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Scale-model Measurements

Author: Edmund A. Laport

The search for the simplest and most economical antenna configuration that will give a desired value of radiation resistance with top loading is complicated by many factors.

Computations are practical only for rather simple systems, such as for inverted-L, T, and X configurations. For large, complicated capacitance areas made up of many wires of different lengths and orientations it is easier to make and measure small-scale models than to attempt to compute the values.

Antenna scale models make use of the fact that there is a fixed relationship in systems of identical electrical dimensions. An accurate scale model one-fiftieth the size of the original will have the same electrical dimensions when operated at fifty times the frequency. Scales as small as one one-thousandth have been successfully used for low-frequency design engineering.

The technique is to lay out a large metallic surface, very large with respect to the area of the proposed model, to serve as a ground plane. Towers, guys, antenna rigging, and placement of insulators are modeled carefully, including the relative cross section of the wires, sags, etc. Usually the only measurements necessary from a model are the fundamental frequency and the reactances in the region of the scaled operating frequency. The fundamental frequency can be measured by using shielded coupled buzzer excitation with a calibrated receiver as a frequency meter. The reactances can be measured with a Q meter, taking necessary precautions with body effect of the operator in some cases. With small expense and very little expenditure of time, a good model will yield the desired information and permit studies of such things as insulated versus grounded towers, placement of guy insulators, and alternative wire configurations. All the data needed to compute the performance of a full-scale system can be obtained except those factors associated with the ground system. The characteristics of ground systems and flow of ground currents in imperfect dielectrics do not follow the same principle of similitude as do fundamental frequency, reactance, and radiation resistance.

The value of scale-model measurements in low-frequency-antenna design cannot be overemphasized, both for economy of engineering time and for precise forecasting of performance. By this technique it is possible to obtain a great deal of empirical information that cannot be reliably calculated. System costs can be minimized by experimentally developing the most conservative structure for a given performance.

Last Update: 2011-03-19