Radiation Characteristics

Author: Edmund A. Laport

The horizontal rhombic antenna is pictured in Fig. 3.77 (unidirectional) and Fig. 3.80 (reversible). This construction, using two or three wires per side, is the preferred form for both transmitting and receiving. The original single-wire type is suitable only for receiving. The multiwire type has a flatter impedance-frequency response curve and lower characteristic impedance, which is of great importance for transmitting, and smaller pickup of precipitation static when used for receiving. The latter can be improved further by using insulated wires to prevent charged particles from coming in direct contact with the metal of the antenna and feeder system.

Each leg of a rhombic antenna has a radiation pattern which is the result of an attenuated traveling wave, as indicated in Fig. 3.68. The length per leg l is usually great enough to cause its pattern to have many secondary radiation lobes (one for each half wavelength of length, if the main lobe is included). The over-all pattern is that due to interference between the radiations from the four legs and their images. The height above ground h, the ground constants, the length of the legs, and their included angles are the parameters that control the pattern for each frequency. These parameters are optimum for only one frequency.

With fixed physical dimensions, the elevation of the peak of the main lobe, which is directed along the major axis of the array, is lowered as the operating frequency is raised. If the frequency is increased too much, the main beam will split. At the same time, the horizontal width of the main beam decreases, and there is an increase in the number of secondary lobes of appreciable amplitude appearing near the main lobe. In extreme cases, the main beam is broken into several beams of almost equal amplitude and spread over a considerable range of horizontal and vertical angles, as is seen in Fig. 3.78c.

Radiation along the major axis of the array is horizontally polarized. Radiations off of this axis contain both horizontally and vertically polarized components, the proportion of the latter increasing with angle from the axis.

There is one other parameter that can be varied in addition to those named. That is the plane of the rhombus, which can be tilted out of the horizontal plane about the minor axis. The principal effect of tilting is to increase the vertical beam width of the main lobe. The value of this characteristic depends upon propagation conditions, and under some propagation circumstances it can be a disadvantage.

The rhombic antenna is best suited for long-distance circuits where low-angle transmission or reception is wanted. When the beam angle is higher than 30 or 35 degrees, the gain is low and the desired performance can usually be obtained more economically with dipoles.

Figure 3.81 shows the optimum parameters for horizontal rhombic antennas for one frequency only. These data were compiled from stereographic charts which will be described later. The parameters were computed for the conditions that maximize the main beam and minimize the first side lobes to obtain best gain, using idealized factors for the system, including perfectly conducting ground.

The data represent about as much information as can be dependably presented in one figure for the guidance of the designer of a rhombic antenna. The main beam is less sensitive to the influence of variations in the geometric parameters and in the several empirical factors than are the other lobes.

It is always desirable to know the complete radiation pattern for an antenna for every application. While it is very seldom that this pattern is exactly what it is supposed to be, one may at least have an idea of the true state of affairs. This is difficult with rhombic antennas because one must submit to a great amount of labor to compute a single array at a single frequency.

FIG. 3.81. Optimum parameters for the horizontal rhombic antenna, for maximum gain and minimum side-lobe amplitudes.

As a consequence, rhombic-antenna performances are customarily taken for granted far more frequently than they are known. This naturally has led to many errors of judgment in rhombic-antenna applications.

If one wishes to compute rigorously the radiation pattern from a rhombic antenna, taking into account the attenuation of the traveling waves of current in the system and the complex reflectivity of the soil, the formulas of Cafferata are perhaps the most complete. Harper provides formulas which allow for the earth reflectivity but not for the attenuation of the currents in the wires. The method of Foster permits location of the main and all the minor lobes of a rhombic-antenna pattern relatively quickly by a graphical method which is very useful in forming an idea of the over-all pattern without excessive labor. The original sources should be consulted for details of formulation and method.