To K9UWA, ON4UN and Other Topbanders:
In the past there have been discussions on the effectiveness of top
loading a vertical element with horizontal wires.
One school of thought says the effect of one or two horizontal wires
is the same, so that an inverted L or a Tee antenna has the same
loading wire length from attachment at the top of vertical element to
end of top loading wire(s), and to shorten the top loading, three or
more wires are needed at the top of the vertical element.
Another school of thought says using two wires provides more top
loading, so each side should be about 60 - 70 % as long as a single
inverted L horizontal top wire, and adding more top loading wires
will simulate a solid disc, which may make the length of each top
loading wire about 50% of a single wire length from an inverted L.
Over this past weekend W8UVZ, W0CD and I did some work on a top
loaded wire vertical element as part of a multi-element array for
160 meters. We thought we should share the dimensional information
for future reference.
The Tee element is part of a cantenary wire from the top of a 126.5
foot tall tower to a ground anchor 247.5 feet away. The feed point
for the wire is 67.5 feet from the tower. The tower is insulated
at the base, and is guyed with Philystran (r) to avoid extra metal
in the near field.
We started with a 78 foot vertical wire connected to the center of
a 96 foot long Tee top loading wire. Both are made of 3/16
galvanized EHS guy wire. The approximate resonant frequency was
1465 Khz, with a feed point impedance of about 26 ohms.
After twice trimming the top loading wire of this Tee element we
ended up with the same vertical wire, 78 feet high, and the Tee top
wire 67 feet long. Since it is part of a cable from the top of a
tower to a ground anchor, it is tilted about 33 degrees from
horizontal.
The ground system is 120 radials as much as 210 feet long on the
outside perimeter of the array under construction. The interior
radials all tie to buss wires and radials from future element feed
points.
The resonant frequency is 1811 khz with Z ant = 30.8 + 0j ohms.
Note: the Tee top length is about two feet longer than a computer
model that seems to agree with the second school mentioned above.
Incidentally, the tower fed against ground comes in at 1811 or 1812
Khz, with base insulators one foot above ground, and 125.5 feet actual
height of Rohn 25G from feed point to top of the plate on the top
section.
Hope this is of interest.
George, K8GG for W8UVZ and W0CD.
PS, my home modem took a hit, so I am sending this from the office.
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