On Thu, 6 May 2004 12:47:58 -0400, "Jack Painter"
wrote:

"Richard Clark" wrote

The electric dipole moment is clearly bridged by a conductor, by
definition. As such, at the interface, it must collapse completely
into a current which gives rise to counter emf, the two waves cancel
as a function of phase - the proof again is found in the Lambertian
distribution that vanishes completely with the removal of ground (why
horizontal antennas are held up in the air). The more remote the
ground, the greater the variation of phase and the distribution, and
yet the low angles never fully recover (the death embrace of ground is
always there).

Richard, would the dipole's performance thus be improved by bedding the
ground with sand, and hurt by adding ground radials? Same true if the dipole
was at some compromise between 1/4 wave and the desired 1/2 wave above
ground?

Hi Jack,

A good question, and one that brings out the one of my elliptical
statements about having disproven you don't have to worry, because
there is nothing you can do.

In fact you can do something, however, it separates the discussion of
ground insofar as near field and far field issues.

IF you add a ground screen below a horizonal antenna, you CAN improve
your communications efficiency (your contact, with sufficient
resolution, could see an improved, stronger signal).

This, of course, has no strength in its argument in the far field, the
same problem exists of the complete collapse of the electric field
through its polarization giving rise to a canceling current. The near
field application (where the media does NOT exhibit a 377 Ohm
characteristic) is one of shielding the source from loss (which is
largely a dielectric loss, not a conductive, Ohmic loss).

Richard Harrison, KB5WZI, has already recalled Terman's treatment, but
having no reference handy, he hadn't really pulled it together.

The point of the matter is that for a conductive ground, the electric
fields are laid across a short. The obvious occurs and that electric
field collapses into a magnetic field (through the short circuit
current that necessarily follows) at the interface. This simple
statement is enough to evidence the reversal of fortune (magnetic
replacing electric in the face of its initiating source spells short
circuit city).

At a distance (along the magic 0° DX launch angle), BOTH the source
and its reflection (or image) in the ground below it, are at an equal
distance to the observer. Thus the distant observers (if they could)
see TWO sources that are 180° out of phase. Thus everywhere along
this meridian, those two signal completely cancel. With tongue in
cheek, let's call this 100dB down. This happens ONLY for horizontal
polarized signals. By shielding ground beneath the horizontal
antenna, you are doing nothing to change this star fixed fate; but you
are improving efficiency with a net positive gain, relatively
speaking. You simply have two stronger signals canceling.

At higher angles, lets call them 5° or higher (sometimes much higher)
the path lengths of the two sources diverge from equality (a phase
shift is introduced) as the signal strength attempts to pull toward
the free space value, some 30dB higher. If you pull your attention
successively higher, you eventual come to the point where the two path
lengths introduce enough phase difference that they combine to a net
signal that is greater than the free space value. This, by the way,
does not constitute DX opportunity and is crowed about as the great
NVIS advantage (in other words, the sufferer has no options and is
content to make lemonade). This exercise describes the Lambertian
distribution, a classic example of Optical sources.

Raising the horizontal is much the same gain story. It removes itself
from the cold embrace of earth's loss, and it introduces a new phase
combination. Thus the lobes may lower from the Zenith, but you will
never see them pulled all the way down to the horizon, such is the
fate of horizontality. ;-)

73's
Richard Clark, KB7QHC