Art Unwin KB9MZ wrote:
Roy Lewallen wrote in message ...

It's really quite simple and fundamental.


Appreciate your response Roy, but the fact is the
matter is not simple to me. I am comparing horizontally
polarisation patterns in all cases thus I am having
difficulty with your explanation!

Ok, I'll try again. To determine the relative field strength at a
distant point at an elevation angle of, say 20 degrees, put the antenna
at the height you're interested in. Draw a line from the antenna to the
ground, at a downward angle of 20 degrees, then reflecting upward,
resulting in a ray going upward at an elevation angle of 20 degrees.
Draw another line from the antenna at an upward angle of 20 degrees. You
now have two parallel lines, a "direct ray" and a "reflected ray". At
some distant point, draw a line perpendicular to those rays. Measure the
distance from the antenna to the new line via each of the two paths, one
direct and the other reflected. You'll be adding these two rays, and the
difference between the two paths tells you the relative phases of these
two components you'll be adding. For example, if the antenna is a half
wavelength high, you'll find that at an elevation angle of 30 degrees,
the reflected ray travels exactly one wavelength farther than the direct
ray, so the two rays will exactly add in phase. At higher or lower
angles, they won't. When adding the two rays, you've also got to factor
in the free-space radiation pattern of the antenna to see just how much
the antenna is radiating at those angles (say, 20 degrees down and 20
degrees up from horizontal, for the pattern at 20 degrees). In the case
of a dipole, the free-space radiation pattern broadside to the antenna
is circular, so rays at all angles are equal. Thus, 30 degrees is the
"takeoff angle" for a dipole up a half wavelength. You do also have to
include a factor for the reflection coefficient of the reflected ray
from the ground. But for horizontally polarized waves at moderate to low
angles, it's very close to one. (But it's not, for vertically polarized
signals, so it should always be computed for vertical antennas.) This is
the way that AO, NEC, EZNEC, MININEC, and similar programs compute the
elevation pattern.

Now suppose that an antenna has a skinny elevation pattern in free
space. The W8JK is an example. At, say, 30 degrees up or down, the
signal is weaker than at 20 degrees. So the elevation pattern will be
correspondingly stronger than a dipole at 20 degrees relative to 30
degrees. This will lower the "takeoff angle" -- the elevation angle at
which the pattern is maximum.

These patterns can be pretty easily created with a calculator and either
some trigonometry or graph paper if you have the free-space pattern, but
modern programs can do the work for you.

It comes to mind also that an antenna used for listening
( beverage ?) also comprises of stacked collinear
horizontally polarised radiators where the vertical
radiators appears to cancel
themselves out.

No, the radiation from a Beverage is primarily vertically polarized, off
the end.

So it would appear to be a case where
a beam that is close to the ground ( coupled maybe to a
radiator other than the ground) is also capable of
decreasing the TOA even more than such an arrangement
at 1WL height.

I dunno. Look at the method I described, and try it on your theoretical
antenna to see if that's true or not.

Odd that you also brought into the picture the W8JK
antenna that also relies on critical coupling for
its extrorninary gain which you suggest also provides
for a low TOA when compared to the Yagi.

Egad, the magical "critical coupling". The W8JK has mutual impedance and
coupling between the elements like any other antenna. At 4 or so dB for
a couple of elements (if you keep losses down), I wouldn't call its gain
"extraordinary", either. It follows the same rules as all other
antennas, and its gain and other characteristics can be predicted with
great accuracy using the same ordinary methods used for all other antennas.

I will have to get the Kraus book from the library
for myself to read and hopefully there will be a
graph of some sort that will outline its advantages
and limitations.

It's described in _Antennas_, all editions I believe.

. . .

Roy Lewallen, W7EL