Topband: desperate newcomer modeling

[Eric Gustafson]

Hi Kele,

Be careful in drawing conclusions from these models.  Both of
these antennas are strongly coupled to earth in the near field.
But none of the NEC based programs that I have dealt with so far
(up through NEC2) handle near field losses well (or at all).
Most don't handle horizontal antennas near earth well (including
NEC2 below their published minimum height) even neglecting
storage field effects.  At least with the vertical you can
mitigate the near field losses with an adequate radial screen
under it.

But besides the modeling program limitations, there is another
issue.  For frequencies as low as topband, there is an earth's
magnetic field effect to consider.  Depending on where you are on
the earth, either a horizontal or a vertically polarized antenna
may be the most efficeint at launching energy into the
ionosphere.  For the U.S., verticals generally win big.  In other
parts of the world, horizontals have a clear advantage.  Find out
what kind of geomagnetic field effects are operational in your
area before deciding what is the best antenna option.  There was
an article about this in a recent issue of Communications
Quarterly.  It was very interesting.

73, Eric  N7CL




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