General Considerations of Tower Antennnas for Broadcast Use

Abstract

The factors influencing the action of towers when used as radiators are considered. It is shown that the results predicted from the simple theory of sinusoidal distribution of current on the tower differ to a major extent from the actual results. A series of measurements using small models of actual antenna structures resulted in data correlating closely with the performance of the full-sized structures. These measurements showed that departures from the simple theory are due to nonsinusoidal current distribution. Several types of recently installed antenna towers are shown to be less effective than the simple theory prediction, particularly with regard to reduction of sky wave and fading. Means for correcting the current distribution and thereby improving the performance are pointed out. The statement that low base capacity is essential to high antenna efficiency is shown to be a fallacy providing simple precautions are taken to reduce conduction losses. The ground system and earth currents are considered from both a theoretical and experimental viewpoint. A simple method of measuring the earth currents is described and it is pointed out that such measurements indicate whether the antenna current is sinusoidal or not. Appendix A gives a method of computing the radiation characteristics and the radiation resistance when the current distribution is known but is not expressed analytically. Appendix B contains the theory behind the ground current measurements. Appendix C shows the influence of the base insulator capacitance on the operating characteristics.