Furnished to Dtic Contained

The possibilities for tunable electrically small antennas are explored and analyzed. Although experimental determinations were made of impedance characteristics from 30 to 220 MHz, the objective was to study the possibility of tuning and bandwidth control for antennas in the range 30 to 100 MHz. A considerable part of the discussion is devoted to acceptable circidt representations or models for the antennas. Both the electric monopole and the folded monopole, each over a conducting ground plane, are considered in detail. The first of these tends to become capacitive as it is operated at progressively lower frequencies whereas the second tends to become inductive as the frequency is decreased. The practicality of having tunable small antennas is directly associated with the possibilities of remote tuning and remote bandwidth control. Both of these topics were investigated. It appears entirely possible to have such remote control units operable at 30 MHz and above. The present state of the art of transistors would make this more difficult for frequencies approaching 100 MHz. Transistors, however, are currently being produced which show great promise for control units even at this frequency. Fortunately, some phase shift in a transistor is tolerable for the tuning reactance units and the bandwidth control units. The range required for the tuning unit was thoroughly explored and two examples are given of antennas which are properly tuned by an external tuning unit. The electric monopole studied was electrically small, being less than 1/4 of its usual height, when operated at the lower end of the frequency range. Data are presented on the tuning of a folded monopole and show the possibility of operation of this type of antenna with a similar reduction in size. It is apparent that greater reduction factors could be obtained with both of these antennas.