Basic Definitions and Standards

In this section, some basic standards used for both skywave and ground wave propagation are collected. The standard reference antenna for skywave propagation is the Isotrope antenna in free space. For ground wave propagation, the standard reference antenna is the short vertical over a perfectly conducting ground. (For the equations used for these antennas, see 'Computation of Linear Communication Antennas').

The Isotrope antenna in free space is an omni-directional antenna, i.e., radiates equally in all directions, and is placed in free space, a homogeneous, non-absorbing medium of dielectric constant unity. This is, of course, a fictitious situation, unrealizable in practice, but used to standardize the antenna gains and transmitted power. The field intensity or power density in free space is given by (Norton, 1959). At a standard distance of 1 km and a power of 1 kW, the field strength is 173.2 mV/m or 104.8 dBu. This is for a full sphere; if a half-sphere, i.e., over a perfectly conducting plane, the field intensity is doubled, 245 mV/m or 107.8 dBu.

Following Ramo and Whinnery (1960), the gain, g, relative to an Isotrope source is defined as the ratio of power required from the Isotrope source to produce the given intensity in the desired direction to that required from the actual antenna.

The antennas modeled for skywave patterns, are referred to an Isotrope radiator, and are to be used in the system loss calculation. Most of the variation in the system loss calculation is due to those variables, which depend upon the state of the Ionosphere. The effect on the use of the antenna models is the change of usable frequency with the resultant changes in elevation angle. The problem to be solved by the user of the antenna models is then to select or design an antenna system that will, with some acceptable probability, provide the necessary power/gain pattern over the required period of time and area of coverage.

The ground wave reference standard is a short, vertical antenna over perfect ground radiating 300 mV/m at 1 km. For a standard power of 1 kW and distance of 1 km, the field strength is 300 mV/m or 109.54 dBu. The gain of this antenna is 4.8 dBi.

The radiation resistance and efficiency terms will set the maximum gain at each frequency but will not affect the variation with elevation angle and azimuth. Thus a knowledgeable user can adjust the power/gain input to overcome suspected deficiencies in the radiation resistance or efficiency calculations.

Notes on Gain

Basically there are three quantities in the evaluation of the antenna gain: the power gain as a function of elevation and azimuth angles (and frequency, of course), the input resistance looking into the antenna feed point (i.e., the radiation resistance), and the ability of the antenna to radiate the power supplied to it (i.e., the antenna efficiency). Both the power pattern and the radiation resistance depend greatly upon the current distribution along the antenna, the environmental conditions, and the mutual interaction among antennas if there are more than one in the system. The antenna model follows the common convention of assuming a sinusoidal current distribution on all the wire antennas. With this assumption, the calculation of power patterns including the ground effect is rather straightforward. Evaluation of radiation resistance's with the ground influence included is, however, still somewhat difficult for some antenna models. The basic reference for the models used has been taken as the ITS-74 report (Ma and Walters, 1969). Any questions of difficulties with the equations have been resolved by taking the values given in that report as standard.