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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