>Ok, this post seems to tell me I also could have an array on 40 on a hill
>side!!
>I believed such arrays had to be on level ground to operate as expected.
>What a shock this is to learn.
(snip...)
> Is there a max slope/grade down which
>one should not try arrays??
I think it depends on the slope of the hill. I was told by several people,
including Jim Miller of Comtek (the 4-square expert and provider of the only
commercial phasing boxes), that it is important for the element feedpoints
to be at the same height. If you can manage that, I don't think the shape of
the terrain in the near field will have all that much effect, unless you're
on a *really* steep hill, have mountains right in your face on one or more
sides, or live in a box canyon. My comments, however, apply to installations
with an adequate number of ground-mounted radials (60+) per element. I have
no idea how terrain would affect an elevated radial system (e.g., radials
sloping steeply uphill, etc.)
I'll try to describe the terrain here:
My house in New Hampshire (elevation 680 feet) sits nearly at the local top
of the eastern bank of the Connecticut River (elevation 380 feet.) To the
north, northwest, west, and southwest), the land falls away sharply down the
river bank, dropping 300 vertical feet in 1500 horizontal feet. This affords
nearly 180-degrees of wide-open view to the west, across the river valley to
the green hills of Vermont, which don't start to rise above the elevation of
my house for several miles (the sunsets are great.) To the northeast, east,
southeast and south, the terrain rises very gradually, less than 200 feet in
one mile. After a couple of miles, the land rises steeply to about 1400
feet, drops again, then rises to 2300 ft on Moose Mountain, about 3 miles to
the southeast.
The 4-square is located about 150 horizontal feet down the hill from the
house, about 40-50 feet lower in elevation (it, the crankup tower, and all
my other antennas are down there hidden behind a screen of trees, per the
XYL -- otherwise it wouldn't be downhill to my antenna farm!) That means the
4-square has about 50 vertical feet of steep hill above it to the northeast,
east, southeast and south, and the element tips are well below the top of
grade. The land on the western side of the compass drops away sharply.
*However*, the array elements sit on a little 50' x 50' section of land that
happens to be considerably more level than the rest of the hillside. Through
careful analysis of the placement alternatives (see below), three of the
elements (SW, NW, and NE) are very nearly level with each other (i.e., the
feedpoints and tips are within a foot or less in height.) The SE element is
up the hill a bit, so that its feedpoint is about 2 feet higher than the NE
and SW elements and about 3 feet higher than the NW element. That means the
hill drops about 3 feet in about 50 feet across the square, so it's
considerably less steep than the general grade of the river bank (about 1 in
17 versus 1 in 5). The radials to the eastern side of the compass tend to be
on steeper ground rising up, while the radials on the western side of the
compass tend to be on more level ground, gradually dropping down.
The placement issues were these: 1) try to keep the element feedpoints as
level with each other as possible, 2) keep the elements at least 1/2
wavelength from the tower and inverted vee, 3) place the array to minimize
incursion of radials across the property line (about 10 of them threatened
to go over by about 5 feet -- not a big deal since the neighbor lives way
down by the river), and 4) optimize beam directions for DX and contesting (a
BASIC program was used with beam headings from the CT countries file for
this.) I drew the antenna farm to scale on graph paper, then cut out a
square of graph paper the size of the array (plus radials) so I could slide
it around and see how it worked out at various angles. Then I did trial
placements of the elements (using stakes) at the best DX/contesting angles,
and checked out the relative feedpoint heights and where the radials would
end up. It turned out that placing the NE element pointing at 55 degrees
worked out the best.
I could have put all the feedpoints at exactly the same elevation by raising
three of the elements up 2-3 feet higher on their supports. I decided not to
do this because I felt that the difference in feedpoint height wasn't all
that bad and that raising some of the elements a different height above the
ground screen might result in significant differences in feedpoint
impedance. So, I elected to put all the feedpoints about 8 inches above the
ground. As it is, dumped power is running well below 5% on all four
elements, with only slight differences per element.
As I said in my last post, the array works very well, and I haven't been
able to detect any serious differences in performance between the elements.
I think performance is excellent to Europe/Russia/Asia, North Africa, South
America/Southeast USA, Southwest USA/VK/ZL, Pacific, and Northwest USA/JA.
If I have a bad direction, it's bound to be east/southeast towards central
and southern Africa. Two of the "nulls" intersect there and the local hills
rise up steepest in that direction. I haven't heard any booming signals from
that area on either the 4-square or the tribander, but haven't done enough
analysis to be sure whether it's lack of activity in that part of the world
or a limitation of my local terrain.
Hope this helps.
73, Dick, WC1M
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