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the 6' > height . . . . I'm not sure who first mentioned the Cebik article, but I looked it up. You can find it at http://www.cebik.com/fdim/fdim4.html. For a 40m vertical halfwave dipole he notes that as the base is raised more that 10 ft above ground, a second lobe begins to develop at high incidence angle. His comment on this is as follows: "The pattern of a vertical antenna at a low height shows a single lobe when viewed with respect to the field elevation. Notice that the antenna is relatively insensitive to radiation coming from higher elevation angles. As we increase the height of the vertical, a second lobe emerges at a higher elevation angle. This lobe peaks in the vicinity of a 60 degrees elevation angle--too high for the reception of almost anything except atmospheric noise. "Those who use vertical antennas by choice rather than the necessities of a particular antenna site often select them knowing that the gain will not compete with a horizontally polarized antenna they might use instead. However, the signal-to-noise ratio is often improved because atmospherics received from high elevation angles are reduced. Some of that reception advantage disappears if we place the antenna too high. and the second lobe of the elevation pattern achieves full development." To translate this into Sigma V terms, 10 ft agl is 3m. A quarter wave on 40m is 10.7m, so the dipole feedpoint is 13.7m AGL, or about 0.3 wavelengths. For a Sigma V on 20m, 0.3 wavelengths is 6.3 m, so the base (which is 4.5 ft or 1.4 m below the feedpoint) is at approx 4.9 m AGL, which is in line my own findings. Cebik also mentions the point raised by Guy K2AV, that in the real world there are other considerations: "There is a counterweight to this facet of vertical antenna behavior that is especially apt to urban, suburban, and wooded locations. I cannot demonstrate it with a model, but only from the collective experience of many vertical users, including myself. The phenomenon is the dreaded "Rf-eating shrubbery." In the open fields of America's great farming states, a ground-mounted vertical has its best home, with nothing but open fields for many wavelengths in any direction. In crowded locations, the presence of significant structures--both natural and man-made--appears to prevent a ground-mounted vertical from achieving its full performance potential. Therefore, the elevated location of a vertical monopole--for example, a roof-top- -becomes a better location. The higher location is especially apt to the compact, multi-band vertical monopoles produced by many commercial companies." 73, Andrew -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force12 Web Site: http://www.force12inc.com To Submit Message to the List: Force12Talk@qth.com To unsubscribe and view the Message Archive: see http://qth.com/force12/list For problems with the list: contact n4zr@qth.com |
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