I've received an authoritative answer about NEC plane wave excitation.
When a 1 V/m incident plane wave is specified via a plane wave source
and a ground plane is present, the field strength at all points is 2
V/m, not 1 V/m as I had assumed. The rationale is that the "incident
wave" is reflected by the ground plane, doubling its strength.

The conclusions from this are that:

1. NEC reports that the voltage from the base of a 1 meter electrically
short vertical wire to perfect ground in the presence of a 1 V/m field
is 0.5 volt, not 1 volt as I said in my earlier posting in response to a
question by Reg. I apologize for the error.

2. The power intercepted by the matched dipole in the problem recently
posed by Reg is approximately 3 mW, not 12. The EZNEC calculation I
described, which does not use a plane wave source, is correct. The same
result can be obtained with NEC by using two antennas as in EZNEC, or
with a 212 V/m (peak, equal to 150 V/m RMS) plane wave source which
produces a 300 V/m RMS field at the loaded antenna.

This is a good place to give an additional caution to people using NEC
for calculations. NEC uses peak, not RMS values for all voltages and
currents. Power results will be off by a factor of two or four if a user
mistakenly assumes RMS values. EZNEC uses RMS values throughout. When
Reg posed the dilemma about the factor of four disparity in reported
powers, my first thought was that this was the cause. As it turned out,
it wasn't, but caution is needed. Results should always be given a
reality check, as Reg has done. Any model -- and this doesn't exclude
the mathematical models we often consider "theory" -- can be subject to
many errors, including but not limited to misapplication,
misinterpretation, and limitations of an approximation or numerical
calculation.

Roy Lewallen, W7EL