Joel Kolstad wrote:
"I thought the entire point of the ground plane was that the induced
currents are necessary to make up for the current sources that are
supposed to have come from the "missing" half of the antenna?"

The currents need to be induced in low-loss material for efficiency.

The performance is as if a vertical and its reflection made up a dipole
in place of a monopole and its reflection. The equivalence is very good
in most respects.

According to Laport, G.H. Brown, the inventor of the "ground plane
antenna" and one of the famous "Brown, Lewis, and Epstein Trio" at RCA
is responsible for using electrostatic principles for describing a
ground or image plane and writing the equations to quantify the
potentials due to the charges on a system of cylindrical conductors to
be used as in a transmission line to determine characteristic impedance
in terms of capacitance per unit length and the velocity of propagation
of a TEM wave. See page 513 of "Radio Antenna Engineering" for details.

A vertical radiator driven against the earth or a ground plane needs to
complete its electrical circuit between the radiator and ground through
existing capacitance via capacitive (displacement) current. To minimize
loss, ground current in the lossy earth is minimized by using enough
radials of sufficient length to capture nearly all displacement current
before it can flow through any significant length of earth.

If the vertical radiator is high above the earth and far away from
ground, only a few radials suffice to capture nearly all displacement
current. Earth far below is out of the current loop.

When the radials are near the surface of the earth, many radials are
needed to hide the earth`s surface from displacement current with the
vertical radiator. Fortunately, when the radials are closely spaced, any
current induced in the earth doesn`t usually travel any significant
distance before being collected by a nearby radial and this keeps the
earth-current loss down. For the FCC, 120 radials of 1/4-wavelength
evenly distributed around a 1/4-wave vertical radiator are just about
perfect for the medium wave broadcast band.

Some of B, L.& E.`s performance charts are on page 119 of "Radio Antenna
Engineering".

Best regards, Richard Harrison, KB5WZI