When the Sommerfeld/Norton ground option is requested on the GN card, NEC reads interpolation tables from the file TAPE21. This file must be created prior to the NEC run by running the separate program SOMNEC. SOMNEC reads a single data card with the parameters:

• EPR, SIG, FMHZ, IPT (format 3E10.3, I5)

The three decimal numbers end in columns 10, 20, and 30 and the integer IPT must end in column 35. The parameters are:

• EPR=relative dielectric constant of ground (Er)
• SIG=conductivity of ground in mhos/m (S)
• FMHZ=frequency in MHz
• IPT=1 to print the interpolation table
• =0 for no printed output.

The interpolation tables depend only on the complex dielectric constant

• Ec=Er - j(S/WE0),
• Er=EPR, S=SIG.

If SIG is input as a negative number, the program sets

• Ec=EPR - j|SIG| ,

and frequency is not used. The tables are written on the file TAPE21. The central processor time to generate the tables on a CDC 7600 computer is about 15 seconds.