Steerable Antenna Vertical DirectivityJuly 23rd Fact-of-the-Day
HF dipoles, Yagi's, log-periodic arrays, and other types of antennas are often made horizontally rotatable to make their radiation patterns steerable in azimuth. Antennas with steerable vertical directivity are rarely used by amateurs, but are sometimes used in international broadcast, military, ionospheric research, and other applications. Vertical rotation of a Yagi or log-periodic array changes its vertical radiation pattern, but less than horizontal rotation changes its azimuth pattern, because HF vertical radiation patterns are influenced strongly by ground reflections. In general, the more gain a directional antenna has, the more its vertical radiation pattern will change with vertical rotation. A greater change can be achieved by adjusting the relative RF phase between vertically stacked or horizontally displaced antennas that are also rotatable vertically. However, horizontally displaced antennas must be displaced in the direction of transmission to make their combined vertical radiation pattern steerable by means of phase adjustment. ©2005 Martek International All rights reserved.
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