Bill,
I modelled your antenna for a 20 meter high vertical with 4 X 10 meter (25 feet
is too short) long sloping topload wires. All 2mm.
The radiation resistance was about 14 ohm when resonant (no ‘’J’’). Depending
on the quality of your radial system you need to add 5…..100 Ohm (5 Ohm, very
good; 100 ohm, only two wires, 20 meter long and poor efficiency).
Even with a less then optimal radialnet it is unlikely that the impedance will
be as high as 50 Ohm and most likely it will be complex.
So you will need a network to convert this low (and complex) impedance to 50
Ohm.
My advise: 4 X 10 meter topload, good radial system, measure the impedance,
calculate the values of the components (calculator can be found on internet)
and construct a tuner. Do not take the mast down, you will not get 50 Ohm by
changing the lenght of the wires (at least not with that perfect radial system
you are going to make…..)
Kees, PE5T
Verzonden vanuit Mail voor Windows 10
Van: cqtestk4xs--- via Topband
Verzonden: maandag 5 februari 2018 18:14
Aan: topband@contesting.com
Onderwerp: Topband: 160 top loaded vertical question
Planning on putting up a 160 vertical, but of course a 1/4 wave vertical is out
of the question here in HI since we have a height limitation.
Here's the question: I'm putting up around 50 feet of 3 inch aluminum tubing
(left over from various antenna takedowns). I plan on putting up on top of
that around another 10 or 15 feet of smaller diameter stinger for a total
height of around 60-65 feet. I obviously need to make up for the lack of
height and I was planning on making a top hat of four wires coming down from
the top of the antenna at around a 45 degree angle. The problem is how long to
make those wires. My guess is around 25 feet based on my past experience in
making Tee-topped 80 meter verticals. However, it is only a guess and I don't
feel like yanking the 3 inch pipe up and down a bunch of times to get it just
right.
I know some trimming will be necessary since this is not an exact science. I
just need an educated guess for where to start with the length. Any ideas?
Bill K4XS/KH7XS
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