> To my
> > amazement this worked well. Have you ever looked at this combination or
> can you
> > analyze its performance?
>
> The answer is no, concerning if I ever tested a shunt feed inside the
> tower. The answer it's yes about a possible analysis with
modeling.
I've tried to use them on large BC towers, and they were a good
demonstration of Faraday's laws about fields and currents inside
closed cylinders being equal and opposite, and the outside world
having no effect on them and vice versa.
The best way to view this problem is by considering the ultimate
"tower with a wire inside", a piece of common but very fat coaxial
cable.
Short a piece of coax and try, without inverting the shield and
center or cutting the outside conductor all the way around some
distance down or near the feedpoint, to receive or transmit on the
coax. It will be a dead system, even if (at the short) you connect
more wire.
If you tie the shield of the feedline to the tower at the point of entry
into the tower, and have no breaks in the tower all the way up to
the point where the wire attaches, the system will be stone dead.
The only thing that can radiate is current that "spills over" on the
outside of the tower through some connection defect where the
"shunt wire" enters the tower or higher.
For example, if I ground the feedline to an external ground and
slope or run the wire up the inside of the tower that is grounded to
another ground or another point on the same ground, the system
will radiate. All excitation comes from the small area where the
connections are made, and that connection can excite the whole
tower.
In any case, it behave nothing like a conventional shunt system. It
radiates only if some flaw, intentional or accidental, where the
feedwire enters causes current to flow on the outside of the tower.
In any case, the result is about the same as using the wire entry
point as a tap for a gamma feedpoint, with some modification in
reactance.
You might be better off intentionally feeding the tower as high up as
possible (if you can't get to the 50 ohm point) and be done with it
rather than depending on accidents or flaws in the feed system
causing it to work.
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
|