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.     

MF AM folded monopole characteristics

Folded monopole antennas have been used for many years in medium frequency amplitude modulation broadcast applications. These antennas offer a grounded radiating structure with a height approximating a quarter wavelength. Folded monopoles have been studied exhaustively during a previous effort in an attempt to characterize the input impedance and directivity characteristics as a function of monopole height. As a result it was observed that several impedance regions can be achieved as a function of antenna height and cage diameter: Some of these regions permit a bandwidth operation sufficiently broad to support a high fidelity response for analog or digital transmissions. This type of antenna can be useful at different heights including a self matching region at the standard 50 ohms impedance. By choosing the appropriate antenna physical dimensions, antenna tuners can be avoided and thus it increases efficiency. Measurements have been made on a full scale 72 meter tower with a 6 wire cage in order to validate the calculated antenna input impedance and directivity. The cage and supporting mast make up the equivalent of a transmission line. For this reason different standard triangular towers were analyzed in order to determine their effect on the input impedance. Tower effective radius is determined taking into account the tower width and the tower leg diameter. At the same time, a minimum antenna height with maximum efficiency has been analyzed to solve the hard problem for installations at places where antenna height is a challenge for the designer.     

MF AM grounded dipole for stereo and digital transmissions

Modern MF AM transmitter output impedance has been chosen to be 50 ohms unbalanced. For this reason high power 50 ohms coaxial lines have been designed and manufactured, rigid or almost flexible in order to connect the transmitter in the station building and the far away radiating system at one or more wavelengths. At MF frequencies the radiating system is generally a standard classical monopole where a matching system close to the monopole is needed to obtain maximum radiation efficiency. The proposal here is to use a radiating system made up of a grounded tower close to half a wavelength tall with a cylindrical skirt around the tower's lower part. The skirt is fed directly from the 50 ohm coaxial line at a point whose impedance is very close to 50 ohms, avoiding the matching system and improving the radiation efficiency within a good bandwidth suitable for stereo or digital transmissions. Design permits a low standing wave ratio within the information passband necessary to obtain the lowest information distortion. Scale models have been designed, constructed and measured obtaining similar results as predicted by calculations in the MF frequencies. The advantages of a grounded tower are discussed     

MF AM asymmetric vertical-dipole-antenna measurements

A medium-frequency AM transmitting vertical dipole for a 50 kW broadcast station was constructed, during May and June, 1994, at Buenos Aires, Argentina. This kind of vertical dipole operates without the classic, 120-wire, buried artificial ground plane. In order to analyze its behavior, extensive measurements of the near and far fields were carried out. A comparison to standard monopoles gave a better understanding of the performance and efficiency parameters. The nocturnal service area was determined by measurements, in order to observe the antenna's high-angle radiation, and to compare it with the theoretical radiation pattern. Comparison with the previous monopole antenna confirmed that substantial improvements were obtained with this new installation, both antennas being of similar height. The measurements showed an improved coverage of both diurnal and nocturnal service areas, estimated to be four times the previous service areas     

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 soil's conductivity; and (2) the natural ground plane or bare soil up to a circular boundary a half wavelength from the antenna's 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 plane's 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     

FM wide band panel dipole antenna

It is very common that when a broadcaster needs to install an FM transmitting antenna within a large metropolitan area he places it on the tallest structure or building available. When the rooftop is already occupied by a large number of other types of transmitting and receiving antennas, the panel dipole antenna should be chosen. This antenna is secured to the side walls of the upper floors with the panel oriented to obtain full coverage of the most desirable areas of the city. For the Buenos Aires area this orientation avoids radiating toward Uruguay and specifically toward Montevideo, some 140 miles away. A wide band antenna operation permits placing the station on the air and at the same time allows future stations to share it without the installation of new antennas. Details of model and full model impedance and radiation pattern measurements during the antenna development are presented in order to show its technical characteristics. The radiation patterns were measured on a scale model in an anechoic chamber. The full scaled model was measured in an outdoor antenna range. Both E and H plane radiation patterns were measured along the FM band in order to observe pattern variations on both planes. Practically no difference in a panel radiation beamwidth from 88 to 108 MHz was observed and at the same time good input impedance was maintained. A really wide band antenna in pattern and VSWR is obtained. Power division for the antenna system is obtained designing an eight port power divider using the microstrip line technique. In this case, however due to high power operation the ground plane and strip are contained in a sealed metallic box and are separated by high pressure dry air like into the 3" feeding coaxial line.     

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 Joule's law. It is theoretically valid at any frequency. Nevertheless, some simplifications can be applied in the low-and medium-frequency bands, where the Earth's soil behaves like a good conductor. In the analysis, the antenna's 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 antenna's 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 monopole's 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.     

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 coverage area for MF AM broadcaststations

It is shown that the coverage area of medium- and high-frequency stations using ground-wave propagation beyond the horizon, especially MF AM stations, can be calculated by a simplified method. Recent field measurements have shown that the cumbersome FCC and CCIR calculations for estimating service area can be simplified to allow calculations with a PC, or even a programmable pocket calculator. The ground-wave electrical field intensity calculations are simplified by the introduction of a shadow or diffraction factor in the Norton planar earth expression. This factor permits the ground E field to be calculated well beyond the geometric and radio horizon, where E-field values are close to the atmospheric noise level. The approach permits engineers or technicians to calculate their own E =field curves for any frequency and any kind of soil