocations.

Spherical retrodirective array

This paper derives the characteristics of a spherically-shaped retrodirective space array. The array can receive an incoming signal over 4\pi steradians, amplify, frequency translate, and modulate this signal, and then return it in the direction of the incident wave. The directivity of the array for every angle of incidence corresponds to the area illuminated by the incoming wave. In the array the retrodirective characteristic is obtained by a phase inversion technique that is realized in tunnel-diode image frequency converters. A circular cross section of the spherically-shaped retrodirective array is analyzed. The analysis is extended to the spherically-shaped array. It is shown that the effective aperture of a spherical array is smaller than the illuminated area. This reduction occurs because of a tapered amplitude distribution and a symmetrical phase error proportional to the frequency difference between incident wave and reradiated wave at the image frequency. The output spectrum of the converters in the array contains not only the image frequency signal but additional frequency components as well. Because of the phase characteristics of the spectral components, only the image frequency signal when reradiated by the array elements will add coherently in direction of the incident wave. The reradiation of the other frequency components is not collimated.

The absorption of energy by a wireless aerial

If a receiving aerial is placed in the field of a wave produced by a distant transmitter, currents are induced in the receiving aerial and these dissipate energy in the ohmic resistances of the aerial. This energy may be thought of as being absorbed directly from the incident wave, but this viewpoint gives no insight into the mechanism by which the absorption takes place. In this paper the following alternative way of regarding the phenomenon is considered. The currents induced in the aerial cause it to re-radiate a secondary wave. The electric and magnetic fields of this secondary wave combine with those of the unabsorbed incident wave to give resultant fields which have, in general, a direction different from those of the incident wave. These resultant fields give rise to energy flows which are different from those in the incident wave, both in magnitude and direction. In this way a flow of energy into the aerial is produced, by the superposition of the incident and the re-radiated fields. It is important to notice that if this viewpoint is adopted we must add the secondary re-radiated wave to the unabsorbed main wave, the addition, in itself, producing the requisite absorption.