The characteristics of electrically short, umbrella top-loaded antennas

Experimental measurements of the electrical characteristics of umbrella top-loaded, electrically short antennas have been performed. Electrically short antennas can be represented by a series RLC circuit with good accuracy up to and including their first series resonant frequency. Electrically short antennas generally have very high Q´s and, consequently, narrow bandwidths. Top-loading, such as umbrella top-loading of electrically short monopole antennas, leads to antennas which have lower Q´s and larger bandwidths. The experimental measurements were made using a scale model facility. The antennas measured on the scale model facility were one-hundredth scale models of the anticipated full scale antennas. The equivalent inductance, capacitance, resistance, and effective height of these scale model antennas were determined and compared with values of static capacitance, static effective height, and static resistance obtained from a computer calculation [1], [2] and good correlation was obtained. Additionally, the dynamic characteristics of the antennas were determined from the model studies. The dynamic parameters determined were the antenna inductance, the dynamic radiation resistance, and the dynamic effective height. The measured data obtained from the model study are summarized in graphs and tables and compared with other reported experimental results as well as with computer calculations. Nomograms summarizing the computed data have been constructed and are presented.