As Wes N5WA pointed out, coupling to your tower could be a real issue.
It sounds like the height of your tower, plus the capacitive loading of
the 40m yagi, could make it resonant near 160m. With further info on
tower and yagi dimensions, it wouldn't be hard to add them to a model to
get a fairly good idea of the impact. The biggest unknown would be
ground losses from the tower's connection to earth, if there is a
sizable current in the tower.
My model of your antenna (without the tower) over "average ground" shows
about 34 ohms at resonance, using two 28 foot sloping hat wires and two
130 foot long, 10 foot high elevated radials (all #14 bare copper). The
gain is 0.46 dBi broadside and -0.01 dBi off the ends of the wires at 22
degree elevation.. If the top hat wires are instead horizontal, they
only need to be 24 feet each and the feed Z rises to 37.4 ohms, with
0.53 dBi broadside and 0.09 dBi off the wire ends at 22 degree
elevation. There isn't much difference, since the hat wires are
relatively short in wavelengths.
73, Terry N6RY
On 2009-11-23 9:01 PM, Robert Pack wrote:
> I have a bit of a dilemma for the short term. While the construction is
> going on to erect a true elevated 160m vertical (36 foot high base, insulator
> then 120 feet above that) I need an alternative vertical for 160. My
> preferred choice at the moment is a top loaded T but current circumstances
> just are not going to allow me to string up such an antenna between towers.
> I had to remove the one I had up......or better yet........mother nature
> removed it for me and I cannot put it back up right now.
> I do not want an inverted L although it is probably the easiest thing to
> do....but I do want a vertical. I also do not want to shunt feed a tower and
> put down miles of radial wire.
> With a top loaded T antenna in mind.........I put the vertical wire on
> supports up my 40m tower with the feedpoint 10 feet above the ground to a
> point at 105 feet up the 40m tower (95 feet of vertical wire) and instead of
> the top loading being in a T configuration I just slope those two wires at 45
> degree angles downward.
> Does it matter if the top loading wires are really horizontal or can they be
> sloped as described above?
> I am assuming the feedpoint will be about 35 ohms so a pair of 75 ohm lines
> should accommodate that ok.
>
> Anyone have any valuable input on this issue?
> If I can make it work it will be a nice backup antenna for the future.
>
> Bob NX5M
>
>
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