Analysis and design of a minimal profile MW transmitting antenna

The performance of a vertically polarized low-profile antenna element with minimum vertical dimension is investigated for use by AM broadcast stations at MW frequencies. The radiation patterns for a sample lossless DDRR (directional discontinuity ring radiator) antenna over a perfectly conducting ground plane near the center of the AM broadcast band are presented. The height above ground of the lambda /4 horizontal ring is 3 m, or 0.0097 lambda at 970 kHz. Comparison of the vertically polarized radiation patterns to those of a lossless lambda /4 monopole (77 m in height at this frequency) shows reasonably equivalent performance. The horizontal plane pattern exhibits excellent circularity. The reduced gain in the horizontal plane is attributed to additional gain at the higher elevation angle. Independent control of the vertical plane radiation pattern would allow the designer to maximize the groundwave field while restricting high angle radiation, and therefore skywave interference, toward other cochannel stations. The DDRR is highly promising in this regard, and optimization of the design appears to provide some degree of control.<
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