Topband: radials for verticals

[Maurizio Panicara]

A vertical dipole still needs a good ground to be efficient at low angles
and have no advantages over a quarter wave radiator.
The center feed is an arbitrary feature and an half wave dipole can be also
feed at bottom, or upper edge, with no difference in dipole current
distribution or efficiency.
An half wave radiator close to ground should also have ground radials to
minimize losses, radials that have the bad requirement to be the long and
and more "dense" off the antenna base to be effective.
The reason for this is in the current distribution along the antenna that in
an half wave radiator sets the maximum current density, and consequently the
return path, 1/4 Wl off the antenna base.
Rising off the ground a vertical dipole minimizes the requirement of radials
but a cancellation at low angles occurs and make its gain at a number of
strategical angles inferior to a classical ground plane.
Concerning short radiators, it's not only useless but wrong to load twice
the available element lenght to make it half wave resonant. This technique
increases the loading losses with no benefits.
Loading the available antenna height as a quarterwave against ground is
definitely a more efficient solution.
First because of the less loading amount required (intuitively explaining,
the missing half of the antenna is not loaded), second because the point of
max current in the "doubly short" half wave is never enough elevated from
ground. (best compromise would be around 3/8 wl, but then a full size
quartewave GP would be 1/8 WL shorter)
In case of  extremely short radiators, the current distribution along the
antenna can be considered constant while there are no practical differences,
losses apart,  between half wave and quarter wave loaded antennas in term of
radiating properties.

73
Mauri I4JMY



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