> On Sun, 7 Oct 2007 16:58:21 +0000, Kristinn Andersen
> wrote:
>
>>Am I correct that there will be high RF voltages along the
>>entire
>>vertical, all the way from the "top end" of the base
>>inductor?
>
> Your vertical and the feedline to the dipole CAN interact
> with each
> other, and the result can be bad. A VERY easy fix is to
> add a
> serious common mode choke to the feedline to the dipole
> near the
> transmitter, and another one to the feedline for the
> vertical near
> the base of the
Kristin and all,
We have to be careful!!
As someone else pointed out there can be many thousands of
volts between the antenna element and ground. The electric
fields are extremely strong. Even the capacitance of the
feedline to the mast can negate most of the desired effects
of the choke or balun, and if that doesn't happen the choke
has to withstand thousands of volts.
It is very difficult, if not impossible, for the average
person to build a choke like that.
What I would suggest is to embrace the loading effect
instead of fighting it. Use a loading wire pair at the very
top as a dipole. Intentionally connect the dipole to the
mast on 160 with a low impedance. This can be done with a
small air wound inductor between the dipole terminals with a
center tap tied to the mast.
Then the problem is only decoupling the feeder at the
antenna bottom. I would suggest a small coax run inside
copper tubing, with the copper tubing as a loading and
matching coil. The feedline can exit at the coil bottom
where it can be grounded, or at least where it is a near
zero J point. At that point on towards the house
conventional chokes will work.
Other than that there is almost no way to decouple the
antenna.
I had just such an antenna many years ago. Two of the
loading wires were a 40 meter dipole, two were a 20 meter
dipole. They were in parallel like a fan dipole with wide
spread. The mast would load on 160 and 80, the dipoles on
40, 20, and 15.
5 bands with one 40 ft high push up mast.
73 Tom
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