Ladies and gentlemen:
My recent post about the new 160m array at KC1XX
generated a number of questions. As a result I
compiled the FAQ (below) that I thought would be of
interest to this group. In case you missed it
previously, you can see the photos at:
http://www.kc1xx.com/gallery/New-160m-array.
73, John W1FV
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Q. Why didn't you just employ all ground-mounted
radials if you were going to the trouble of installing
a ground screen plus elevated radials?
A. There were 2 deciding factors:
(1) The vertical at the southwest end sits very close
to KC1XX's property boundary and it is not possible to
run radials of any significant length over the boundary
in that direction. However, we could run 2 elevated
radials parallel to the boundary and still be inside
the property line. The ground screen runs up to the
property boundary. As I mentioned before, the modeling
shows a significant improvement in gain with the screen
vs. without, using a "very poor" ground model. New
England ground at inland locations is generally poor to
very poor. The benefits of a ground screen when using
elevated radials have been noted in ON4UN's book and in
some of the previous posts to this reflector.
(2) Although it's not obvious from the photos, the
verticals sit on the side of a hill, with the land
sloping up in the northeast direction. The vertical at
the bottom of the hill is nearly 40 feet lower in
elevation than the vertical at the top. We figured
that elevating the feedpoints would help offset this
disadvantage, although I can't quantify the effect.
(The YT and TA terrain analysis programs can't handle
this scenario).
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Q. How did you construct the screen? Is it a real
screen or radials?
A. The screen is made of individual wires, laid out in
a rectangular grid, about 400'x300'. The individual
grid squares are 15' on a side. All the wire
intersections are soldered.
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Q. Are the elevated radials and the screen connected
together?
A. No, there is no connection between them.
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Q. How did you decide on 2 elevated radials per
element and not more?
A. Modeling showed that the best pattern and gain was
obtained with 2 elevated radials per element,
perpendicular to the axis of the array. Adding any
more radials degraded the pattern, probably because the
coupling between the radials distorted the current
distributions from the ideal. There was no geometry
that I could find that mitigated this effect except
with 2 radials. The importance of elevated radial
geometry with multi-element arrays is also discussed in
ON4UN's book.
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Q. Why the spacing of 112 feet? Is that the spacing
between individual elements or the total end-to-end
spacing?
A. 112 feet is the spacing between each element, so
it's 224 feet end-to-end. Actually, this choice was
quite simple. The array makes use of two pre-existing
towers that were 224 feet apart. We simply added a
third tower in the middle. This spacing works out well
in that it produces decent gain in both the end-fire
and broadside modes, so that 4 directions of switched
coverage are obtained.
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