RADIATION FROM A POINT SOURCE IN AN ANISOTROPIC MEDIUM
A procedure is presented for the derivation of formal solutions to the problem of radiation from finite sources in the presence of gyrotropic regions. The boundary surface between adjacent gyrotropic regions constitute a set of parallel planes. The gyrotropic axis is perpendicular to the boundary planes. The procedure was applied to the problems of radiation from a current element in an infinite and a semi-infinite plasma region. In both cases, the current element was oriented along the direction of the d-c magnetic field. Asymptotic expressions for the far field were derived by application of the saddle point method. In the case of an infinite plasma region with an infinite d-c magnetic field, explicit expressions were derived which are valid everywhere in space. The physical interpretation of the results is discussed in detail. In the case of an infinite plasma region, the radiation field due to a point source consists of several ray contributions. At the vicinity of a given observation point, each ray contribution behaves like a plane wave whose power flow is directed along the straight line from the point source to the observation point. A graphical construction is presented for the evaluation of the wave nurnbers of a plane wave from a given direction of the power flow. It was found that for some frequency ranges, the physical space can be divided into several regions by circular cones whose common axis is parallel to the d-c magnetic field and passes through the point source. The number of ray contributions is different in each of the conical regions. The configuration of the boundary cones is strongly affected by a change in the applied frequency. The ratio of the amplitudes of dhfferent rays at the same observation point, and of the same ray in different directions, depends on the nature of-the point source. In the case of a semi-infinite plasma, a graphical construction for the optical paths is presented, by which the point of observation can be reached from the point source. This constraction is based on r flection snd refraction laws that relate the directions of the power flow in the incident and reflected (refracted) rays, rather than the direction of the phase gradients. The radiation field at a given observation point consists of ray contributions whose power flow is directed along the above optical paths. The amplitudes of the incident rays can be found from the results for the corresponding infinite region. The amplitudes of the reflected (refracted) rays are related to those of the incident rays via reflection or transmission dyadics, which are the analogs of the corresponding scalar coefficients employed in the isotropic case. A phenomenon of backward refraction was observed, whereby the incident and the refracted rays are located on the same side of the perpendicular to the boundary plane. In certain frequency ranges, where the point source is located inside the semiinfinite plasma region, curved caustic surfaces appear in the isotropic region. These surfaces are envelopes of welldefined groups of rays. The number of ray contributions at an observation point changes as the observation point moves from one side of the caustic surface to the other side. The shape of the caustic surfaces depends on the frequency range and on the location of the point source. (auth)
- Research Organization:
- Brooklyn. Polytechnic Inst. Microwave Research Inst.
- NSA Number:
- NSA-15-023002
- OSTI ID:
- 4037261
- Report Number(s):
- PIBMRI-861-60; AFCRL-303
- Country of Publication:
- United States
- Language:
- English
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