The Theory of Thin Antennas and Its Use in Antenna Engineering

The great value of the theory of thin antennas is substantiated. Models of a linear radiator shaped as a straight perfectly conducting filament with zero and finite radii and as a straight circular thin-wall cylinder are described. Methods of calculation, which were applied before resorting to integral equations, are presented, in particular the induced emf method, its first and second formulations. Results of its application to symmetrical dipoles, to radiators with displaced feed point, to radiators with constant and piecewise constant surface impedance and lumped loads, to folded and multiradiators antenna are given.

Integral equations for currents in one and two straight metal radiators with exact and approximate kernel and methods of solving these equations with the help of an iterative process, perturbation method and Moment Method are considered. Results are generalized to the case of radiators with constant and piecewise-constant impedance and with lumped loads. The Moment Method with piecewise-sinusoidal basic and weighting functions is shown to correspond to the physical content of a problem and be equivalent to division of the radiator into isolated dipoles, the self- and mutual impedances of which are calculated by the method of induced emf.

Additional methods of analysis are considered. It is shown that reducing three-dimensional conic problem to the two-dimensional one and using the complex potential method enables one to calculate the capacitance per unit length and the wave impedance for a dipole with inclined arms and also the same for an infinite long line and a metal radiator of two convergent filaments or conic shells. The theory of electrically coupled lines permits analyzing multiple-wire structures of antennas and cables. The mathematical programming method allows selecting the loads to create an antenna with the characteristics as close to the given one as possible. The compensation method is proposed to protect living organisms and electronic devices from strong electromagnetic fields in the near region of an antenna.

The performance of a slot antenna situated on infinite conic metal surface with circular cross-section is shown to be similar to that of situated on the infinite flat metal surface. The proof is based on the transition from a V-shaped magnetic radiator to a double-sided slot cut in the mentioned surface. If, to simplify the design, metal and slot radiators are placed along the regular pyramid sides, the antenna performance changes but remains close to that of the self-complementary antenna. Flux density distribution over long line cross section is analyzed. The transition from the parabolic problem to the flat one is done.

The theory of electrically coupled lines is applied as the rigorous method to the calculation of multi-conductor cables in order to determine the cause of the emergence of the electromagnetic interference (crosstalk) in communication channels and the common mode currents in the lines. The causes of the cable asymmetry, which leads to crosstalk, are shown to be the construction of each line as a twisted pair and the different distances between wire pairs of two adjacent long lines. The cause of emergence of the common mode is the asymmetry of excitation and loads. Calculation of capacitance between two wire elements in the presence of the third one permits to determine the equivalent lengths of a long line and monopole with unequal lengths of wires.

Application of the impedance line method to radiators with concentrated loads permits to create antennas with required characteristics, in particular, the wideband monopole, an antenna with the given current distribution, etc. Wideband radiators must have an exponential or linear in-phase current distribution, created by capacitive loads that vary, in particular, along the radiator length in accordance with linear law. In order to retain in-phase current distribution in a wider frequency range, the capacitances of these loads should vary in inverse proportion to the squared frequency. Using of capacitive loads in V-dipoles yields similar results.

Issues of applying the compensation method application are studied, including the heterogeneous nature of the media, optimal placement of main and auxiliary radiators, the dark spot dimensions, the reduction factor and variation of the total and local SAR, retention of patterns and field strength. Results of calculations, performed by different methods, are compared with each other and with experiment. A technique of protecting system against external action is described; the automatic adjustment circuit is developed. Recommendations for using the compensation method in a wide frequency range are given.

Main circuits of antenna arrays and their characteristics are described. Flat log-periodic structure, reflect arrays and adaptive systems are considered in greater details including the use of algorithms of adaptation and adaptive control of weighting coefficients, which is performed by the adaptive processor.

The chapter treats particular antenna problems, namely, transparent antennas, ship antennas, rectangular loop, and ground resistance. A rigorous method to calculate a transparent antenna based on solving the integral equation for the current permits to develop the antenna design. An analysis of electrical characteristics of a rectangular loop shows how its pattern differs from that of a circular loop. An application of the theorem on the oscillating power enables to simplify and define more precisely the magnitudes of the monopole ground resistance. Section dedicated to ship antennas deals with specific difficulties in their designing and service, the influence of superstructures and other metal bodies on their electrical characteristics, and the creation of the main ship antenna with inductance-capacitance load.