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The ability of aircraft cap-type high frequency antennas to act as efficient radiators is a subject of much current interest. In most instances the fixed-wire type hf antenna is incorrectly assumed to be superior to flush cap-type antennas. In this paper a performance comparison of five different hf communication antenna configurations for a large high speed aircraft is presented. These antennas include two fixed-wire types, a conventional tail-cap type, and two ``L-gap'' tail-cap configurations. The impedance vs frequency characteristic is explored over the 2-24 mc range for each antenna. The antenna system efficiency of each antenna is computed using the antenna impedance characteristic and considering the transmission line, coupling network, antenna feed and lightning protection system, and the antenna radiation pattern. This antenna system efficiency is then used as a basis for comparison of the five antennas.