A Simple Method for Predicting Field Intensity Near a Rod Antenna

Abstract

This paper presents the results of a theoretical study of the electromagnetic field in the vicinity of a rod antenna transmitting over a conductive ground plane. In the course of studying compatibility problems one occasionally finds it necessary to predict accurately the field strength within a few wavelengths of a radiating element. This problem i s more difficult than one might expect, especially if the length of the radiating element is greater than one tenth of a wavelength. In fact, the equations involved are sufficiently complex to effectively prohibit their use as a practical engineering tool. What is needed is a simple and rapid method for applying these equations. This paper presents such a method. The three general equations which describe the three components of the field are presented and discussed. A digital computer is used to obtain solutions for a specific antenna height and several specific frequencies. The graphical computer results are presented and discussed. A method for normalizing the three general equations is developed. It i s shown that the data obtained from the computer (for specific values of the variables) can be transformed into a normalized presentation in which all distances are measured in terms of wavelength. It therefore becomes possible to construct normalized field strength graphs. These graphs can easily be applied to obtain theoretical field strength values for the three components of the field over a wide range of parameters. Examples are presented, illustrating the method of application. Figure 1 is a typical normalized field strength graph. It is constructed for the case in which antenna height (h ) is equal to 0.685 wavelength. The horizontal axis represents horizontal distance from the base of the antenna, measured in wavelengths. Each of the curves on the graph corresponds to certain height above the ground plane, again measured in wavelengths. The vertical axis is field strength expressed in decibels above one microvolt per wavelength per ampere of antenna current. This graph i s for the vertical component of electric field. Similar graphs are constructed for the horizontal component of electric field, and for the magnetic field. The complete family of graphs is made up of a set of three graphs for each ratio of antenna height-to-wavelength to be evaluated.