Hi Bill,
> One of the best ways to isolate an inverted-vee from a shunt-fed
> tower is to use a Resonant Balun which eliminates common-mode currents and
> decouples both sides of the wire antenna. At last year's 160 BBQ at
> K9RJ's (W9DXCC gathering), Jim K9RJ gave me a paper he wrote for inclusion
> in ON4UN's new book. He claims no change in the feed-point impedance of
> his shunt-fed tower when the inverted vee is connected. The top loading
> effect of the wire antenna and the heating effects encountered with
> conventional baluns were completely eliminated.
A few words of caution.
Electrical stresses in such installations can vary greatly from
installation to installation. Even though this works well in one case,
it might be totally unworkable in another.
1.) The operating Q of the parallel resonant circuit depends on the
impedance at the attachment point and the size of the components
used in the balun. Using as much L and least C possible will make
operating Q the lowest possible for any given case, and you want
the lowest operating Q possible.
With high operating Q (not much L and a lot of C), bandwidth of the
vertical will become narrower and the choke will become less
effective sooner with frequency change.
2.) Voltage across the balun is essentially the voltage at that point
on the tower. That can be MANY kilovolts even with modest power,
if there isn't considerable hardware (like yagi's) above the
attachment point of the balun.
Remember the voltage at any point on a monopole is twice the
voltage of a similar dipole. So traps have to be extra heavy-duty
when used to section off (and that's what this does) a monopole.
That means you need large transmitting style HV capacitors in the
trap, and in some cases it could even require vacuum caps.
The best case would be a single layer choke that is self-resonant
on 160 meters. Then you won't need a capacitor, and bandwidth
will be maximized.
By the way, I had a ten inch diameter two foot long resonant choke
of RG-8 feeding an 80 meter dipole near my 160 vertical. This
choke was used to detune the dipole's feedline. It wasn't even
attached to the tower. With 1500 watts during a light rain, it arced
over across a one inch path of plastic to a grounded metal support
post and started the form and coax on fire!
Keep all this in mind when using resonant chokes. Electrical
stresses can be enormous.
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|