Ron, et. al.,
This issue was addressed recently on this reflector with a tall tower and
big yagi that provided enough top loading to make the tower plus yagi about
1/2 wavelength tall. The three best solutions seemed to be:
1a. Make a wire sleeve around the bottom half of the tower to isolate it
and feed it at the junction of the sleeve and tower. KV4FZ suggested at
least 4 vertical down wires, more would be better. Sloping parasitics can
be hung off such a tower. The problem is switching them in and out of the
field, since they are up in the air and a coil or stub has to be added to go
from a reflector to a director, and the parasitics have to be detuned
perpendicular to the direction of interest if the array, assuming the array
is going to be stearable in more than front and back directions.
1b. Alternately, feed the tower 1/4 wave down from the top and make a
sloping radial skirt as K9AY suggested. With the SSV style tower (with wide
base) this may be a lot easier than an isolation sleeve.
2. Make a K8UR style 4-Square. This is 4 vertically oriented inverted
Vees with the insulated tie points attached to the top and bottom of the
tower. Each is pulled out so the sine wave current centers are 1/4 wave
apart, diagonally. I believe ComTec, who makes the phasing boxes, has the
details. This is better than plain slopers, as the horizontal components of
the sloping dipoles are cancelled.
3. Look up sloping 4-square arrays in ON4UNs low band DXing books. Since
you have a tall tower, it should be fairly easy to make a 4-square with
sloping elements guyed off near the top of the tower without having to hang
spreaders out to keep the slope steep.
Good luck, 73, George K8GG
----- Original Message -----
From: "Ron Feutz" <feutz@wctc.net>
To: <topband@contesting.com>
Sent: Tuesday, August 19, 2003 10:17 PM
Subject: Topband: 1/2 wave tower
> Topbanders,
>
> I have been toying with the idea of building some antennas for 160
> contesting at one of my tower sites, a 240' SSV. (I'm in the two-way land
> mobile business).
>
> Is it practical to shunt feed a 1/2^ vertical? ON4UN's book doesn't
> address feeding this monster, at least not that I could find. Obviously,
> it is grounded by the footings.
--- snip----
> 73, Ron - WA9IRV
>
--- snip----
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