Thermal Radiation Fields and Antenna Parameters in Magnetoplasma

Spectral densities of components of the electromagnetic fields of thermal radiation in a uniform magnetoplasma are shown to be related to the radiation resistances of electrically small antennas radiating in the magne toplasma. For antennas of arbitrary size and shape the conventional concepts used to describe antenna per­ formance in isotropic media, i.e., gain, effective area and effective length are generalized to anisotropic media and relationships between them and radiation resistance established. A thermodynamic method is used in the derivation of the relationships. A new derivation of the radiation resistance for an electrically small dipole in magnetoplasma is given based only on the far-zone radiated fields. ' In recent years there has been considerable interest in the measurement of field strengths or radiation intensities of radio waves in magnetoplasmas, for example for radio astronomy measure­ ments from vehicles in the topside ionosphere. Two related aspects of this problem are treated in this paper. First, several papers have been published w_hich deal either with the strengths of thermal fields or with the radiation resistances of small dipole antennas in magnetoplasma. Simple relationships exist between these quantities in an isotropic plasma (Hugill, 1961). In this paper, these quantities are discussed for magnetoplasma and general relationships between them are established. The last section is concerned with properties of antennas of arbitrary size and form in magnetoplasma, with electrically small antennas entering only as an explicit, special case. In section 2, we consider infinitesimal electric and magnetic dipoles radiating into an em­ bedding magnetoplasma. By equating the power fed to each antenna to the power fed by the antenna into the medium directly surrounding the antenna, Kogelnik (1960) and Kogelnik and Motz (1963) obtained integral expressions for the respective radiation resistances Re and Rm. Using these formulas, the ratios of Re or Rm to the corresponding free-space values can be written in the form: (l.l) where tfJ is the angle between the vector dipole moment and the static magnetic field, subscript p stands for either e or m, an4 subscript 0 stands for free space. The Ptp and Pai> are functions of the usual ionospheric parameters: X= [(plasma frequency)/(operating frequency)J 2 , Y =(gyro frequency)/(operating frequency). Extensive numerical values are available (Weil and Walsh, 1964, 1965) for the quantities ! {p3p ± Ptp) computed on the assumption of a cold, collisionless plasma. In this case there are separate sets of curves, one for radiation in each of the two modes of propagation, ordinary and extraordinary, from which may be obtained the net radiation resist­ ance which is the sum of the contributions due to propagation of energy from the antenna in each of these modes. By a general argument, PtJJ and p3p are shown in section 2 to be proportional to the spectral densities of certain components of the thermal electromagnetic fields in an isothermal enclosure filled with a uniform magnetoplasma. Such radiation is equivalent, as far as local radiation in­ tensity is concerned, to radiation within the topside of the ionosphere (a non-uniform plasma)