Valentino Trainotti

On the crossed field antenna performance

Lately, short antennas and Crossed Field Antennas (CFA) have attracted broadcast and amateur community attention. The CFA antenna has been developed in the last decade of the 20th century, trying to obtain a compact transmitting antenna for low and medium frequency AM bands. The CFA is intended to be used in order to get a low profile antenna and a supposed performance similar or better compared to a quarter-wave monopole. The CFA has a short monopole and a metallic disk close to the monopole base, both mechanical structures being fed by means of two separated generators. Thus, the CFA has two ports and can be analyzed from the Network Theory point of view. In this paper, the CFA has been studied exhaustively using the Transmission Line Method (TLM) in order to obtain an equivalent network and the antenna performance. Due to the lack of theoretical data to explain the CFA antenna behavior, the TLM has been validated by means of Moment Method simulations and some available experimental data

Short low- and medium-frequency antenna performance

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 antennas 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 antennas 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 antennas 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 Earths 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.

Vertically Polarized Dipoles and Monopoles, Directivity, Effective Height and Antenna Factor

The effective length (Le) of linear antennas like dipoles and the effective height (He) of monopoles shorter than a wavelength are determined using a transmitting Hertz Dipole as a reference. Effective Receiving Area (AeR) has been calculated in the receiving mode and hence the Antenna Receiving Directivity (DR) or Gain (GR)) can be determined. From these calculations, it can be seen that the Received Power (WR) in a link between two dipole antennas in free space or between two monopole antennas over a perfect ground is of the same value in the far field region. As a result, the directivity or gain of the ideal dipole antenna in free space has the same value in transmission and reception; but, a different value for monopole and dipole antennas installed over a perfect ground plane. This result is fulfilled in the ideal as well as the real case where losses and mismatchings are involved.

Short medium frequency AM antennas

Short antennas have again attracted broadcaster attention. These kinds of antennas have been used since the 1920s. At that time it was the logical antenna as a new application of this service after more than twenty years of telegraphic transmissions. Telegraphic transmissions were the most important radio communication service at that time, and because of the long range needed the lowest frequencies as possible were employed. For this reason very short antennas were used even if their size was enormous. Top loaded monopoles were very popular and this technique was employed for broadcast use before the vertical transmitting mast exhaustive study was carried on in the thirties. Nowadays a short antenna would be useful for low power applications and specially to be mounted on building tops. Of course this kind of antennas is not intended to replace the optimum monopoles or vertical dipole where maximum efficiency, maximum gain and antifading properties were achieved after exhaustive studies and after long experience theoretically and practically achieved. CFAs, short monopoles, short dipoles and short folded monopoles have been analyzed from the theoretical and practical point of view in order to choose the simplest and most efficient. model to fulfill downtown stringent requirements.

Simplified calculation of ground losses in low- and medium-frequency antenna systems

Calculations of ground losses are paramount in obtaining the best performance of a monopole antenna in the low- (LF) and medium-frequency (MF) bands. Ground losses are usually computed numerically, due to difficulties in the mathematical formalism. The novel approach here permits obtaining simple analytical expressions for ground-loss calculations that can be useful for determining the behavior of the ground plane. As a first approximation, the monopole antenna is placed on a perfect electrically conducting (PEC) ground plane in order to obtain the antenna current distribution and the near magnetic field, taking into account the non-zero-radius effect of the monopole. Next, the near magnetic field is used to determine the surface-current density on the ground plane below the antenna. This is divided into two zones: (1) the artificial ground plane, where either a radial-wire ground screen or a metallic layer is used to increase the soils conductivity; and (2) the natural ground plane or bare soil up to a circular boundary a half wavelength from the antennas base. The power dissipation is calculated from the artificial and natural ground-plane surface-current densities, and the ground-plane loss resistance is obtained. Also, an effective conductivity is defined as a measure of the ground planes effectiveness, and the cases of quarter-wave monopoles and short top-loaded antennas are analyzed. Some results are validated by means of numerical computations and moment method simulations

Electromagnetic Compatibility (EMC) Antenna Gain and Factor

A method for calculating and measuring antenna gain and factor over perfect ground is presented. The EMC antenna factor is used to determine the spurious electric field of device under evaluation. The radio link employed to calibrate the receiving antenna gain and factor will produce completely different values for the transmitting and receiving antennas, and the principle of reciprocity does not hold true, in this particular case, even if both antennas are identical.

Accurate Evaluation of Magnetic- and Electric-Field Losses in Ground Systems

The efficiency of ground-based antennas is highly determined by the power dissipated in the ground plane, which can be separated into H-field and E-field losses. In this paper, a new approach is presented for the separation of ground losses that is based on Joules law. It is theoretically valid at any frequency. Nevertheless, some simplifications can be applied in the low-and medium-frequency bands, where the Earths soil behaves like a good conductor. In the analysis, the antennas ground plane has been divided into two zones: a) the artificial ground plane, where a radial-wire ground screen was used; and b) the natural ground plane or bare soil, up to a circular boundary a half wavelength from the antennas base. In order to avoid overestimating the penetration of fields in the artificial ground plane, the previous theory has been extended by introducing the concept of effective skin depth. The monopoles nonzero equivalent radius effect has been taken into account by means of a modified current distribution. Also, the case of short top-loaded antennas has been treated. H-field and E-field losses have been analyzed by means of equivalent resistances and computed numerically as functions of frequency in the LF and MF bands for different antenna dimensions, ground screens, and soil physical conditions. Some results have also been obtained by Moment-Method simulations.

Overhead Quasi-Coaxial Transmission Lines

Design expressions for overhead quasi-coaxial transmission lines are developed and presented in a simplified form, suitable for engineering purposes. The electrostatic logarithmic potential method is used for low-, medium-, and high-frequency applications, where the line dimensions in wavelengths are quite low, so it is not necessary to take into account the retarded potential. This kind of transmission line is very useful and convenient for high-power broadcast transmissions, due to the high power capacity and very low attenuation of the line. This is especially important when the distance between the transmitting building and the radiating system is quite large.

Radio Link Power Density and Antenna Factor

Generally antenna characteristics have been defined always in the transmitting case. In free space radio link identical antennas characteristics show that they have exactly the same gain, area, and factor. Contrary to current usage and practices transmitting and receiving antenna characteristics are not the same, since the surface of the earth acts as a reflecting plane. This investigation was conducted to show the difference in gain, area, and factor of two identical antennas in the transmitting and receiving role, through calculations, simulation, and measurements necessary for electromagnetic compatibility.

Electromagnetic compatibility (EMC) antenna gain and factor

A method for calculating and measuring antenna gain and factor over perfect ground is presented. The electromagnetic compatibility antenna factor is used to determine the spurious electric field of the device under evaluation. The radio link employed to calibrate the receiving antenna gain and factor will produce completely different values for the transmitting and receiving antennas, and the principle of reciprocity does not hold true, in this particular case, even if both antennas are identical.