Topband: 160 Antenna's Modeled

Maurizio Panicara
Thu, 19 Apr 2001 19:35:49 +0200

Inherent antenna efficiency takes in account for losses in the antenna
itself and in the nearby area but an efficiency of this kind doesn't insure
for optimal pattern if the distant ground, that one where the important
reflections occur, hasn't the needed features to be optimal.
Often a low efficiency antenna but with a good pattern for a stated
application largely outperforms a different type of antenna that have an
high efficiency but also a poor radiation at desired elevation or azimuth.
In spite of claims that called almost useless the very low angles at lower
HF frequency, this is actually the reason why verticals over nearly perfect
ground often outperform other antennas or a similar ones but in another
location with a poorer ground.
Because of a much higher power density at very low elevation angles infact,
the number of hops is reduced and this is surely important when the power
used is small like in amateur applications and each Hop is a lot of dB loss
The fact that the average useful angle at a stated frequency is higher, is
surely useful for commercial applications but of secondary importance when
the "anomaly" is what's needed to allow a very distant contact with
constrained aerial environment and little used power.
Verticals over salt water are often so great because the others over poor or
fair ground "suffer" and do not radiate enough at very low elevations,
almost independently by losses caused by an imperfect radial system.
Below a certain angle and with imperfect ground that extends for
wavelenghts, the vertical antenna and it's image are not in phase and thus a
consistent energy cancellation occurs at very low elevations. This
pheneomena is actually caused by imperfect ground diffraction and the angle
below which the diffraction starts to happen is named "pseudo Brewster".

Mauri I4JMY

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