ALL THE RIGHT ANGLES ?
YES and NO !
N6BV used 100 ft elevation for his 80, 40 and 20 meter models,
and 60 ft elevation for 10 and 15 meters, at both ends of the path.
Such elevations have pattern NULLS inside the range of angles
the ionosphere will support. Of course his results show NO
propagation at those angles because of the ANTENNA PATTERN.
If one really wants to understand the IONOSPHERE, it is necessary
to illuminate ALL angles equally, or use several different heights
to check path loss at each angle that is supported between the
two given locations.
Most simplistic models ONLY give the LOWEST angle that will
propagate between two locations. When the MUF exceeds the
frequency in use, several different angles may be supported,
yielding 3, 4, 5 or more hop modes.
If one of the stations is using a LOW antenna with reduced radiation
at the lower angles, received signal strength may very well be greated
at the higher angles, favoring a LOW antenna both locations. This is
NOT recognized in ALL THE RIGHT ANGLES.
An intestesting confirmation is that on 20M N6BV noted some very
high angle paths (over 30 degrees if I remember correctly) to high
MUF regions of the world, but NOTHING at intermediate angles
in the 20 degree range. This is a direct result of the PATTERN
NULL of the antennas used and has nothing to do with the angles
the IONOSPHERE will support. N6BV dismissed this as a aberation.
There is a very simple method to determine the MAXIMUM angles
that are supported by the ionosphere: simply note the CLOSEST
signals heard via the F-layer. These will represent the highest
angle supported by the F- layer. The ARRL Antenna Book and
handbooks have charts showing WAVEANGLE vs. distance for
the different layers.
On TEN meters, a study of SS Logs showed that a 900 mile
skip zone was typical around the country. This represents a
maximum supported waveangle of 22 degrees, well ABOVE
the highest angles reported in most propagation materials.
The highest angle supported on 15M is around 30 degrees,
and the highest angle supported on 20M is around 40 degrees.
One method of determining received waveangle is to have
multiple antennas at different heights. The antenna with
the LEAST signal provides the best clue: look at the angle
for the first NULL. Nulls tend to be narrow, limiting the
range of possible angles, and therefore are more useful
in determining received angles than the broad main lobes.
MANY observers report that LOW antennas outperform
their higher antennas during daylight hours on the high
bands, especially during higher sunspot activity. Even
during LOW sunspot activity I often see stronger signals
from Europe and Africa on my 40 ft high TH7 than my
higher monobanders.
Because of ground reflections, the vertical patterns of
horizonatal antennas have ranges of pattern enhancements
and cancellations. These nulls make it IMPOSSIBLE for
ANY SINGLE antenna to cover ALL of the angles supported
by the ionosphere. A tribander at 35 to 40 ft does an
excellent job of covering the higher angles with higher
antennas necessary to drive the main lobe down for the
lower angles useful at band openings and closings.
de Tom N4KG
............................
On Tue, 11 Aug 1998 13:00:37 -0400 "Jim White, K4OJ" <k4oj@ij.net>
writes:
>
>......try
>
>All The Right Angles
>
>by N6BV
>
>....if you really wanna get into this - this is the book to read!
>
>73,
>
>Jim, K4OJ
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