Topband: Inverted L matching problem

[Guy Olinger, K2AV]

----- Original Message ----- 
From: "Dennis W0JX" <w0jx at yahoo.com>

Your SWR results suggest that you should not connect the braid to the tower 
base. The tower's ground rods do little for your ground loss issues under 
the vertical portion of the antenna. That is affected mostly by the number 
and length of radials you put out under the vertical portion of the "L."
------------------------------

The SWR is not predictive of the efficiency of the tower/L system.  And it 
will be a system.

The electrical relationship between the tower and the inverted L is fixed by 
separation and parallel run length, regardless of what you do at the base, 
***more like two windings of the same transformer.***  The question remains 
as to what happens when the connections are switched at the base.  Most 
towers are NOT insulated, but set in concrete and then besides attached to a 
lightning ground rod system not at all designed for RF efficiency. The 
conductors up and down the tower (coax to beam, rotator control wires) are 
in the same close coupled relationship.

Since the original poster reported a significant impedance change when the 
radials were disconnected from the tower, one must assume that the base 
impedance just looking the tower into the ground rods is quite different 
than looking into the ground rods AND the radials. Changing this connection 
changes the tower's "acceptance" of energy from the tightly coupled  L and 
therefore changes the effective Z seen at the wire, the same way a change in 
the impedance on the secondary of a transformer changes the impedance seen 
at the primary.

The tower ground rods et al are quite lossy at RF and if there is no 
alternative less lossy path, the ground loss though the rods is imposed on 
the tightly coupled L, resulting in a less effective antenna, even if the 
happenstance impedance is accidentally closer to a 50 ohm match.

The original poster needs to view the tower as an unremovable attachment to 
the L as long as the vertical part is within an 1/8 wave or so.  Inverted 
L's away from towers with a decent radial system will act in the predicted 
manner.  L's within 1/4 wave or so can have the tower acting as an 
unintended parasitic element in what has become a two element array.

73, Guy.
K2AV