Short low- and medium-frequency antenna performance

Abstract

Lately, short antennas have attracted the attention of the broadcast and communication communities. This kind of antenna has been used since the 1920s. Top-loaded monopoles are the logical antennas to be used in order to get a low-profile antenna and performance according to the needs of the broadcaster and for communication. In this paper, top-loaded monopoles have been studied exhaustively using the transmission-line technique. Improved expressions for the antenna's radiation resistance have been obtained, taking into account the top-base current relationship and under different top-loading conditions. This idea, of using an equivalent transmission-line technique, has been used since the 1920s in order to obtain the antenna's input reactance. Using this old idea, the novel approach here permits obtaining the near- and far-field expressions from the current distribution on the antenna structure. A near-field calculation is used to determine the surface-current density on the ground plane. The power dissipation is calculated from the artificial and natural ground-plane surface-current densities, and the ground plane equivalent loss resistance is obtained. In all cases, as a first approximation, a half-wavelength ground-plane radius has been used, because this is the maximum distance covered by the ground surface current under the antenna, closing the antenna's electric circuit. Beyond this distance, the ground currents do not return to the antenna generator, and are taken into account in the surface-wave propagation calculations. The half-wavelength ground-plane surface is partially occupied by the metallic radial ground system, and the remainder consists of the natural soil. Artificial ground-plane behavior is paramount in obtaining the best performance for a short antenna. This kind of antenna could perform very close to a standard quarter-wave monopole, if it has optimum dimensions. For these reasons, a short antenna and the corresponding artificial ground plane have been analyzed, modifying the number of radials and their lengths, in order to achieve optimum performance, or to obtain maximum field strength with several soil conditions for the Earth's surface. A very simple and efficient antenna can be obtained, giving to the broadcast and communication communities a product that can fulfil the required performance of radiating a high-quality AM signal or digital transmission in the MF band, and good speech quality in the LF band.