Topband: Inverted L question

[Guy Olinger K2AV]

On Thu, Dec 27, 2012 at 10:12 AM, Ashton Lee <Ashton.R.Lee at hotmail.com>
 wrote:

> Several of us here in Western Colorado run slopers off the tower, which I
> believe is essentially loading the tower with an elevated lead. It works
> for us.


Sloper is really not the same thing as a parallel vertical wire at four
feet as with the original poster.  The vertical wire parallel to the tower
at four feet has a much higher degree of coupling to the tower than your
sloper.

The tower is induced to a degree either way, but if the tower to dirt
resistance is high, the impedance of the sloper is not changed nearly to
the extent that the L's is.  The close coupling of the L works harder on
the tower's base resistance.   This is very easy to model.  Varying a load
resistance at the base of the tower in either case will show the dependence
of either antenna.  The worst losses occur when the tower, plus all its
antennas viewed as top loading, is close to natural resonance on 160.

Once the tower current gets down to a third or a fourth of the current in
the driven antenna, the antenna should be out of the deep woods for induced
loss.  As usual, those with really good conductive dirt are far less
affected than those with poor soil.

It is a very good exercise to model the tower and its entire antenna and
its conductors, along with all the other antennas, with feedlines
explicitly included.  Once you get that model done, a bit of a PITA the
first time, you hang on to it, and keep it current. You turn on the
feedpoints (EZNEC "sources") one at a time and see what happens.  There
should be a "load" in the bottom of the tower to account for the base
resistance to the dirt.   The devil is in the details.   Some folks come
out fairly clean, others have surprising interactions paid for in loss.  If
you are the latter, it's a very useful thing to know.

73, Guy