NEC-LIST: NEC-4 bugs

NEC-4 users:

Roy Lewallen has uncovered a couple of bugs in the NEC-4 incident
wave coding. Actually he reported these back in January, so they
have been fixed in codes that we have sent out since 1/24/06.

For other users who want to correct them in their codes, the first
involves the sense of rotation of the elliptically polarized incident
plane wave. Right-hand and left-hand sense were reversed in the
incident wave. The corrected code lines in subroutine SORPWA are:

       CX=EMAX*(PX-FJ*AXRAT*QX)
       CY=EMAX*(PY-FJ*AXRAT*QY)
       CZ=EMAX*(PZ-FJ*AXRAT*QZ)

The original code had +FJ*... in each line. I have a vague memory of
a question of the polarization sense coming up ten years or more ago,
but we did not spot the error, or at least it did not get changed.
The polarization sense for either incident or scattered waves refers
to thumb pointing in the direction that the wave is propagating.
With the change to "-FJ*" crossed dipoles with an incident
right-hand elliptically polarized wave scatter a left-hand polarized
wave, as expected. Also, NEC-4 is now in agreement with NEC-2 which
has always been correct on this.

The other bug involves the reported scattering cross section when a
wave with elliptic polarization or amplitude other than 1 V/m is
incident. A normalization constant GCONN was computed in subroutine
RDPAT as:

C
C PLANE-WAVE SOURCE
C
          PRAD=0.
          GCONN=4.*PI/(1.+PSOR6(NSORC)**2)*CABS(XKU*XKU)/(4.*PI*PI)
          GCOP=GCONN

The correct form for GCONN is:

          GCONN=ABS(XKU*XKU)/(PI*(1.+PSOR5(NSORC)**2))/PSOR4(NSORC)**2

This problem is probably less likely to have been encountered than
the incident polarization sense, but it is surprising that it has
gone undetected for so long.

There have been a number of bugs reported in the ACES Newsletter over
the years, so NEC-4 users who got their codes more than about three
years ago and have not kept up on the patches should contact us for
an up to date set of files.

Regards,
Jerry Burke
LLNL

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