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#7
July 1st 06, 11:01 PM
posted to rec.radio.amateur.antenna
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HFTA-ARRL-Space
Richard Clark wrote:
On Sat, 01 Jul 2006 11:09:22 -0400, jawod wrote: Here's my question: At 25 to 30 degrees elevation response, aren't these waves leaving the ionosphere (i.e., refracted instead of reflected)? Hi John, You are confusing models of propagation with models for antenna lobe characteristics. The lobes certainly have a major impact on propagation, but the antenna modeler is not concerned with that. The terrain modeler is not a propagation modeler. For that, consult VOACAP or WINCAP. They are properly concerned with ionospheric issues, but they are also driven by antenna models too. Am I right to consider this component of propagation to have left the earth? To some degree, yes, but it has very little bearing on what you are focused on here. This would indicate a substantial fraction of each amateur transmission is sent into space. In fact, no. The apportionment of the energy into lobes is simply robbing Peter to pay Paul. The nulls were developed from energy lost to the peaked lobes. This is very loose analogy because energy was never lost, it is merely the combination from many sources that makes this lobe shape appears as it does. An antenna radiates from every portion of its structure and in every direction. When all contributions are viewed from a distance, some portions of the structure are out of phase with respect to others portions. When those two contributions are 180° out of phase, that remote point at where they combine perceives a null (a destructive combination). At some other remote point, those same two contributions may combine constructively for a peak response. Same energies all around, but path lengths shift the wave phases and how they combine constructs the characteristic lobe shape. If you took college physics, you must have seen how two charges separated by a distance combine their effects at remote points to offer an "electric dipole." Same logic. 73's Richard Clark, KB7QHC Thanks for setting me right! John AB8WH |
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