In daylight, with vertical LF and MF antennas, all radiation at an
angle above 0 dgrees and that reflected from the ground, whuch is most
of it, is wasted and generally unwanted.

Only the groundwave is useful.

At night a virtue is sometimes made out of a vice.


"Roy Lewallen" wrote in message
...
wrote:
Hi to all,

From the help file of EZNEC, I copy this sentence:

''The ground reflections which cause the low-angle pattern
reflections
take place farther from the antenna than most common radial ground
systems extend, so ground radial systems have little or no effect
on
the ground reflection phenomenon.''

My question: Is this phenomenon dependent of the frequency? In
other
words, will the first reflexion point be farther away with
lowering of
the frequency or about at the same distance, independently of the
wavelength**?

**In that case, the ''long'' radials of AM broadcasting verticals
'may'
also become reflectors...


For a given antenna height and elevation angle, it's the same
distance
from the antenna.

Imagine yourself at some point on the antenna, say at the middle of
a
vertical antenna. Direct your gaze at an angle of 10 degrees below
horizontal. Radiation traveling from the antenna at that angle will
strike the ground at the point where your gaze does, and after
reflection it will be going upward at an angle of 10 degrees above
horizontal. The overall elevation pattern at 10 degrees elevation
angle
is formed by the sum of a direct ray going upward at 10 degrees and
one
going downward 10 degrees below horizontal, reflecting off the
ground,
and adding to it at a distant point. The lower the angle or the
higher
on the antenna you are, the farther the reflection point. Antenna
programs repeat this process for every "segment", that is, points
all
along the antenna, to get the total field.

So where does that reflection take place? Let's use your example of
an
AM broadcast antenna. Some broadcasters, I believe, use verticals
which
are around a half wavelength high. Suppose the frequency is 1 MHz,
so
the wavelength is 300 meters, and a half wave antenna would be about
150
meters high. Broadcast radials are, I think, around a half
wavelength
long. So the field from half way up the antenna would just strike
the
end of the radial field, using this simplified model, at a downward
angle of arctan(75/150) ~ 26 degrees. Any radiation from the center
of
the vertical at an angle below 26 degrees would reflect beyond the
radial field, so the elevation pattern for all angles below 26
degrees
would be independent of the radial field. Likewise, radiation at 26
degrees from any portion of the antenna above half way up would
strike
the ground even farther away. Ironically, vertically polarized
reflection at angles above this aren't too bad off plain dirt, so
the
radials don't help as much as you might think even at high angles.
It's
low angle reflections that really suffer when polarization is
vertical,
and those are the ones missing the radial field.

This model is oversimplified, not taking into account the fact that
the
wavelength is large relative to some of the dimensions being
considered
or the considerable skin depth of the ground. The "NEC radial"
option of
NEC and EZNEC uses just this kind of analysis with the ground in the
region of the radial field being assumed uniformly conductive, and I
discourage its use. It's more accurate to model very slightly
elevated
radial wires where the fields from the wire currents make their true
contribution. But when you do, you'll find that the radials still
don't
have much impact on the lower angle portion of the pattern.

Roy Lewallen, W7EL