Topband: Inverted Ground Plane

[Larry Higgins]

> Stand by for the usual barrage about the need for radial
> grounds on the ground beneath it.

There's good reason for the barrage.  In the case of the normal vertical,
fed at the bottom, it's easy to see that current will flow into the radials
because they are connected to the shield of the coax  which is one side of
the feedline.  What's not so obvious is that in the inverted GP, the bottom
of the antenna is at a high impedance and is therefore at high rf voltage.
As the voltage alternates, the electrons in the lossy ground below are
alternately attracted and repelled.  And that is an induced current.  It's
easy to fall into the trap of thinking that all induction involves a
magnetic field; but there is also electrostatic induction as we have here.

The current which flows in the lossy ground heats the ground.  And the
transmitter is supplying the lost power.  As W8JI points out, the last thing
you want to do is induce a current in the lossy ground--whether by magnetic
fields or electrostatic fields.

73

Larry, N9DX