General design considerations for Tacan transponder antennas

During the course of development and production of the TACAN System, much effort was devoted to the improvement of the Transponder Antenna. This work has led to the realization and definition of certain design parameters and their effect on the operational characteristics of the antenna. This paper defines these various parameters and discusses how these affect the radiation characteristics. Specifically, the paper includes discussions of the following factors: 1. The general derivation of the horizontal and vertical radiation pattern of a circular symmetrical array of radiators. 2. The modulation components generated by a rotating symmetrical array are defined by Bessel Coefficients. The broad limitations imposed by the Bessel Function are discussed. 3. A reference radiator is required to provide the carrier component for the system. The r-f phase of this reference radiator, as related to the phase of the symmetrical array, is an important consideration in the resulting pattern. The techniques for determining and adjusting this phase are discussed. These techniques include the adjustment of the diameter of the symmetrical array (for certain types of design) and the vertical displacement of the center of radiation of the symmetrical array with respect to the reference radiator. 4. The specific problems involved in obtaining optimum vertical coverage are discussed and in part, involve the factors mentioned above. In addition, however, the coverage also depends on the relative amplitude of the carrier and side-band components which, in turn are determined by the individual radiation patterns. The optimum formation of these patterns is discussed. 5. Techniques have been developed which utilize the carrier component of the horizontal symmetrical array to obtain vertical gain. This permits the design of a miniaturized antenna which possesses both vertical gain and good modulation characteristics. The proper utilization of the above factors permits the specific design of a TACAN antenna to meet any special operational requirements.