I like the inverted “V” configuration for the
40-meter and 80-meter bands. In the past I had used trap dipoles strung between
the tops of trees. It was not only difficult to get the antenna’s high in the
tree, but when the wind is gusty, the tops of trees would sway 10 to 20 feet.
That would require a pulley system with weights to avoid snapping the antenna.
The wire would become very heavy and the pulleys would freeze up in icing
conditions. The center feed of a dipole would add to the sag.
With the inverted “V”, the center of the span
is supported by the mast just 12 feet above where the rigid mast that is in turn
clamped to the even more rigid house. Both arms of the antenna take the form of
a catenary, exiting the center feed choke at an angle about 30 degrees below the
horizon and ending horizontally about 12 feet below. That form only sags about 3
feet in each 60-foot arm. The force on the mast is just the weight of the
antenna, choke, and feed line. The horizontal tension on the ends of the antenna
is equal to the weight of the entire antenna. The tension on the antenna where
it joins the choke is only 1.12 times the weight of the entire antenna.
The wide band, dual band, trap less antenna is
slightly different from the ideal described above because the weight is not
evenly distributed. But it is close enough to get a feel for the forces
involved. Each arm actually consists of two catenary segments.
The shape of the simpler catenary is found by
Antenna Height = A
( Cosh ( X / A) – 1)
Wire Length = A ( Sinh ( X / A))
where:
X = Horizontal distance from low point
A = Adjusted to achieve 30 degree exit
from antenna choke balun (109.25)
Note that the choice of 30 degrees is arbitrary.
There are an infinite number of possible catenary curves.
Similarly, having the low point tension
horizontal is also arbitrary. For example, if I pull on the ends with twice the
force, the sag becomes about half as much and the tension at the low point would
pull down by about 15 degrees.
By itself, the antenna only weighs 8.2 lbs (6.5 lbs wire, 1.7 lbs spacers). However, we have seen as much as an inch of ice form on the wires (2" diameter), which brings the weight closer to 500 lbs. as well as adding wind resistance. In practice, if more than 3/8 inch of ice coating (70 lbs) is expected, we untie the line at the base of the mast and in a short time most of the antenna is safely on the ground.
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