I have written a deck for NEC2 which uses and incident plane wave as the
excitation scenario.

The deck:

CMSmall square untuned loop
CMEXTENDED THIN WIRE KERNEL USED
CM1. FREE SPACE
CE2. Plane wave excitation
GW 1 9 -0.25 0 1 -0.25 0 1.5 .0007
GW 2 9 -0.25 0 1.5 +0.25 0 1.5 .0007
GW 3 9 +0.25 0 1.5 +0.25 0 1 .0007
GW 4 9 +0.25 0 1 -0.25 0 1 .0007
GE 1
EK
FR 0 1 0 0 7.1
EX 1 1 1 0 90 0 0 0 0 0
LD 4 1 1 1 50 0
GN -1
XQ
EN

I get an antenna factor of 35.8dB/m by multiplying the current in wire
1, seg 1 (the location of the 50 ohm load) by 50 ohms, and taking
-20*log of that voltage.

Owen

PS: This won't work if you are working with one of the original column
formatted Fortran inputs, but it does work with nec2++ (
http://www.si-list.org/NEC_Archives/swindex.html )