W8JI wrote:
>I suspect 90 degrees is far too high a wave angle for any DX.
I think 90 degrees may be best under certain conditions.
If you are familiar with the Pederson ray propagation mode, I
think that best describes what may be going on. Imagine the
following:
-----------/--------
/ F2 layer
/^>>>>>>>>>>>>>>>>>>>>>
/ ^
-------/ ^
D layer ^
^
^
^
Signal
Daylight Darkness
Twilight
Transition
Area
During daylight, DX signals are absorbed by the D layer which
is the lowest of all ionospheric regions. During darkness, the F1
and F2 layers predominate since D (completely) and E (mostly) disappear
in darkness. Assuming we go through some sort of transition zone
(probably a combination of D/E as they dissipate and F1/F2 as they
change altitudes) between these layers at sunrise and sunset, isn't it
possible that a high takeoff angle signal is refracted (not reflected)
by the transition area at a very low effective radiation angle?
Page 1-8 of my old 1984 ARRL Antenna Book states the following:
"...the wave is thought to propagate inside the ionosphere throughout
some portion of the path length, tending to be ducted in the ionized
layer. This theory is supported by the results of propagation studies
that have shown that a medium-angle ray sometimes reaches the earth at
greater distance from the transmitter than a low-angle ray as shown
in figure 9B (similar to my crude figure above). This higher angle ray,
named the Pederson ray, is believed to penetrate the layer farther than
lower angle rays. In the less densely ionized upper edge of the layer,
the amount of refraction nearly equals the curvature of the layer itself
as it encircles the earth. This non-hopping theory is further supported
by studies of propagation times for signals that travel completely
around the world. The time required is significantly less than would
be necessary to hop between the earth and the ionoshere 10 or more times
while circling the earth."
While I don't know exactly what is going on in the ionosphere,
or whether this Pederson ray theory even applies, I do think it is
possible that a 90 degree takeoff angle can perform better than a
low takeoff angle under certain conditions. Don't get me wrong, a
low angle will be better 99% of the time, especially in darkness, as
I know from my own experience using both antennas for many years. But
I also know that there are times when a high angle antenna will easily
outperform a vertical/Beverage combination on both transmit and
receive.
73, Bill W4ZV
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