Topband: 1/2 wave tower

[Tree]

On Tue, Aug 19, 2003 at 09:17:07PM -0500, Ron Feutz wrote:

> 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.

It is tricky.  The problem is that the bottom of the tower wants to be
a high voltage point - not very nice when it is grounded.

You would have to try something I played with a while back - which is 
trying to make part of your tower "disappear" using a shunt wire and
capacitor in series.  The idea is to make it resonant so that it acts
like a high impedance circuit.  I think W8JI put a chapter on his web
page about this.

It is also described in ON4UN's book - with one minor error.  He says
to tune for minimum current in the shunt wire.  It should be maximum
current.  

You might need to experiement with where to put the shunt.  My guess
would be to try doing it for the whole bottom quarter wave of the 
tower, but that might be too long to tune,

A better antenna is probably a half wave sloper using coax feed in 
the middle.  A similar array to yours was used at N5AU with great
success.  They had 3 or 4 dipoles going off in different directions
and got some good directivity.  It would be a lot easier to make work,
but does require that you can string wires out from the tower.

Tree N6TR
tree at kkn.net