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"Richard Harrison" wrote:
The earth`s attenuation of low-angle radiation from a 1/4-wave vertical antenna has a significant effect on the vertical radiation pattern. ee Fig. 54-1 on page 465 of B. Whitfield Griffith`s "Radio-Electronic Transmission Fundamentals". This figure shows field intensity curves versus vertical angle from a 1/4-wave vertical antenna radiating 1 kilowatt over earth of average conductivity. Anything below about 5-degrees is gone, eaten by the earth`s losses. ________________ This certainly is not true for frequencies below about 2 MHz. If it was true, MW broadcast stations would have no groundwave coverage -- which of course is the only useful coverage they _do_ have in the daytime. A monopole vertical radiator of any length up to 5/8-wave, when used with a ground system of ~120 buried radials each ~1/2-wave long, radiates its peak field very nearly in the horizontal plane regardless of the conductivity of the ground in which the radials are buried. This gain is within a few percent of the theoretical peak gain for these radiators when working against an infinite, perfectly conducting ground plane, as was demonstrated by the field tests of Brown, Lewis & Epstein in 1937. This principle has been accepted and used by the FCC and other regulating agencies, and has been field-proven in thousands of installations going back many decades. Once "launched," the groundwave signal is affected by ground conductivity along the propagation path, earth curvature, obstructions etc. Groundwave path loss increases with increasing frequency, and above some frequency in the low HF range, the groundwave is unable to serve a practical purpose. But that doesn't necessarily mean that the transmit antenna did not generate the groundwave in the first place, ie, that it radiated zero field in the horizontal plane and at very low elevation angles. RF Visit http://rfry.org for FM transmission system papers. |
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