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>Stephen, > >As noted on page 8 of our catalog when discussing ANTENNA SPECIFICATIONS, >"(The net gain numbers will be fairly constant for antennas installed at 35' >and higher)." However, as the antenna will be reacting to signals arriving >at higher angles when installed at lower heights, the f/b may be lessened >somewhat. Of course the height above ground that the array cares about is >the effective height above ground which can be quite different than physical >ground. The effective height above ground depends on many factors, >including the presence of nearby structures and the composition of the soil. >Unfortunately, there is not an easy way to determine what height the antenna >thinks it is at without detailed soil and near field analysis. I'm not so sure if this really is changes in "effective height", or if it is more a question of the physical soil characteristics altering the ground reflection coefficients. In other words, the ground plane right under the antenna is at the same place, but its loss profile is different, so it has a different effect. I also note that Leeson says in his book that, for horizontal antennas, the reflection occurs at the discontinuity between air and ground, so that the ground characteristics in the Fresnel zone have relatively much less effect on pattern formation than one would otherwise suspect. Reminds me of a cute trick that John Belrose came up with. to determine ground characteristics at a given location, he would very precisely cut a dipole and mount it at a low height above ground. Then he would tweak the ground parameters in his modeling software till it agreed with the empirical behavior of the dipole, et voila! -- he knew the key ground parameters and could then plug them into all his models for antennas to be used there. 73, Pete Smith N4ZR n4zr@contesting.com Don't forget to update your entry on the World Contest Station Database, at http://206.102.70.3/search.htm -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force 12 Web site: http://www.qth.com/force12 Submissions: send to Force12Talk@qth.com To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk For problems with the list, contact force12@qth.com |
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