In the inverted L or any antenna with a horizontal wire, there's
coupling between the wire and ground. The field from the horizontal wire
induces current in the ground under it. If the wire is low, the loss
produced by this current can be substantial. By putting an elevated wire
under the horizontal wire, you've changed this coupling to the ground,
plus you've introduced a new conductor into the antenna. Mutual coupling
between this conductor and the other wires will change the impedance.

Modeling will give a lot of insight into what all is going on.

Roy Lewallen, W7EL

Harold E. Johnson wrote:

Roy, I've always maintained that when I don't know that the ground
resistance is zero, I want as little current flowing in it as possible. My
inverted "L" is a voltage fed half wave on 160 meters, about 70 feet up and
170 feet out courtesy of a couple of strategically placed Oaks . I have
measured that feed impedance as being in excess of 2600 ohms and feed it
with a remotely tuned "L" network. I used it for several years just fed
against 60 feet of 6 inch well casing, and then, bowing to conventional
wisdom as advertised on 1850 KHz, added an elevated counterpoise beneath the
whole thing. (About 12 feet in the air) I didn't notice any change in signal
reports, but that impedance sure changed a lot. Did I change something
besides the ground resistance?

Regards

W4ZCB