The well-known electromagnetic fields generated by infinitesimal dipoles in the presence of the earth are reviewed briefly with special reference to contributions by H. Bremmer. The fields due to currents in extended conductors can be expressed in terms of an integral over the occupied volume but its evaluation is possible only when the currents are known in their dependence on the properties of the earth. This is true of the horizontal-wire antenna quite close to the earth both when driven by a localized emf and when acting as a scatterer in an incident field. It is also true of horizontal antennas in the earth not too close to the surface. Expressions for the most useful component of the radiated or scattered field are formulated for an end-driven Beverage-type antenna, for the horizontal wire as a scatterer in the presence of the earth, and for the embedded insulated traveling-wave antenna.

Electromagnetic waves and antennas above and below the surface of the earth

The theory of the linear, insulated antenna embedded in an electrically dense medium is applied to microwave antennas used for hyperthermia cancer therapy. The pattern of power deposition is computed for a square array of four antennas with a side length of 3 cm under the assumption of no coupling among antennas. The driving frequency is set to seven values between 300 and 915 MHz, and the antenna halflength is set to three values: 3 cm, 6 cm, and the resonant value.

The Effects of Driving Frequency and Antenna Length on Power Deposition Within a Microwave Antenna Array Used for Hyperthermia

The propagation constant of an electromagnetic wave on a long horizontal thin wire is determined from a modal equation for various situations: an elevated wire; a buried bare wire; a bare wire located in the air-earth interface; and a buried insulated wire. Analytical expressions are derived under the assumption that the height or depth of the wire is much less than the skin depth of the conducting earth and that the angular variation around the wire can be ignored. Although not restrictive in frequency, these conditions are satisfied for extremely low frequency (ELF) and very low frequency (VLF) applications except possibly for deeply buried wires when the effect of the interface is negligible anyway. We show that the propagation constant of a buried bare wire is significantly different from an elevated one for almost all heights of practical interest. When the bare wire is in the interface or buried in close proximity of the earth surface, we find that the square of the wave propagation constant is the average of the squares of the propagation constants in the two media. Of particular interest is the fact that a buried insulated wire has a much smaller propagation constant than a bare wire at the same location. Thus, a horizontal ELF radiating structure, such as the Sanguine antenna, could behave in much the same manner whether it is elevated or located in a buried cylindrical insulating cavity. This conclusion is strictly valid when the length of the structure is small compared with the wavelength of the propagating current in either case, and when adequate grounding of the structure can be achieved.

Extremely Low Frequency (ELF) Propagation Along a Horizontal Wire Located Above or Buried in the Earth

An electrically thin dielectric insulating shell on an antenna composed of electrically thin circular cylindrical wires is examined. A moment method solution is obtained, and the insulating shell is modeled by equivalent volume polarization currents. These polarization currents are related in a simple manner to the surface charge density on the wire antenna. In this way the insulating shell causes no new unknowns to be introduced, and the size of the impedance matrix is the same as for the uninsulated wires. The insulation is accounted for entirely through a modification of the symmetric impedance matrix. This modification influences the current distribution, impedance, efficiency, field patterns, and scattering properties. The theory is compared with measurement for dielectric coated antennas in air.

Dielectric coated wire antennas

The applications and theories of the insulated dipole and monopole are reviewed. A recent analysis of the insulated antenna in a relatively dense ambient medium is summarized and its predictions are shown to be in good agreement with an extensive new set of measurements of the distributions of current and charge and the driving‐point admittances of air‐insulated antennas in fresh and salt water. Four different thicknesses of insulation and lengths up to one wavelength are used. The attenuation due to radiation into fresh water is comparable with and even exceeds the attenuation due to dissipation in salt water. It increases with decreasing radius of the insulating cylinder. The effect of the junction between the feeding coaxial line and the insulated antenna is discussed.

Insulated linear antenna: Theory and experiment. II

The properties of an insulated dipole antenna are derived when the wavenumber of the surrounding infinite, homogeneous, and isotropic medium is large compared with that of the insulation. It is shown that even when the surrounding medium is a perfect dielectric, the distributions of current and charge along the antenna and its driving-point admittance are substantially like those of a section of transmission line with a distributed radiation loss appearing as a part of the series impedance per unit length.

R. W. P. King

Papers

The insulated dipole antenna in a relatively dense medium