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7
Author: Wilmotte Raymond M.
Title: The nature of the field in the neighbourhood of an antenna
Released: 1928
The problem dealt with is the calculation of the effective e.m.f. induced in a receiving antenna from the field of a neighbouring transmitting antenna. It is assumed that the distribution of the current in the transmitting antenna is known. In this connection the author has shown that, for the simple antenn? usually met with in practice, the current distribution can be readily calculated. The various parts of the problem are taken separately. First, the formul? for the field at a point due to any element of current are given, and it is shown that the wave-form of the field varies with the distance from the antenna. The next point deals with the polar diagram at large distances of an antenna. With regard to the wave-form, it is pointed out that the usual spectrum of the current of a radio station consists of a number of peaks at frequencies which are whole multiples of the first, each peak having a fairly rapidly decreasing intensity for a comparatively small range of frequency on each side. The peaks correspond to the fundamental and the harmonics, and on each side are the side bands of the modulation. It is pointed out that the polar diagram of the harmonics may differ very considerably from each other, but that, for the frequencies of the side bands, the directions of the maxima of radiation would not be appreciably different from the directions of the maxima of the corresponding harmonic. The effect of the conductivity and dielectric constant of the earth is next considered, the image due to any given element of current being taken as the exact image of the current element, but having K times the intensity. The factor K depends on the angle of incidence at the earth, the nature of the polarization of the incident field, the electric constants of the earth, and the frequency. This, again, will therefore produce an effect on the wave-form of a received signal. Finally, a formula is given for the effective e.m.f. induced in a simple receiving antenna when the field - at any point along the antenna is known. This formula applies even if the field is not uniform. In conclusion, the problem of interference is considered. It is suggested that a criterion for the interference of a radio station dependent on the current and potential spectrum at the foot of the antenna would be convenient. A new spectrum calculated from these would give the variation of the sum of the squares of the current and potential with frequency, for the harmonics and average working modulation of the station. From a rough calculation the maximum energy of radiation for a unit current at the various frequencies of the spectrum could be calculated and, possibly, combined in some suitable arbitrary way with the maximum energy of radiation along the ground. The product of this arbitrary relation and the spectrum distribution, obtained as explained, would form a convenient basis of a criterion for the measurement of interference.
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