----- Original Message -----
From: "Dennis W0JX" <w0jx@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
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